New doc/building.md

1 % Building the JDK
2
3 ## TL;DR (Instructions for the Impatient)
4
5 If you are eager to try out building the JDK, these simple steps works most of
6 the time. They assume that you have installed Mercurial (and Cygwin if running
7 on Windows) and cloned the top-level JDK repository that you want to build.
8
9 1. [Get the complete source code](#getting-the-source-code): \
10 `hg clone http://hg.openjdk.java.net/jdk/jdk`
11
12 2. [Run configure](#running-configure): \
13 `bash configure`
14
15 If `configure` fails due to missing dependencies (to either the
16 [toolchain](#native-compiler-toolchain-requirements), [build tools](
17 #build-tools-requirements), [external libraries](
18 #external-library-requirements) or the [boot JDK](#boot-jdk-requirements)),
19 most of the time it prints a suggestion on how to resolve the situation on
20 your platform. Follow the instructions, and try running `bash configure`
21 again.
22
23 3. [Run make](#running-make): \
24 `make images`
25
26 4. Verify your newly built JDK: \
27 `./build/*/images/jdk/bin/java -version`
28
29 5. [Run basic tests](##running-tests): \
30 `make run-test-tier1`
31
32 If any of these steps failed, or if you want to know more about build
33 requirements or build functionality, please continue reading this document.
34
35 ## Introduction
36
37 The JDK is a complex software project. Building it requires a certain amount of
38 technical expertise, a fair number of dependencies on external software, and
39 reasonably powerful hardware.
40
41 If you just want to use the JDK and not build it yourself, this document is not
42 for you. See for instance [OpenJDK installation](
43 http://openjdk.java.net/install) for some methods of installing a prebuilt
44 JDK.
45
46 ## Getting the Source Code
47
48 Make sure you are getting the correct version. As of JDK 10, the source is no
49 longer split into separate repositories so you only need to clone one single
50 repository. At the [OpenJDK Mercurial server](http://hg.openjdk.java.net/) you
51 can see a list of all available repositories. If you want to build an older version,
52 e.g. JDK 8, it is recommended that you get the `jdk8u` forest, which contains
53 incremental updates, instead of the `jdk8` forest, which was frozen at JDK 8 GA.
54
55 If you are new to Mercurial, a good place to start is the [Mercurial Beginner's
56 Guide](http://www.mercurial-scm.org/guide). The rest of this document assumes a
57 working knowledge of Mercurial.
58
59 ### Special Considerations
60
61 For a smooth building experience, it is recommended that you follow these rules
62 on where and how to check out the source code.
63
64 * Do not check out the source code in a path which contains spaces. Chances
65 are the build will not work. This is most likely to be an issue on Windows
66 systems.
67
68 * Do not check out the source code in a path which has a very long name or is
69 nested many levels deep. Chances are you will hit an OS limitation during
70 the build.
71
72 * Put the source code on a local disk, not a network share. If possible, use
73 an SSD. The build process is very disk intensive, and having slow disk
74 access will significantly increase build times. If you need to use a
75 network share for the source code, see below for suggestions on how to keep
76 the build artifacts on a local disk.
77
78 * On Windows, if using [Cygwin](#cygwin), extra care must be taken to make sure
79 the environment is consistent. It is recommended that you follow this
80 procedure:
81
82 * Create the directory that is going to contain the top directory of the
83 JDK clone by using the `mkdir` command in the Cygwin bash shell.
84 That is, do *not* create it using Windows Explorer. This will ensure
85 that it will have proper Cygwin attributes, and that it's children will
86 inherit those attributes.
87
88 * Do not put the JDK clone in a path under your Cygwin home
89 directory. This is especially important if your user name contains
90 spaces and/or mixed upper and lower case letters.
91
92 * Clone the JDK repository using the Cygwin command line `hg` client
93 as instructed in this document. That is, do *not* use another Mercurial
94 client such as TortoiseHg.
95
96 Failure to follow this procedure might result in hard-to-debug build
97 problems.
98
99 ## Build Hardware Requirements
100
101 The JDK is a massive project, and require machines ranging from decent to
102 powerful to be able to build in a reasonable amount of time, or to be able to
103 complete a build at all.
104
105 We *strongly* recommend usage of an SSD disk for the build, since disk speed is
106 one of the limiting factors for build performance.
107
108 ### Building on x86
109
110 At a minimum, a machine with 2-4 cores is advisable, as well as 2-4 GB of RAM.
111 (The more cores to use, the more memory you need.) At least 6 GB of free disk
112 space is required.
113
114 Even for 32-bit builds, it is recommended to use a 64-bit build machine, and
115 instead create a 32-bit target using `--with-target-bits=32`.
116
117 ### Building on aarch64
118
119 At a minimum, a machine with 8 cores is advisable, as well as 8 GB of RAM.
120 (The more cores to use, the more memory you need.) At least 6 GB of free disk
121 space is required.
122
123 If you do not have access to sufficiently powerful hardware, it is also
124 possible to use [cross-compiling](#cross-compiling).
125
126 ### Building on 32-bit arm
127
128 This is not recommended. Instead, see the section on [Cross-compiling](
129 #cross-compiling).
130
131 ## Operating System Requirements
132
133 The mainline JDK project supports Linux, macOS, AIX and Windows.
134 Support for other operating system, e.g. BSD, exists in separate "port"
135 projects.
136
137 In general, the JDK can be built on a wide range of versions of these operating
138 systems, but the further you deviate from what is tested on a daily basis, the
139 more likely you are to run into problems.
140
141 This table lists the OS versions used by Oracle when building the JDK. Such
142 information is always subject to change, but this table is up to date at the
143 time of writing.
144
145 Operating system Vendor/version used
146 ----------------- -------------------------------------------------------
147 Linux Oracle Enterprise Linux 6.4 / 7.6
148 macOS Mac OS X 10.13 (High Sierra)
149 Windows Windows Server 2012 R2
150
151 The double version numbers for Linux are due to the hybrid model
152 used at Oracle, where header files and external libraries from an older version
153 are used when building on a more modern version of the OS.
154
155 The Build Group has a wiki page with [Supported Build Platforms](
156 https://wiki.openjdk.java.net/display/Build/Supported+Build+Platforms). From
157 time to time, this is updated by contributors to list successes or failures of
158 building on different platforms.
159
160 ### Windows
161
162 Windows XP is not a supported platform, but all newer Windows should be able to
163 build the JDK.
164
165 On Windows, it is important that you pay attention to the instructions in the
166 [Special Considerations](#special-considerations).
167
168 Windows is the only non-POSIX OS supported by the JDK, and as such, requires
169 some extra care. A POSIX support layer is required to build on Windows.
170 Currently, the only supported such layers are Cygwin and Windows Subsystem for
171 Linux (WSL). (Msys is no longer supported due to a too old bash; msys2 would
172 likely be possible to support in a future version but that would require effort
173 to implement.)
174
175 Internally in the build system, all paths are represented as Unix-style paths,
176 e.g. `/cygdrive/c/hg/jdk9/Makefile` rather than `C:\hg\jdk9\Makefile`. This
177 rule also applies to input to the build system, e.g. in arguments to
178 `configure`. So, use `--with-msvcr-dll=/cygdrive/c/msvcr100.dll` rather than
179 `--with-msvcr-dll=c:\msvcr100.dll`. For details on this conversion, see the section
180 on [Fixpath](#fixpath).
181
182 #### Cygwin
183
184 A functioning [Cygwin](http://www.cygwin.com/) environment is required for
185 building the JDK on Windows. If you have a 64-bit OS, we strongly recommend
186 using the 64-bit version of Cygwin.
187
188 **Note:** Cygwin has a model of continuously updating all packages without any
189 easy way to install or revert to a specific version of a package. This means
190 that whenever you add or update a package in Cygwin, you might (inadvertently)
191 update tools that are used by the JDK build process, and that can cause
192 unexpected build problems.
193
194 The JDK requires GNU Make 4.0 or greater in Cygwin. This is usually not a
195 problem, since Cygwin currently only distributes GNU Make at a version above
196 4.0.
197
198 Apart from the basic Cygwin installation, the following packages must also be
199 installed:
200
201 * `autoconf`
202 * `make`
203 * `zip`
204 * `unzip`
205
206 Often, you can install these packages using the following command line:
207 ```
208 <path to Cygwin setup>/setup-x86_64 -q -P autoconf -P make -P unzip -P zip
209 ```
210
211 Unfortunately, Cygwin can be unreliable in certain circumstances. If you
212 experience build tool crashes or strange issues when building on Windows,
213 please check the Cygwin FAQ on the ["BLODA" list](
214 https://cygwin.com/faq/faq.html#faq.using.bloda) and the section on [fork()
215 failures](https://cygwin.com/faq/faq.html#faq.using.fixing-fork-failures).
216
217 #### Windows Subsystem for Linux (WSL)
218
219 Windows 10 1809 or newer is supported due to a dependency on the wslpath utility
220 and support for environment variable sharing through WSLENV. Version 1803 can
221 work but intermittent build failures have been observed.
222
223 It's possible to build both Windows and Linux binaries from WSL. To build
224 Windows binaries, you must use a Windows boot JDK (located in a
225 Windows-accessible directory). To build Linux binaries, you must use a Linux
226 boot JDK. The default behavior is to build for Windows. To build for Linux, pass
227 `--build=x86_64-unknown-linux-gnu --host=x86_64-unknown-linux-gnu` to
228 `configure`.
229
230 If building Windows binaries, the source code must be located in a Windows-
231 accessible directory. This is because Windows executables (such as Visual Studio
232 and the boot JDK) must be able to access the source code. Also, the drive where
233 the source is stored must be mounted as case-insensitive by changing either
234 /etc/fstab or /etc/wsl.conf in WSL. Individual directories may be corrected
235 using the fsutil tool in case the source was cloned before changing the mount
236 options.
237
238 Note that while it's possible to build on WSL, testing is still not fully
239 supported.
240
241 ### macOS
242
243 Apple is using a quite aggressive scheme of pushing OS updates, and coupling
244 these updates with required updates of Xcode. Unfortunately, this makes it
245 difficult for a project such as the JDK to keep pace with a continuously updated
246 machine running macOS. See the section on [Apple Xcode](#apple-xcode) on some
247 strategies to deal with this.
248
249 It is recommended that you use at least Mac OS X 10.13 (High Sierra). At the time
250 of writing, the JDK has been successfully compiled on macOS 10.12 (Sierra).
251
252 The standard macOS environment contains the basic tooling needed to build, but
253 for external libraries a package manager is recommended. The JDK uses
254 [homebrew](https://brew.sh/) in the examples, but feel free to use whatever
255 manager you want (or none).
256
257 ### Linux
258
259 It is often not much problem to build the JDK on Linux. The only general advice
260 is to try to use the compilers, external libraries and header files as provided
261 by your distribution.
262
263 The basic tooling is provided as part of the core operating system, but you
264 will most likely need to install developer packages.
265
266 For apt-based distributions (Debian, Ubuntu, etc), try this:
267 ```
268 sudo apt-get install build-essential
269 ```
270
271 For rpm-based distributions (Fedora, Red Hat, etc), try this:
272 ```
273 sudo yum groupinstall "Development Tools"
274 ```
275
276 ### AIX
277
278 Please consult the AIX section of the [Supported Build Platforms](
279 https://wiki.openjdk.java.net/display/Build/Supported+Build+Platforms) OpenJDK
280 Build Wiki page for details about which versions of AIX are supported.
281
282 ## Native Compiler (Toolchain) Requirements
283
284 Large portions of the JDK consists of native code, that needs to be compiled to
285 be able to run on the target platform. In theory, toolchain and operating
286 system should be independent factors, but in practice there's more or less a
287 one-to-one correlation between target operating system and toolchain.
288
289 Operating system Supported toolchain
290 ------------------ -------------------------
291 Linux gcc, clang
292 macOS Apple Xcode (using clang)
293 AIX IBM XL C/C++
294 Windows Microsoft Visual Studio
295
296 Please see the individual sections on the toolchains for version
297 recommendations. As a reference, these versions of the toolchains are used, at
298 the time of writing, by Oracle for the daily builds of the JDK. It should be
299 possible to compile the JDK with both older and newer versions, but the closer
300 you stay to this list, the more likely you are to compile successfully without
301 issues.
302
303 Operating system Toolchain version
304 ------------------ -------------------------------------------------------
305 Linux gcc 9.2.0
306 macOS Apple Xcode 10.1 (using clang 10.0.0)
307 Windows Microsoft Visual Studio 2017 update 15.9.16
308
309 All compilers are expected to be able to compile to the C99 language standard,
310 as some C99 features are used in the source code. Microsoft Visual Studio
311 doesn't fully support C99 so in practice shared code is limited to using C99
312 features that it does support.
313
314 ### gcc
315
316 The minimum accepted version of gcc is 5.0. Older versions will generate a warning
317 by `configure` and are unlikely to work.
318
319 The JDK is currently known to be able to compile with at least version 9.2 of
320 gcc.
321
322 In general, any version between these two should be usable.
323
324 ### clang
325
326 The minimum accepted version of clang is 3.2. Older versions will not be
327 accepted by `configure`.
328
329 To use clang instead of gcc on Linux, use `--with-toolchain-type=clang`.
330
331 ### Apple Xcode
332
333 The oldest supported version of Xcode is 8.
334
335 You will need the Xcode command lines developers tools to be able to build
336 the JDK. (Actually, *only* the command lines tools are needed, not the IDE.)
337 The simplest way to install these is to run:
338 ```
339 xcode-select --install
340 ```
341
342 It is advisable to keep an older version of Xcode for building the JDK when
343 updating Xcode. This [blog page](
344 http://iosdevelopertips.com/xcode/install-multiple-versions-of-xcode.html) has
345 good suggestions on managing multiple Xcode versions. To use a specific version
346 of Xcode, use `xcode-select -s` before running `configure`, or use
347 `--with-toolchain-path` to point to the version of Xcode to use, e.g.
348 `configure --with-toolchain-path=/Applications/Xcode8.app/Contents/Developer/usr/bin`
349
350 If you have recently (inadvertently) updated your OS and/or Xcode version, and
351 the JDK can no longer be built, please see the section on [Problems with the
352 Build Environment](#problems-with-the-build-environment), and [Getting
353 Help](#getting-help) to find out if there are any recent, non-merged patches
354 available for this update.
355
356 ### Microsoft Visual Studio
357
358 The minimum accepted version of Visual Studio is 2010. Older versions will not
359 be accepted by `configure`. The maximum accepted version of Visual Studio is
360 2019. Versions older than 2017 are unlikely to continue working for long.
361
362 If you have multiple versions of Visual Studio installed, `configure` will by
363 default pick the latest. You can request a specific version to be used by
364 setting `--with-toolchain-version`, e.g. `--with-toolchain-version=2015`.
365
366 If you get `LINK: fatal error LNK1123: failure during conversion to COFF: file
367 invalid` when building using Visual Studio 2010, you have encountered
368 [KB2757355](http://support.microsoft.com/kb/2757355), a bug triggered by a
369 specific installation order. However, the solution suggested by the KB article
370 does not always resolve the problem. See [this stackoverflow discussion](
371 https://stackoverflow.com/questions/10888391) for other suggestions.
372
373 ### IBM XL C/C++
374
375 Please consult the AIX section of the [Supported Build Platforms](
376 https://wiki.openjdk.java.net/display/Build/Supported+Build+Platforms) OpenJDK
377 Build Wiki page for details about which versions of XLC are supported.
378
379
380 ## Boot JDK Requirements
381
382 Paradoxically, building the JDK requires a pre-existing JDK. This is called the
383 "boot JDK". The boot JDK does not, however, have to be a JDK built directly from
384 the source code available in the OpenJDK Community. If you are porting the JDK
385 to a new platform, chances are that there already exists another JDK for that
386 platform that is usable as boot JDK.
387
388 The rule of thumb is that the boot JDK for building JDK major version *N*
389 should be a JDK of major version *N-1*, so for building JDK 9 a JDK 8 would be
390 suitable as boot JDK. However, the JDK should be able to "build itself", so an
391 up-to-date build of the current JDK source is an acceptable alternative. If
392 you are following the *N-1* rule, make sure you've got the latest update
393 version, since JDK 8 GA might not be able to build JDK 9 on all platforms.
394
395 Early in the release cycle, version *N-1* may not yet have been released. In
396 that case, the preferred boot JDK will be version *N-2* until version *N-1*
397 is available.
398
399 If the boot JDK is not automatically detected, or the wrong JDK is picked, use
400 `--with-boot-jdk` to point to the JDK to use.
401
402 ### Getting JDK binaries
403
404 JDK binaries for Linux, Windows and macOS can be downloaded from
405 [jdk.java.net](http://jdk.java.net). An alternative is to download the
406 [Oracle JDK](http://www.oracle.com/technetwork/java/javase/downloads). Another
407 is the [Adopt OpenJDK Project](https://adoptopenjdk.net/), which publishes
408 experimental prebuilt binaries for various platforms.
409
410 On Linux you can also get a JDK from the Linux distribution. On apt-based
411 distros (like Debian and Ubuntu), `sudo apt-get install openjdk-<VERSION>-jdk`
412 is typically enough to install a JDK \<VERSION\>. On rpm-based distros (like
413 Fedora and Red Hat), try `sudo yum install java-<VERSION>-openjdk-devel`.
414
415 ## External Library Requirements
416
417 Different platforms require different external libraries. In general, libraries
418 are not optional - that is, they are either required or not used.
419
420 If a required library is not detected by `configure`, you need to provide the
421 path to it. There are two forms of the `configure` arguments to point to an
422 external library: `--with-<LIB>=<path>` or `--with-<LIB>-include=<path to
423 include> --with-<LIB>-lib=<path to lib>`. The first variant is more concise,
424 but require the include files and library files to reside in a default
425 hierarchy under this directory. In most cases, it works fine.
426
427 As a fallback, the second version allows you to point to the include directory
428 and the lib directory separately.
429
430 ### FreeType
431
432 FreeType2 from [The FreeType Project](http://www.freetype.org/) is not required
433 on any platform. The exception is on Unix-based platforms when configuring such
434 that the build artifacts will reference a system installed library,
435 rather than bundling the JDK's own copy.
436
437 * To install on an apt-based Linux, try running `sudo apt-get install
438 libfreetype6-dev`.
439 * To install on an rpm-based Linux, try running `sudo yum install
440 freetype-devel`.
441
442 Use `--with-freetype-include=<path>` and `--with-freetype-lib=<path>`
443 if `configure` does not automatically locate the platform FreeType files.
444
445 ### CUPS
446
447 CUPS, [Common UNIX Printing System](http://www.cups.org) header files are
448 required on all platforms, except Windows. Often these files are provided by
449 your operating system.
450
451 * To install on an apt-based Linux, try running `sudo apt-get install
452 libcups2-dev`.
453 * To install on an rpm-based Linux, try running `sudo yum install
454 cups-devel`.
455
456 Use `--with-cups=<path>` if `configure` does not properly locate your CUPS
457 files.
458
459 ### X11
460
461 Certain [X11](http://www.x.org/) libraries and include files are required on
462 Linux.
463
464 * To install on an apt-based Linux, try running `sudo apt-get install
465 libx11-dev libxext-dev libxrender-dev libxrandr-dev libxtst-dev libxt-dev`.
466 * To install on an rpm-based Linux, try running `sudo yum install
467 libXtst-devel libXt-devel libXrender-devel libXrandr-devel libXi-devel`.
468
469 Use `--with-x=<path>` if `configure` does not properly locate your X11 files.
470
471 ### ALSA
472
473 ALSA, [Advanced Linux Sound Architecture](https://www.alsa-project.org/) is
474 required on Linux. At least version 0.9.1 of ALSA is required.
475
476 * To install on an apt-based Linux, try running `sudo apt-get install
477 libasound2-dev`.
478 * To install on an rpm-based Linux, try running `sudo yum install
479 alsa-lib-devel`.
480
481 Use `--with-alsa=<path>` if `configure` does not properly locate your ALSA
482 files.
483
484 ### libffi
485
486 libffi, the [Portable Foreign Function Interface Library](
487 http://sourceware.org/libffi) is required when building the Zero version of
488 Hotspot.
489
490 * To install on an apt-based Linux, try running `sudo apt-get install
491 libffi-dev`.
492 * To install on an rpm-based Linux, try running `sudo yum install
493 libffi-devel`.
494
495 Use `--with-libffi=<path>` if `configure` does not properly locate your libffi
496 files.
497
498 ## Build Tools Requirements
499
500 ### Autoconf
501
502 The JDK requires [Autoconf](http://www.gnu.org/software/autoconf) on all
503 platforms. At least version 2.69 is required.
504
505 * To install on an apt-based Linux, try running `sudo apt-get install
506 autoconf`.
507 * To install on an rpm-based Linux, try running `sudo yum install
508 autoconf`.
509 * To install on macOS, try running `brew install autoconf`.
510 * To install on Windows, try running `<path to Cygwin setup>/setup-x86_64 -q
511 -P autoconf`.
512
513 If `configure` has problems locating your installation of autoconf, you can
514 specify it using the `AUTOCONF` environment variable, like this:
515
516 ```
517 AUTOCONF=<path to autoconf> configure ...
518 ```
519
520 ### GNU Make
521
522 The JDK requires [GNU Make](http://www.gnu.org/software/make). No other flavors
523 of make are supported.
524
525 At least version 3.81 of GNU Make must be used. For distributions supporting
526 GNU Make 4.0 or above, we strongly recommend it. GNU Make 4.0 contains useful
527 functionality to handle parallel building (supported by `--with-output-sync`)
528 and speed and stability improvements.
529
530 Note that `configure` locates and verifies a properly functioning version of
531 `make` and stores the path to this `make` binary in the configuration. If you
532 start a build using `make` on the command line, you will be using the version
533 of make found first in your `PATH`, and not necessarily the one stored in the
534 configuration. This initial make will be used as "bootstrap make", and in a
535 second stage, the make located by `configure` will be called. Normally, this
536 will present no issues, but if you have a very old `make`, or a non-GNU Make
537 `make` in your path, this might cause issues.
538
539 If you want to override the default make found by `configure`, use the `MAKE`
540 configure variable, e.g. `configure MAKE=/opt/gnu/make`.
541
542 ### GNU Bash
543
544 The JDK requires [GNU Bash](http://www.gnu.org/software/bash). No other shells
545 are supported.
546
547 At least version 3.2 of GNU Bash must be used.
548
549 ## Running Configure
550
551 To build the JDK, you need a "configuration", which consists of a directory
552 where to store the build output, coupled with information about the platform,
553 the specific build machine, and choices that affect how the JDK is built.
554
555 The configuration is created by the `configure` script. The basic invocation of
556 the `configure` script looks like this:
557
558 ```
559 bash configure [options]
560 ```
561
562 This will create an output directory containing the configuration and setup an
563 area for the build result. This directory typically looks like
564 `build/linux-x64-server-release`, but the actual name depends on your specific
565 configuration. (It can also be set directly, see [Using Multiple
566 Configurations](#using-multiple-configurations)). This directory is referred to
567 as `$BUILD` in this documentation.
568
569 `configure` will try to figure out what system you are running on and where all
570 necessary build components are. If you have all prerequisites for building
571 installed, it should find everything. If it fails to detect any component
572 automatically, it will exit and inform you about the problem.
573
574 Some command line examples:
575
576 * Create a 32-bit build for Windows with FreeType2 in `C:\freetype-i586`:
577 ```
578 bash configure --with-freetype=/cygdrive/c/freetype-i586 --with-target-bits=32
579 ```
580
581 * Create a debug build with the `server` JVM and DTrace enabled:
582 ```
583 bash configure --enable-debug --with-jvm-variants=server --enable-dtrace
584 ```
585
586 ### Common Configure Arguments
587
588 Here follows some of the most common and important `configure` argument.
589
590 To get up-to-date information on *all* available `configure` argument, please
591 run:
592 ```
593 bash configure --help
594 ```
595
596 (Note that this help text also include general autoconf options, like
597 `--dvidir`, that is not relevant to the JDK. To list only JDK-specific
598 features, use `bash configure --help=short` instead.)
599
600 #### Configure Arguments for Tailoring the Build
601
602 * `--enable-debug` - Set the debug level to `fastdebug` (this is a shorthand
603 for `--with-debug-level=fastdebug`)
604 * `--with-debug-level=<level>` - Set the debug level, which can be `release`,
605 `fastdebug`, `slowdebug` or `optimized`. Default is `release`. `optimized`
606 is variant of `release` with additional Hotspot debug code.
607 * `--with-native-debug-symbols=<method>` - Specify if and how native debug
608 symbols should be built. Available methods are `none`, `internal`,
609 `external`, `zipped`. Default behavior depends on platform. See [Native
610 Debug Symbols](#native-debug-symbols) for more details.
611 * `--with-version-string=<string>` - Specify the version string this build
612 will be identified with.
613 * `--with-version-<part>=<value>` - A group of options, where `<part>` can be
614 any of `pre`, `opt`, `build`, `major`, `minor`, `security` or `patch`. Use
615 these options to modify just the corresponding part of the version string
616 from the default, or the value provided by `--with-version-string`.
617 * `--with-jvm-variants=<variant>[,<variant>...]` - Build the specified variant
618 (or variants) of Hotspot. Valid variants are: `server`, `client`,
619 `minimal`, `core`, `zero`, `custom`. Note that not all
620 variants are possible to combine in a single build.
621 * `--enable-jvm-feature-<feature>` or `--disable-jvm-feature-<feature>` -
622 Include (or exclude) `<feature>` as a JVM feature in Hotspot. You can also
623 specify a list of features to be enabled, separated by space or comma, as
624 `--with-jvm-features=<feature>[,<feature>...]`. If you prefix `<feature>`
625 with a `-`, it will be disabled. These options will modify the default list
626 of features for the JVM variant(s) you are building. For the `custom` JVM
627 variant, the default list is empty. A complete list of valid JVM features
628 can be found using `bash configure --help`.
629 * `--with-target-bits=<bits>` - Create a target binary suitable for running
630 on a `<bits>` platform. Use this to create 32-bit output on a 64-bit build
631 platform, instead of doing a full cross-compile. (This is known as a
632 *reduced* build.)
633
634 On Linux, BSD and AIX, it is possible to override where Java by default
635 searches for runtime/JNI libraries. This can be useful in situations where
636 there is a special shared directory for system JNI libraries. This setting
637 can in turn be overriden at runtime by setting the `java.library.path` property.
638
639 * `--with-jni-libpath=<path>` - Use the specified path as a default
640 when searching for runtime libraries.
641
642 #### Configure Arguments for Native Compilation
643
644 * `--with-devkit=<path>` - Use this devkit for compilers, tools and resources
645 * `--with-sysroot=<path>` - Use this directory as sysroot
646 * `--with-extra-path=<path>[;<path>]` - Prepend these directories to the
647 default path when searching for all kinds of binaries
648 * `--with-toolchain-path=<path>[;<path>]` - Prepend these directories when
649 searching for toolchain binaries (compilers etc)
650 * `--with-extra-cflags=<flags>` - Append these flags when compiling JDK C
651 files
652 * `--with-extra-cxxflags=<flags>` - Append these flags when compiling JDK C++
653 files
654 * `--with-extra-ldflags=<flags>` - Append these flags when linking JDK
655 libraries
656
657 #### Configure Arguments for External Dependencies
658
659 * `--with-boot-jdk=<path>` - Set the path to the [Boot JDK](
660 #boot-jdk-requirements)
661 * `--with-freetype=<path>` - Set the path to [FreeType](#freetype)
662 * `--with-cups=<path>` - Set the path to [CUPS](#cups)
663 * `--with-x=<path>` - Set the path to [X11](#x11)
664 * `--with-alsa=<path>` - Set the path to [ALSA](#alsa)
665 * `--with-libffi=<path>` - Set the path to [libffi](#libffi)
666 * `--with-jtreg=<path>` - Set the path to JTReg. See [Running Tests](
667 #running-tests)
668
669 Certain third-party libraries used by the JDK (libjpeg, giflib, libpng, lcms
670 and zlib) are included in the JDK repository. The default behavior of the
671 JDK build is to use the included ("bundled") versions of libjpeg, giflib,
672 libpng and lcms.
673 For zlib, the system lib (if present) is used except on Windows and AIX.
674 However the bundled libraries may be replaced by an external version.
675 To do so, specify `system` as the `<source>` option in these arguments.
676 (The default is `bundled`).
677
678 * `--with-libjpeg=<source>` - Use the specified source for libjpeg
679 * `--with-giflib=<source>` - Use the specified source for giflib
680 * `--with-libpng=<source>` - Use the specified source for libpng
681 * `--with-lcms=<source>` - Use the specified source for lcms
682 * `--with-zlib=<source>` - Use the specified source for zlib
683
684 On Linux, it is possible to select either static or dynamic linking of the C++
685 runtime. The default is static linking, with dynamic linking as fallback if the
686 static library is not found.
687
688 * `--with-stdc++lib=<method>` - Use the specified method (`static`, `dynamic`
689 or `default`) for linking the C++ runtime.
690
691 ### Configure Control Variables
692
693 It is possible to control certain aspects of `configure` by overriding the
694 value of `configure` variables, either on the command line or in the
695 environment.
696
697 Normally, this is **not recommended**. If used improperly, it can lead to a
698 broken configuration. Unless you're well versed in the build system, this is
699 hard to use properly. Therefore, `configure` will print a warning if this is
700 detected.
701
702 However, there are a few `configure` variables, known as *control variables*
703 that are supposed to be overriden on the command line. These are variables that
704 describe the location of tools needed by the build, like `MAKE` or `GREP`. If
705 any such variable is specified, `configure` will use that value instead of
706 trying to autodetect the tool. For instance, `bash configure
707 MAKE=/opt/gnumake4.0/bin/make`.
708
709 If a configure argument exists, use that instead, e.g. use `--with-jtreg`
710 instead of setting `JTREGEXE`.
711
712 Also note that, despite what autoconf claims, setting `CFLAGS` will not
713 accomplish anything. Instead use `--with-extra-cflags` (and similar for
714 `cxxflags` and `ldflags`).
715
716 ## Running Make
717
718 When you have a proper configuration, all you need to do to build the JDK is to
719 run `make`. (But see the warning at [GNU Make](#gnu-make) about running the
720 correct version of make.)
721
722 When running `make` without any arguments, the default target is used, which is
723 the same as running `make default` or `make jdk`. This will build a minimal (or
724 roughly minimal) set of compiled output (known as an "exploded image") needed
725 for a developer to actually execute the newly built JDK. The idea is that in an
726 incremental development fashion, when doing a normal make, you should only
727 spend time recompiling what's changed (making it purely incremental) and only
728 do the work that's needed to actually run and test your code.
729
730 The output of the exploded image resides in `$BUILD/jdk`. You can test the
731 newly built JDK like this: `$BUILD/jdk/bin/java -version`.
732
733 ### Common Make Targets
734
735 Apart from the default target, here are some common make targets:
736
737 * `hotspot` - Build all of hotspot (but only hotspot)
738 * `hotspot-<variant>` - Build just the specified jvm variant
739 * `images` or `product-images` - Build the JDK image
740 * `docs` or `docs-image` - Build the documentation image
741 * `test-image` - Build the test image
742 * `all` or `all-images` - Build all images (product, docs and test)
743 * `bootcycle-images` - Build images twice, second time with newly built JDK
744 (good for testing)
745 * `clean` - Remove all files generated by make, but not those generated by
746 configure
747 * `dist-clean` - Remove all files, including configuration
748
749 Run `make help` to get an up-to-date list of important make targets and make
750 control variables.
751
752 It is possible to build just a single module, a single phase, or a single phase
753 of a single module, by creating make targets according to these followin
754 patterns. A phase can be either of `gensrc`, `gendata`, `copy`, `java`,
755 `launchers`, or `libs`. See [Using Fine-Grained Make Targets](
756 #using-fine-grained-make-targets) for more details about this functionality.
757
758 * `<phase>` - Build the specified phase and everything it depends on
759 * `<module>` - Build the specified module and everything it depends on
760 * `<module>-<phase>` - Compile the specified phase for the specified module
761 and everything it depends on
762
763 Similarly, it is possible to clean just a part of the build by creating make
764 targets according to these patterns:
765
766 * `clean-<outputdir>` - Remove the subdir in the output dir with the name
767 * `clean-<phase>` - Remove all build results related to a certain build
768 phase
769 * `clean-<module>` - Remove all build results related to a certain module
770 * `clean-<module>-<phase>` - Remove all build results related to a certain
771 module and phase
772
773 ### Make Control Variables
774
775 It is possible to control `make` behavior by overriding the value of `make`
776 variables, either on the command line or in the environment.
777
778 Normally, this is **not recommended**. If used improperly, it can lead to a
779 broken build. Unless you're well versed in the build system, this is hard to
780 use properly. Therefore, `make` will print a warning if this is detected.
781
782 However, there are a few `make` variables, known as *control variables* that
783 are supposed to be overriden on the command line. These make up the "make time"
784 configuration, as opposed to the "configure time" configuration.
785
786 #### General Make Control Variables
787
788 * `JOBS` - Specify the number of jobs to build with. See [Build
789 Performance](#build-performance).
790 * `LOG` - Specify the logging level and functionality. See [Checking the
791 Build Log File](#checking-the-build-log-file)
792 * `CONF` and `CONF_NAME` - Selecting the configuration(s) to use. See [Using
793 Multiple Configurations](#using-multiple-configurations)
794
795 #### Test Make Control Variables
796
797 These make control variables only make sense when running tests. Please see
798 [Testing the JDK](testing.html) for details.
799
800 * `TEST`
801 * `TEST_JOBS`
802 * `JTREG`
803 * `GTEST`
804
805 #### Advanced Make Control Variables
806
807 These advanced make control variables can be potentially unsafe. See [Hints and
808 Suggestions for Advanced Users](#hints-and-suggestions-for-advanced-users) and
809 [Understanding the Build System](#understanding-the-build-system) for details.
810
811 * `SPEC`
812 * `CONF_CHECK`
813 * `COMPARE_BUILD`
814 * `JDK_FILTER`
815 * `SPEC_FILTER`
816
817 ## Running Tests
818
819 Most of the JDK tests are using the [JTReg](http://openjdk.java.net/jtreg)
820 test framework. Make sure that your configuration knows where to find your
821 installation of JTReg. If this is not picked up automatically, use the
822 `--with-jtreg=<path to jtreg home>` option to point to the JTReg framework.
823 Note that this option should point to the JTReg home, i.e. the top directory,
824 containing `lib/jtreg.jar` etc.
825
826 The [Adoption Group](https://wiki.openjdk.java.net/display/Adoption) provides
827 recent builds of jtreg [here](
828 https://ci.adoptopenjdk.net/view/Dependencies/job/jtreg/lastSuccessfulBuild/artifact).
829 Download the latest `.tar.gz` file, unpack it, and point `--with-jtreg` to the
830 `jtreg` directory that you just unpacked.
831
832 To execute the most basic tests (tier 1), use:
833 ```
834 make run-test-tier1
835 ```
836
837 For more details on how to run tests, please see the [Testing
838 the JDK](testing.html) document.
839
840 ## Cross-compiling
841
842 Cross-compiling means using one platform (the *build* platform) to generate
843 output that can ran on another platform (the *target* platform).
844
845 The typical reason for cross-compiling is that the build is performed on a more
846 powerful desktop computer, but the resulting binaries will be able to run on a
847 different, typically low-performing system. Most of the complications that
848 arise when building for embedded is due to this separation of *build* and
849 *target* systems.
850
851 This requires a more complex setup and build procedure. This section assumes
852 you are familiar with cross-compiling in general, and will only deal with the
853 particularities of cross-compiling the JDK. If you are new to cross-compiling,
854 please see the [external links at Wikipedia](
855 https://en.wikipedia.org/wiki/Cross_compiler#External_links) for a good start
856 on reading materials.
857
858 Cross-compiling the JDK requires you to be able to build both for the build
859 platform and for the target platform. The reason for the former is that we need
860 to build and execute tools during the build process, both native tools and Java
861 tools.
862
863 If all you want to do is to compile a 32-bit version, for the same OS, on a
864 64-bit machine, consider using `--with-target-bits=32` instead of doing a
865 full-blown cross-compilation. (While this surely is possible, it's a lot more
866 work and will take much longer to build.)
867
868 ### Cross compiling the easy way with OpenJDK devkits
869
870 The OpenJDK build system provides out-of-the box support for creating and using
871 so called devkits. A `devkit` is basically a collection of a cross-compiling
872 toolchain and a sysroot environment which can easily be used together with the
873 `--with-devkit` configure option to cross compile the OpenJDK. On Linux/x86_64,
874 the following command:
875 ```
876 bash configure --with-devkit=<devkit-path> --openjdk-target=ppc64-linux-gnu && make
877 ```
878
879 will configure and build OpenJDK for Linux/ppc64 assuming that `<devkit-path>`
880 points to a Linux/x86_64 to Linux/ppc64 devkit.
881
882 Devkits can be created from the `make/devkit` directory by executing:
883 ```
884 make [ TARGETS="<TARGET_TRIPLET>+" ] [ BASE_OS=<OS> ] [ BASE_OS_VERSION=<VER> ]
885 ```
886
887 where `TARGETS` contains one or more `TARGET_TRIPLET`s of the form
888 described in [section 3.4 of the GNU Autobook](
889 https://sourceware.org/autobook/autobook/autobook_17.html). If no
890 targets are given, a native toolchain for the current platform will be
891 created. Currently, at least the following targets are known to work:
892
893 Supported devkit targets
894 -------------------------
895 x86_64-linux-gnu
896 aarch64-linux-gnu
897 arm-linux-gnueabihf
898 ppc64-linux-gnu
899 ppc64le-linux-gnu
900 s390x-linux-gnu
901
902 `BASE_OS` must be one of "OEL6" for Oracle Enterprise Linux 6 or
903 "Fedora" (if not specified "OEL6" will be the default). If the base OS
904 is "Fedora" the corresponding Fedora release can be specified with the
905 help of the `BASE_OS_VERSION` option (with "27" as default version).
906 If the build is successful, the new devkits can be found in the
907 `build/devkit/result` subdirectory:
908 ```
909 cd make/devkit
910 make TARGETS="ppc64le-linux-gnu aarch64-linux-gnu" BASE_OS=Fedora BASE_OS_VERSION=21
911 ls -1 ../../build/devkit/result/
912 x86_64-linux-gnu-to-aarch64-linux-gnu
913 x86_64-linux-gnu-to-ppc64le-linux-gnu
914 ```
915
916 Notice that devkits are not only useful for targeting different build
917 platforms. Because they contain the full build dependencies for a
918 system (i.e. compiler and root file system), they can easily be used
919 to build well-known, reliable and reproducible build environments. You
920 can for example create and use a devkit with GCC 7.3 and a Fedora 12
921 sysroot environment (with glibc 2.11) on Ubuntu 14.04 (which doesn't
922 have GCC 7.3 by default) to produce OpenJDK binaries which will run on
923 all Linux systems with runtime libraries newer than the ones from
924 Fedora 12 (e.g. Ubuntu 16.04, SLES 11 or RHEL 6).
925
926 ### Boot JDK and Build JDK
927
928 When cross-compiling, make sure you use a boot JDK that runs on the *build*
929 system, and not on the *target* system.
930
931 To be able to build, we need a "Build JDK", which is a JDK built from the
932 current sources (that is, the same as the end result of the entire build
933 process), but able to run on the *build* system, and not the *target* system.
934 (In contrast, the Boot JDK should be from an older release, e.g. JDK 8 when
935 building JDK 9.)
936
937 The build process will create a minimal Build JDK for you, as part of building.
938 To speed up the build, you can use `--with-build-jdk` to `configure` to point
939 to a pre-built Build JDK. Please note that the build result is unpredictable,
940 and can possibly break in subtle ways, if the Build JDK does not **exactly**
941 match the current sources.
942
943 ### Specifying the Target Platform
944
945 You *must* specify the target platform when cross-compiling. Doing so will also
946 automatically turn the build into a cross-compiling mode. The simplest way to
947 do this is to use the `--openjdk-target` argument, e.g.
948 `--openjdk-target=arm-linux-gnueabihf`. or `--openjdk-target=aarch64-oe-linux`.
949 This will automatically set the `--build`, `--host` and `--target` options for
950 autoconf, which can otherwise be confusing. (In autoconf terminology, the
951 "target" is known as "host", and "target" is used for building a Canadian
952 cross-compiler.)
953
954 ### Toolchain Considerations
955
956 You will need two copies of your toolchain, one which generates output that can
957 run on the target system (the normal, or *target*, toolchain), and one that
958 generates output that can run on the build system (the *build* toolchain). Note
959 that cross-compiling is only supported for gcc at the time being. The gcc
960 standard is to prefix cross-compiling toolchains with the target denominator.
961 If you follow this standard, `configure` is likely to pick up the toolchain
962 correctly.
963
964 The *build* toolchain will be autodetected just the same way the normal
965 *build*/*target* toolchain will be autodetected when not cross-compiling. If
966 this is not what you want, or if the autodetection fails, you can specify a
967 devkit containing the *build* toolchain using `--with-build-devkit` to
968 `configure`, or by giving `BUILD_CC` and `BUILD_CXX` arguments.
969
970 It is often helpful to locate the cross-compilation tools, headers and
971 libraries in a separate directory, outside the normal path, and point out that
972 directory to `configure`. Do this by setting the sysroot (`--with-sysroot`) and
973 appending the directory when searching for cross-compilations tools
974 (`--with-toolchain-path`). As a compact form, you can also use `--with-devkit`
975 to point to a single directory, if it is correctly setup. (See `basics.m4` for
976 details.)
977
978 ### Native Libraries
979
980 You will need copies of external native libraries for the *target* system,
981 present on the *build* machine while building.
982
983 Take care not to replace the *build* system's version of these libraries by
984 mistake, since that can render the *build* machine unusable.
985
986 Make sure that the libraries you point to (ALSA, X11, etc) are for the
987 *target*, not the *build*, platform.
988
989 #### ALSA
990
991 You will need alsa libraries suitable for your *target* system. For most cases,
992 using Debian's pre-built libraries work fine.
993
994 Note that alsa is needed even if you only want to build a headless JDK.
995
996 * Go to [Debian Package Search](https://www.debian.org/distrib/packages) and
997 search for the `libasound2` and `libasound2-dev` packages for your *target*
998 system. Download them to /tmp.
999
1000 * Install the libraries into the cross-compilation toolchain. For instance:
1001 ```
1002 cd /tools/gcc-linaro-arm-linux-gnueabihf-raspbian-2012.09-20120921_linux/arm-linux-gnueabihf/libc
1003 dpkg-deb -x /tmp/libasound2_1.0.25-4_armhf.deb .
1004 dpkg-deb -x /tmp/libasound2-dev_1.0.25-4_armhf.deb .
1005 ```
1006
1007 * If alsa is not properly detected by `configure`, you can point it out by
1008 `--with-alsa`.
1009
1010 #### X11
1011
1012 You will need X11 libraries suitable for your *target* system. For most cases,
1013 using Debian's pre-built libraries work fine.
1014
1015 Note that X11 is needed even if you only want to build a headless JDK.
1016
1017 * Go to [Debian Package Search](https://www.debian.org/distrib/packages),
1018 search for the following packages for your *target* system, and download them
1019 to /tmp/target-x11:
1020 * libxi
1021 * libxi-dev
1022 * x11proto-core-dev
1023 * x11proto-input-dev
1024 * x11proto-kb-dev
1025 * x11proto-render-dev
1026 * x11proto-xext-dev
1027 * libice-dev
1028 * libxrender
1029 * libxrender-dev
1030 * libxrandr-dev
1031 * libsm-dev
1032 * libxt-dev
1033 * libx11
1034 * libx11-dev
1035 * libxtst
1036 * libxtst-dev
1037 * libxext
1038 * libxext-dev
1039
1040 * Install the libraries into the cross-compilation toolchain. For instance:
1041 ```
1042 cd /tools/gcc-linaro-arm-linux-gnueabihf-raspbian-2012.09-20120921_linux/arm-linux-gnueabihf/libc/usr
1043 mkdir X11R6
1044 cd X11R6
1045 for deb in /tmp/target-x11/*.deb ; do dpkg-deb -x $deb . ; done
1046 mv usr/* .
1047 cd lib
1048 cp arm-linux-gnueabihf/* .
1049 ```
1050
1051 You can ignore the following messages. These libraries are not needed to
1052 successfully complete a full JDK build.
1053 ```
1054 cp: cannot stat `arm-linux-gnueabihf/libICE.so': No such file or directory
1055 cp: cannot stat `arm-linux-gnueabihf/libSM.so': No such file or directory
1056 cp: cannot stat `arm-linux-gnueabihf/libXt.so': No such file or directory
1057 ```
1058
1059 * If the X11 libraries are not properly detected by `configure`, you can
1060 point them out by `--with-x`.
1061
1062 ### Creating And Using Sysroots With qemu-deboostrap
1063
1064 Fortunately, you can create sysroots for foreign architectures with tools
1065 provided by your OS. On Debian/Ubuntu systems, one could use `qemu-deboostrap` to
1066 create the *target* system chroot, which would have the native libraries and headers
1067 specific to that *target* system. After that, we can use the cross-compiler on the *build*
1068 system, pointing into chroot to get the build dependencies right. This allows building
1069 for foreign architectures with native compilation speed.
1070
1071 For example, cross-compiling to AArch64 from x86_64 could be done like this:
1072
1073 * Install cross-compiler on the *build* system:
1074 ```
1075 apt install g++-aarch64-linux-gnu gcc-aarch64-linux-gnu
1076 ```
1077
1078 * Create chroot on the *build* system, configuring it for *target* system:
1079 ```
1080 sudo qemu-debootstrap --arch=arm64 --verbose \
1081 --include=fakeroot,build-essential,libx11-dev,libxext-dev,libxrender-dev,libxrandr-dev,libxtst-dev,libxt-dev,libcups2-dev,libfontconfig1-dev,libasound2-dev,libfreetype6-dev,libpng12-dev \
1082 --resolve-deps jessie /chroots/arm64 http://httpredir.debian.org/debian/
1083 ```
1084
1085 * Configure and build with newly created chroot as sysroot/toolchain-path:
1086 ```
1087 CC=aarch64-linux-gnu-gcc CXX=aarch64-linux-gnu-g++ sh ./configure --openjdk-target=aarch64-linux-gnu --with-sysroot=/chroots/arm64/ --with-toolchain-path=/chroots/arm64/
1088 make images
1089 ls build/linux-aarch64-normal-server-release/
1090 ```
1091
1092 The build does not create new files in that chroot, so it can be reused for multiple builds
1093 without additional cleanup.
1094
1095 Architectures that are known to successfully cross-compile like this are:
1096
1097 Target `CC` `CXX` `--arch=...` `--openjdk-target=...`
1098 ------------ ------------------------- --------------------------- ------------- -----------------------
1099 x86 default default i386 i386-linux-gnu
1100 armhf gcc-arm-linux-gnueabihf g++-arm-linux-gnueabihf armhf arm-linux-gnueabihf
1101 aarch64 gcc-aarch64-linux-gnu g++-aarch64-linux-gnu arm64 aarch64-linux-gnu
1102 ppc64el gcc-powerpc64le-linux-gnu g++-powerpc64le-linux-gnu ppc64el powerpc64le-linux-gnu
1103 s390x gcc-s390x-linux-gnu g++-s390x-linux-gnu s390x s390x-linux-gnu
1104
1105 Additional architectures might be supported by Debian/Ubuntu Ports.
1106
1107 ### Building for ARM/aarch64
1108
1109 A common cross-compilation target is the ARM CPU. When building for ARM, it is
1110 useful to set the ABI profile. A number of pre-defined ABI profiles are
1111 available using `--with-abi-profile`: arm-vfp-sflt, arm-vfp-hflt, arm-sflt,
1112 armv5-vfp-sflt, armv6-vfp-hflt. Note that soft-float ABIs are no longer
1113 properly supported by the JDK.
1114
1115 ### Verifying the Build
1116
1117 The build will end up in a directory named like
1118 `build/linux-arm-normal-server-release`.
1119
1120 Inside this build output directory, the `images/jdk` will contain the newly
1121 built JDK, for your *target* system.
1122
1123 Copy these folders to your *target* system. Then you can run e.g.
1124 `images/jdk/bin/java -version`.
1125
1126 ## Build Performance
1127
1128 Building the JDK requires a lot of horsepower. Some of the build tools can be
1129 adjusted to utilize more or less of resources such as parallel threads and
1130 memory. The `configure` script analyzes your system and selects reasonable
1131 values for such options based on your hardware. If you encounter resource
1132 problems, such as out of memory conditions, you can modify the detected values
1133 with:
1134
1135 * `--with-num-cores` -- number of cores in the build system, e.g.
1136 `--with-num-cores=8`.
1137
1138 * `--with-memory-size` -- memory (in MB) available in the build system, e.g.
1139 `--with-memory-size=1024`
1140
1141 You can also specify directly the number of build jobs to use with
1142 `--with-jobs=N` to `configure`, or `JOBS=N` to `make`. Do not use the `-j` flag
1143 to `make`. In most cases it will be ignored by the makefiles, but it can cause
1144 problems for some make targets.
1145
1146 It might also be necessary to specify the JVM arguments passed to the Boot JDK,
1147 using e.g. `--with-boot-jdk-jvmargs="-Xmx8G"`. Doing so will override the
1148 default JVM arguments passed to the Boot JDK.
1149
1150 At the end of a successful execution of `configure`, you will get a performance
1151 summary, indicating how well the build will perform. Here you will also get
1152 performance hints. If you want to build fast, pay attention to those!
1153
1154 If you want to tweak build performance, run with `make LOG=info` to get a build
1155 time summary at the end of the build process.
1156
1157 ### Disk Speed
1158
1159 If you are using network shares, e.g. via NFS, for your source code, make sure
1160 the build directory is situated on local disk (e.g. by `ln -s
1161 /localdisk/jdk-build $JDK-SHARE/build`). The performance penalty is extremely
1162 high for building on a network share; close to unusable.
1163
1164 Also, make sure that your build tools (including Boot JDK and toolchain) is
1165 located on a local disk and not a network share.
1166
1167 As has been stressed elsewhere, do use SSD for source code and build directory,
1168 as well as (if possible) the build tools.
1169
1170 ### Virus Checking
1171
1172 The use of virus checking software, especially on Windows, can *significantly*
1173 slow down building of the JDK. If possible, turn off such software, or exclude
1174 the directory containing the JDK source code from on-the-fly checking.
1175
1176 ### Ccache
1177
1178 The JDK build supports building with ccache when using gcc or clang. Using
1179 ccache can radically speed up compilation of native code if you often rebuild
1180 the same sources. Your milage may vary however, so we recommend evaluating it
1181 for yourself. To enable it, make sure it's on the path and configure with
1182 `--enable-ccache`.
1183
1184 ### Precompiled Headers
1185
1186 By default, the Hotspot build uses preccompiled headers (PCH) on the toolchains
1187 were it is properly supported (clang, gcc, and Visual Studio). Normally, this
1188 speeds up the build process, but in some circumstances, it can actually slow
1189 things down.
1190
1191 You can experiment by disabling precompiled headers using
1192 `--disable-precompiled-headers`.
1193
1194 ### Icecc / icecream
1195
1196 [icecc/icecream](http://github.com/icecc/icecream) is a simple way to setup a
1197 distributed compiler network. If you have multiple machines available for
1198 building the JDK, you can drastically cut individual build times by utilizing
1199 it.
1200
1201 To use, setup an icecc network, and install icecc on the build machine. Then
1202 run `configure` using `--enable-icecc`.
1203
1204 ### Using sjavac
1205
1206 To speed up Java compilation, especially incremental compilations, you can try
1207 the experimental sjavac compiler by using `--enable-sjavac`.
1208
1209 ### Building the Right Target
1210
1211 Selecting the proper target to build can have dramatic impact on build time.
1212 For normal usage, `jdk` or the default target is just fine. You only need to
1213 build `images` for shipping, or if your tests require it.
1214
1215 See also [Using Fine-Grained Make Targets](#using-fine-grained-make-targets) on
1216 how to build an even smaller subset of the product.
1217
1218 ## Troubleshooting
1219
1220 If your build fails, it can sometimes be difficult to pinpoint the problem or
1221 find a proper solution.
1222
1223 ### Locating the Source of the Error
1224
1225 When a build fails, it can be hard to pinpoint the actual cause of the error.
1226 In a typical build process, different parts of the product build in parallel,
1227 with the output interlaced.
1228
1229 #### Build Failure Summary
1230
1231 To help you, the build system will print a failure summary at the end. It looks
1232 like this:
1233
1234 ```
1235 ERROR: Build failed for target 'hotspot' in configuration 'linux-x64' (exit code 2)
1236
1237 === Output from failing command(s) repeated here ===
1238 * For target hotspot_variant-server_libjvm_objs_psMemoryPool.o:
1239 /localhome/hg/jdk9-sandbox/hotspot/src/share/vm/services/psMemoryPool.cpp:1:1: error: 'failhere' does not name a type
1240 ... (rest of output omitted)
1241
1242 * All command lines available in /localhome/hg/jdk9-sandbox/build/linux-x64/make-support/failure-logs.
1243 === End of repeated output ===
1244
1245 === Make failed targets repeated here ===
1246 lib/CompileJvm.gmk:207: recipe for target '/localhome/hg/jdk9-sandbox/build/linux-x64/hotspot/variant-server/libjvm/objs/psMemoryPool.o' failed
1247 make/Main.gmk:263: recipe for target 'hotspot-server-libs' failed
1248 === End of repeated output ===
1249
1250 Hint: Try searching the build log for the name of the first failed target.
1251 Hint: If caused by a warning, try configure --disable-warnings-as-errors.
1252 ```
1253
1254 Let's break it down! First, the selected configuration, and the top-level
1255 target you entered on the command line that caused the failure is printed.
1256
1257 Then, between the `Output from failing command(s) repeated here` and `End of
1258 repeated output` the first lines of output (stdout and stderr) from the actual
1259 failing command is repeated. In most cases, this is the error message that
1260 caused the build to fail. If multiple commands were failing (this can happen in
1261 a parallel build), output from all failed commands will be printed here.
1262
1263 The path to the `failure-logs` directory is printed. In this file you will find
1264 a `<target>.log` file that contains the output from this command in its
1265 entirety, and also a `<target>.cmd`, which contain the complete command line
1266 used for running this command. You can re-run the failing command by executing
1267 `. <path to failure-logs>/<target>.cmd` in your shell.
1268
1269 Another way to trace the failure is to follow the chain of make targets, from
1270 top-level targets to individual file targets. Between `Make failed targets
1271 repeated here` and `End of repeated output` the output from make showing this
1272 chain is repeated. The first failed recipe will typically contain the full path
1273 to the file in question that failed to compile. Following lines will show a
1274 trace of make targets why we ended up trying to compile that file.
1275
1276 Finally, some hints are given on how to locate the error in the complete log.
1277 In this example, we would try searching the log file for "`psMemoryPool.o`".
1278 Another way to quickly locate make errors in the log is to search for "`]
1279 Error`" or "`***`".
1280
1281 Note that the build failure summary will only help you if the issue was a
1282 compilation failure or similar. If the problem is more esoteric, or is due to
1283 errors in the build machinery, you will likely get empty output logs, and `No
1284 indication of failed target found` instead of the make target chain.
1285
1286 #### Checking the Build Log File
1287
1288 The output (stdout and stderr) from the latest build is always stored in
1289 `$BUILD/build.log`. The previous build log is stored as `build.log.old`. This
1290 means that it is not necessary to redirect the build output yourself if you
1291 want to process it.
1292
1293 You can increase the verbosity of the log file, by the `LOG` control variable
1294 to `make`. If you want to see the command lines used in compilations, use
1295 `LOG=cmdlines`. To increase the general verbosity, use `LOG=info`, `LOG=debug`
1296 or `LOG=trace`. Both of these can be combined with `cmdlines`, e.g.
1297 `LOG=info,cmdlines`. The `debug` log level will show most shell commands
1298 executed by make, and `trace` will show all. Beware that both these log levels
1299 will produce a massive build log!
1300
1301 ### Fixing Unexpected Build Failures
1302
1303 Most of the time, the build will fail due to incorrect changes in the source
1304 code.
1305
1306 Sometimes the build can fail with no apparent changes that have caused the
1307 failure. If this is the first time you are building the JDK on this particular
1308 computer, and the build fails, the problem is likely with your build
1309 environment. But even if you have previously built the JDK with success, and it
1310 now fails, your build environment might have changed (perhaps due to OS
1311 upgrades or similar). But most likely, such failures are due to problems with
1312 the incremental rebuild.
1313
1314 #### Problems with the Build Environment
1315
1316 Make sure your configuration is correct. Re-run `configure`, and look for any
1317 warnings. Warnings that appear in the middle of the `configure` output is also
1318 repeated at the end, after the summary. The entire log is stored in
1319 `$BUILD/configure.log`.
1320
1321 Verify that the summary at the end looks correct. Are you indeed using the Boot
1322 JDK and native toolchain that you expect?
1323
1324 By default, the JDK has a strict approach where warnings from the compiler is
1325 considered errors which fail the build. For very new or very old compiler
1326 versions, this can trigger new classes of warnings, which thus fails the build.
1327 Run `configure` with `--disable-warnings-as-errors` to turn of this behavior.
1328 (The warnings will still show, but not make the build fail.)
1329
1330 #### Problems with Incremental Rebuilds
1331
1332 Incremental rebuilds mean that when you modify part of the product, only the
1333 affected parts get rebuilt. While this works great in most cases, and
1334 significantly speed up the development process, from time to time complex
1335 interdependencies will result in an incorrect build result. This is the most
1336 common cause for unexpected build problems.
1337
1338 Here are a suggested list of things to try if you are having unexpected build
1339 problems. Each step requires more time than the one before, so try them in
1340 order. Most issues will be solved at step 1 or 2.
1341
1342 1. Make sure your repository is up-to-date
1343
1344 Run `hg pull -u` to make sure you have the latest changes.
1345
1346 2. Clean build results
1347
1348 The simplest way to fix incremental rebuild issues is to run `make clean`.
1349 This will remove all build results, but not the configuration or any build
1350 system support artifacts. In most cases, this will solve build errors
1351 resulting from incremental build mismatches.
1352
1353 3. Completely clean the build directory.
1354
1355 If this does not work, the next step is to run `make dist-clean`, or
1356 removing the build output directory (`$BUILD`). This will clean all
1357 generated output, including your configuration. You will need to re-run
1358 `configure` after this step. A good idea is to run `make
1359 print-configuration` before running `make dist-clean`, as this will print
1360 your current `configure` command line. Here's a way to do this:
1361
1362 ```
1363 make print-configuration > current-configuration
1364 make dist-clean
1365 bash configure $(cat current-configuration)
1366 make
1367 ```
1368
1369 4. Re-clone the Mercurial repository
1370
1371 Sometimes the Mercurial repository gets in a state that causes the product
1372 to be un-buildable. In such a case, the simplest solution is often the
1373 "sledgehammer approach": delete the entire repository, and re-clone it.
1374 If you have local changes, save them first to a different location using
1375 `hg export`.
1376
1377 ### Specific Build Issues
1378
1379 #### Clock Skew
1380
1381 If you get an error message like this:
1382 ```
1383 File 'xxx' has modification time in the future.
1384 Clock skew detected. Your build may be incomplete.
1385 ```
1386 then the clock on your build machine is out of sync with the timestamps on the
1387 source files. Other errors, apparently unrelated but in fact caused by the
1388 clock skew, can occur along with the clock skew warnings. These secondary
1389 errors may tend to obscure the fact that the true root cause of the problem is
1390 an out-of-sync clock.
1391
1392 If you see these warnings, reset the clock on the build machine, run `make
1393 clean` and restart the build.
1394
1395 #### Out of Memory Errors
1396
1397 On Windows, you might get error messages like this:
1398 ```
1399 fatal error - couldn't allocate heap
1400 cannot create ... Permission denied
1401 spawn failed
1402 ```
1403 This can be a sign of a Cygwin problem. See the information about solving
1404 problems in the [Cygwin](#cygwin) section. Rebooting the computer might help
1405 temporarily.
1406
1407 ### Getting Help
1408
1409 If none of the suggestions in this document helps you, or if you find what you
1410 believe is a bug in the build system, please contact the Build Group by sending
1411 a mail to [build-dev@openjdk.java.net](mailto:build-dev@openjdk.java.net).
1412 Please include the relevant parts of the configure and/or build log.
1413
1414 If you need general help or advice about developing for the JDK, you can also
1415 contact the Adoption Group. See the section on [Contributing to OpenJDK](
1416 #contributing-to-openjdk) for more information.
1417
1418 ## Hints and Suggestions for Advanced Users
1419
1420 ### Setting Up a Repository for Pushing Changes (defpath)
1421
1422 To help you prepare a proper push path for a Mercurial repository, there exists
1423 a useful tool known as [defpath](
1424 http://openjdk.java.net/projects/code-tools/defpath). It will help you setup a
1425 proper push path for pushing changes to the JDK.
1426
1427 Install the extension by cloning
1428 `http://hg.openjdk.java.net/code-tools/defpath` and updating your `.hgrc` file.
1429 Here's one way to do this:
1430
1431 ```
1432 cd ~
1433 mkdir hg-ext
1434 cd hg-ext
1435 hg clone http://hg.openjdk.java.net/code-tools/defpath
1436 cat << EOT >> ~/.hgrc
1437 [extensions]
1438 defpath=~/hg-ext/defpath/defpath.py
1439 EOT
1440 ```
1441
1442 You can now setup a proper push path using:
1443 ```
1444 hg defpath -d -u <your OpenJDK username>
1445 ```
1446
1447 ### Bash Completion
1448
1449 The `configure` and `make` commands tries to play nice with bash command-line
1450 completion (using `<tab>` or `<tab><tab>`). To use this functionality, make
1451 sure you enable completion in your `~/.bashrc` (see instructions for bash in
1452 your operating system).
1453
1454 Make completion will work out of the box, and will complete valid make targets.
1455 For instance, typing `make jdk-i<tab>` will complete to `make jdk-image`.
1456
1457 The `configure` script can get completion for options, but for this to work you
1458 need to help `bash` on the way. The standard way of running the script, `bash
1459 configure`, will not be understood by bash completion. You need `configure` to
1460 be the command to run. One way to achieve this is to add a simple helper script
1461 to your path:
1462
1463 ```
1464 cat << EOT > /tmp/configure
1465 #!/bin/bash
1466 if [ \$(pwd) = \$(cd \$(dirname \$0); pwd) ] ; then
1467 echo >&2 "Abort: Trying to call configure helper recursively"
1468 exit 1
1469 fi
1470
1471 bash \$PWD/configure "\$@"
1472 EOT
1473 chmod +x /tmp/configure
1474 sudo mv /tmp/configure /usr/local/bin
1475 ```
1476
1477 Now `configure --en<tab>-dt<tab>` will result in `configure --enable-dtrace`.
1478
1479 ### Using Multiple Configurations
1480
1481 You can have multiple configurations for a single source repository. When you
1482 create a new configuration, run `configure --with-conf-name=<name>` to create a
1483 configuration with the name `<name>`. Alternatively, you can create a directory
1484 under `build` and run `configure` from there, e.g. `mkdir build/<name> && cd
1485 build/<name> && bash ../../configure`.
1486
1487 Then you can build that configuration using `make CONF_NAME=<name>` or `make
1488 CONF=<pattern>`, where `<pattern>` is a substring matching one or several
1489 configurations, e.g. `CONF=debug`. The special empty pattern (`CONF=`) will
1490 match *all* available configuration, so `make CONF= hotspot` will build the
1491 `hotspot` target for all configurations. Alternatively, you can execute `make`
1492 in the configuration directory, e.g. `cd build/<name> && make`.
1493
1494 ### Handling Reconfigurations
1495
1496 If you update the repository and part of the configure script has changed, the
1497 build system will force you to re-run `configure`.
1498
1499 Most of the time, you will be fine by running `configure` again with the same
1500 arguments as the last time, which can easily be performed by `make
1501 reconfigure`. To simplify this, you can use the `CONF_CHECK` make control
1502 variable, either as `make CONF_CHECK=auto`, or by setting an environment
1503 variable. For instance, if you add `export CONF_CHECK=auto` to your `.bashrc`
1504 file, `make` will always run `reconfigure` automatically whenever the configure
1505 script has changed.
1506
1507 You can also use `CONF_CHECK=ignore` to skip the check for a needed configure
1508 update. This might speed up the build, but comes at the risk of an incorrect
1509 build result. This is only recommended if you know what you're doing.
1510
1511 From time to time, you will also need to modify the command line to `configure`
1512 due to changes. Use `make print-configure` to show the command line used for
1513 your current configuration.
1514
1515 ### Using Fine-Grained Make Targets
1516
1517 The default behavior for make is to create consistent and correct output, at
1518 the expense of build speed, if necessary.
1519
1520 If you are prepared to take some risk of an incorrect build, and know enough of
1521 the system to understand how things build and interact, you can speed up the
1522 build process considerably by instructing make to only build a portion of the
1523 product.
1524
1525 #### Building Individual Modules
1526
1527 The safe way to use fine-grained make targets is to use the module specific
1528 make targets. All source code in the JDK is organized so it belongs to a
1529 module, e.g. `java.base` or `jdk.jdwp.agent`. You can build only a specific
1530 module, by giving it as make target: `make jdk.jdwp.agent`. If the specified
1531 module depends on other modules (e.g. `java.base`), those modules will be built
1532 first.
1533
1534 You can also specify a set of modules, just as you can always specify a set of
1535 make targets: `make jdk.crypto.cryptoki jdk.crypto.ec jdk.crypto.mscapi`
1536
1537 #### Building Individual Module Phases
1538
1539 The build process for each module is divided into separate phases. Not all
1540 modules need all phases. Which are needed depends on what kind of source code
1541 and other artifact the module consists of. The phases are:
1542
1543 * `gensrc` (Generate source code to compile)
1544 * `gendata` (Generate non-source code artifacts)
1545 * `copy` (Copy resource artifacts)
1546 * `java` (Compile Java code)
1547 * `launchers` (Compile native executables)
1548 * `libs` (Compile native libraries)
1549
1550 You can build only a single phase for a module by using the notation
1551 `$MODULE-$PHASE`. For instance, to build the `gensrc` phase for `java.base`,
1552 use `make java.base-gensrc`.
1553
1554 Note that some phases may depend on others, e.g. `java` depends on `gensrc` (if
1555 present). Make will build all needed prerequisites before building the
1556 requested phase.
1557
1558 #### Skipping the Dependency Check
1559
1560 When using an iterative development style with frequent quick rebuilds, the
1561 dependency check made by make can take up a significant portion of the time
1562 spent on the rebuild. In such cases, it can be useful to bypass the dependency
1563 check in make.
1564
1565 > **Note that if used incorrectly, this can lead to a broken build!**
1566
1567 To achieve this, append `-only` to the build target. For instance, `make
1568 jdk.jdwp.agent-java-only` will *only* build the `java` phase of the
1569 `jdk.jdwp.agent` module. If the required dependencies are not present, the
1570 build can fail. On the other hand, the execution time measures in milliseconds.
1571
1572 A useful pattern is to build the first time normally (e.g. `make
1573 jdk.jdwp.agent`) and then on subsequent builds, use the `-only` make target.
1574
1575 #### Rebuilding Part of java.base (JDK\_FILTER)
1576
1577 If you are modifying files in `java.base`, which is the by far largest module
1578 in the JDK, then you need to rebuild all those files whenever a single file has
1579 changed. (This inefficiency will hopefully be addressed in JDK 10.)
1580
1581 As a hack, you can use the make control variable `JDK_FILTER` to specify a
1582 pattern that will be used to limit the set of files being recompiled. For
1583 instance, `make java.base JDK_FILTER=javax/crypto` (or, to combine methods,
1584 `make java.base-java-only JDK_FILTER=javax/crypto`) will limit the compilation
1585 to files in the `javax.crypto` package.
1586
1587 ### Learn About Mercurial
1588
1589 To become an efficient JDK developer, it is recommended that you invest in
1590 learning Mercurial properly. Here are some links that can get you started:
1591
1592 * [Mercurial for git users](http://www.mercurial-scm.org/wiki/GitConcepts)
1593 * [The official Mercurial tutorial](http://www.mercurial-scm.org/wiki/Tutorial)
1594 * [hg init](http://hginit.com/)
1595 * [Mercurial: The Definitive Guide](http://hgbook.red-bean.com/read/)
1596
1597 ## Understanding the Build System
1598
1599 This section will give you a more technical description on the details of the
1600 build system.
1601
1602 ### Configurations
1603
1604 The build system expects to find one or more configuration. These are
1605 technically defined by the `spec.gmk` in a subdirectory to the `build`
1606 subdirectory. The `spec.gmk` file is generated by `configure`, and contains in
1607 principle the configuration (directly or by files included by `spec.gmk`).
1608
1609 You can, in fact, select a configuration to build by pointing to the `spec.gmk`
1610 file with the `SPEC` make control variable, e.g. `make SPEC=$BUILD/spec.gmk`.
1611 While this is not the recommended way to call `make` as a user, it is what is
1612 used under the hood by the build system.
1613
1614 ### Build Output Structure
1615
1616 The build output for a configuration will end up in `build/<configuration
1617 name>`, which we refer to as `$BUILD` in this document. The `$BUILD` directory
1618 contains the following important directories:
1619
1620 ```
1621 buildtools/
1622 configure-support/
1623 hotspot/
1624 images/
1625 jdk/
1626 make-support/
1627 support/
1628 test-results/
1629 test-support/
1630 ```
1631
1632 This is what they are used for:
1633
1634 * `images`: This is the directory were the output of the `*-image` make
1635 targets end up. For instance, `make jdk-image` ends up in `images/jdk`.
1636
1637 * `jdk`: This is the "exploded image". After `make jdk`, you will be able to
1638 launch the newly built JDK by running `$BUILD/jdk/bin/java`.
1639
1640 * `test-results`: This directory contains the results from running tests.
1641
1642 * `support`: This is an area for intermediate files needed during the build,
1643 e.g. generated source code, object files and class files. Some noteworthy
1644 directories in `support` is `gensrc`, which contains the generated source
1645 code, and the `modules_*` directories, which contains the files in a
1646 per-module hierarchy that will later be collapsed into the `jdk` directory
1647 of the exploded image.
1648
1649 * `buildtools`: This is an area for tools compiled for the build platform
1650 that are used during the rest of the build.
1651
1652 * `hotspot`: This is an area for intermediate files needed when building
1653 hotspot.
1654
1655 * `configure-support`, `make-support` and `test-support`: These directories
1656 contain files that are needed by the build system for `configure`, `make`
1657 and for running tests.
1658
1659 ### Fixpath
1660
1661 Windows path typically look like `C:\User\foo`, while Unix paths look like
1662 `/home/foo`. Tools with roots from Unix often experience issues related to this
1663 mismatch when running on Windows.
1664
1665 In the JDK build, we always use Unix paths internally, and only just before
1666 calling a tool that does not understand Unix paths do we convert them to
1667 Windows paths.
1668
1669 This conversion is done by the `fixpath` tool, which is a small wrapper that
1670 modifies unix-style paths to Windows-style paths in command lines. Fixpath is
1671 compiled automatically by `configure`.
1672
1673 ### Native Debug Symbols
1674
1675 Native libraries and executables can have debug symbol (and other debug
1676 information) associated with them. How this works is very much platform
1677 dependent, but a common problem is that debug symbol information takes a lot of
1678 disk space, but is rarely needed by the end user.
1679
1680 The JDK supports different methods on how to handle debug symbols. The
1681 method used is selected by `--with-native-debug-symbols`, and available methods
1682 are `none`, `internal`, `external`, `zipped`.
1683
1684 * `none` means that no debug symbols will be generated during the build.
1685
1686 * `internal` means that debug symbols will be generated during the build, and
1687 they will be stored in the generated binary.
1688
1689 * `external` means that debug symbols will be generated during the build, and
1690 after the compilation, they will be moved into a separate `.debuginfo` file.
1691 (This was previously known as FDS, Full Debug Symbols).
1692
1693 * `zipped` is like `external`, but the .debuginfo file will also be zipped
1694 into a `.diz` file.
1695
1696 When building for distribution, `zipped` is a good solution. Binaries built
1697 with `internal` is suitable for use by developers, since they facilitate
1698 debugging, but should be stripped before distributed to end users.
1699
1700 ### Autoconf Details
1701
1702 The `configure` script is based on the autoconf framework, but in some details
1703 deviate from a normal autoconf `configure` script.
1704
1705 The `configure` script in the top level directory of the JDK is just a thin
1706 wrapper that calls `make/autoconf/configure`. This in turn will run `autoconf`
1707 to create the runnable (generated) configure script, as
1708 `.build/generated-configure.sh`. Apart from being responsible for the
1709 generation of the runnable script, the `configure` script also provides
1710 functionality that is not easily expressed in the normal Autoconf framework. As
1711 part of this functionality, the generated script is called.
1712
1713 The build system will detect if the Autoconf source files have changed, and
1714 will trigger a regeneration of the generated script if needed. You can also
1715 manually request such an update by `bash configure autogen`.
1716
1717 In previous versions of the JDK, the generated script was checked in at
1718 `make/autoconf/generated-configure.sh`. This is no longer the case.
1719
1720 ### Developing the Build System Itself
1721
1722 This section contains a few remarks about how to develop for the build system
1723 itself. It is not relevant if you are only making changes in the product source
1724 code.
1725
1726 While technically using `make`, the make source files of the JDK does not
1727 resemble most other Makefiles. Instead of listing specific targets and actions
1728 (perhaps using patterns), the basic modus operandi is to call a high-level
1729 function (or properly, macro) from the API in `make/common`. For instance, to
1730 compile all classes in the `jdk.internal.foo` package in the `jdk.foo` module,
1731 a call like this would be made:
1732
1733 ```
1734 $(eval $(call SetupJavaCompilation, BUILD_FOO_CLASSES, \
1735 SETUP := GENERATE_OLDBYTECODE, \
1736 SRC := $(TOPDIR)/src/jkd.foo/share/classes, \
1737 INCLUDES := jdk/internal/foo, \
1738 BIN := $(SUPPORT_OUTPUTDIR)/foo_classes, \
1739 ))
1740 ```
1741
1742 By encapsulating and expressing the high-level knowledge of *what* should be
1743 done, rather than *how* it should be done (as is normal in Makefiles), we can
1744 build a much more powerful and flexible build system.
1745
1746 Correct dependency tracking is paramount. Sloppy dependency tracking will lead
1747 to improper parallelization, or worse, race conditions.
1748
1749 To test for/debug race conditions, try running `make JOBS=1` and `make
1750 JOBS=100` and see if it makes any difference. (It shouldn't).
1751
1752 To compare the output of two different builds and see if, and how, they differ,
1753 run `$BUILD1/compare.sh -o $BUILD2`, where `$BUILD1` and `$BUILD2` are the two
1754 builds you want to compare.
1755
1756 To automatically build two consecutive versions and compare them, use
1757 `COMPARE_BUILD`. The value of `COMPARE_BUILD` is a set of variable=value
1758 assignments, like this:
1759 ```
1760 make COMPARE_BUILD=CONF=--enable-new-hotspot-feature:MAKE=hotspot
1761 ```
1762 See `make/InitSupport.gmk` for details on how to use `COMPARE_BUILD`.
1763
1764 To analyze build performance, run with `LOG=trace` and check `$BUILD/build-trace-time.log`.
1765 Use `JOBS=1` to avoid parallelism.
1766
1767 Please check that you adhere to the [Code Conventions for the Build System](
1768 http://openjdk.java.net/groups/build/doc/code-conventions.html) before
1769 submitting patches.
1770
1771 ## Contributing to the JDK
1772
1773 So, now you've built your JDK, and made your first patch, and want to
1774 contribute it back to the OpenJDK Community.
1775
1776 First of all: Thank you! We gladly welcome your contribution.
1777 However, please bear in mind that the JDK is a massive project, and we must ask
1778 you to follow our rules and guidelines to be able to accept your contribution.
1779
1780 The official place to start is the ['How to contribute' page](
1781 http://openjdk.java.net/contribute/). There is also an official (but somewhat
1782 outdated and skimpy on details) [Developer's Guide](
1783 http://openjdk.java.net/guide/).
1784
1785 If this seems overwhelming to you, the Adoption Group is there to help you! A
1786 good place to start is their ['New Contributor' page](
1787 https://wiki.openjdk.java.net/display/Adoption/New+Contributor), or start
1788 reading the comprehensive [Getting Started Kit](
1789 https://adoptopenjdk.gitbooks.io/adoptopenjdk-getting-started-kit/en/). The
1790 Adoption Group will also happily answer any questions you have about
1791 contributing. Contact them by [mail](
1792 http://mail.openjdk.java.net/mailman/listinfo/adoption-discuss) or [IRC](
1793 http://openjdk.java.net/irc/).
1794
1795 ---
1796 # Override styles from the base CSS file that are not ideal for this document.
1797 header-includes:
1798 - '<style type="text/css">pre, code, tt { color: #1d6ae5; }</style>'
1799 ---