1 /*
2 * Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.nio;
27
28 import java.io.FileDescriptor;
29 import java.lang.ref.Reference;
30 import java.util.Objects;
31
32 import jdk.internal.access.foreign.MemorySegmentProxy;
33 import jdk.internal.misc.Unsafe;
34
35
36 /**
37 * A direct byte buffer whose content is a memory-mapped region of a file.
38 *
39 * <p> Mapped byte buffers are created via the {@link
40 * java.nio.channels.FileChannel#map FileChannel.map} method. This class
41 * extends the {@link ByteBuffer} class with operations that are specific to
42 * memory-mapped file regions.
43 *
44 * <p> A mapped byte buffer and the file mapping that it represents remain
45 * valid until the buffer itself is garbage-collected.
46 *
47 * <p> The content of a mapped byte buffer can change at any time, for example
48 * if the content of the corresponding region of the mapped file is changed by
49 * this program or another. Whether or not such changes occur, and when they
50 * occur, is operating-system dependent and therefore unspecified.
51 *
52 * <a id="inaccess"></a><p> All or part of a mapped byte buffer may become
53 * inaccessible at any time, for example if the mapped file is truncated. An
54 * attempt to access an inaccessible region of a mapped byte buffer will not
55 * change the buffer's content and will cause an unspecified exception to be
56 * thrown either at the time of the access or at some later time. It is
57 * therefore strongly recommended that appropriate precautions be taken to
58 * avoid the manipulation of a mapped file by this program, or by a
59 * concurrently running program, except to read or write the file's content.
60 *
61 * <p> Mapped byte buffers otherwise behave no differently than ordinary direct
62 * byte buffers. </p>
63 *
64 *
65 * @author Mark Reinhold
66 * @author JSR-51 Expert Group
67 * @since 1.4
68 */
69
70 public abstract class MappedByteBuffer
71 extends ByteBuffer
72 {
73
74 // This is a little bit backwards: By rights MappedByteBuffer should be a
75 // subclass of DirectByteBuffer, but to keep the spec clear and simple, and
76 // for optimization purposes, it's easier to do it the other way around.
77 // This works because DirectByteBuffer is a package-private class.
78
79 // For mapped buffers, a FileDescriptor that may be used for mapping
80 // operations if valid; null if the buffer is not mapped.
81 private final FileDescriptor fd;
82
83 // A flag true if this buffer is mapped against non-volatile
84 // memory using one of the extended FileChannel.MapMode modes,
85 // MapMode.READ_ONLY_SYNC or MapMode.READ_WRITE_SYNC and false if
86 // it is mapped using any of the other modes. This flag only
87 // determines the behavior of force operations.
88 private final boolean isSync;
89
90 // This should only be invoked by the DirectByteBuffer constructors
91 //
92 MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
93 FileDescriptor fd, boolean isSync, MemorySegmentProxy segment) {
94 super(mark, pos, lim, cap, segment);
95 this.fd = fd;
96 this.isSync = isSync;
97 }
98
99 MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
100 boolean isSync, MemorySegmentProxy segment) {
101 super(mark, pos, lim, cap, segment);
102 this.fd = null;
103 this.isSync = isSync;
104 }
105
106 MappedByteBuffer(int mark, int pos, int lim, int cap, MemorySegmentProxy segment) { // package-private
107 super(mark, pos, lim, cap, segment);
108 this.fd = null;
109 this.isSync = false;
110 }
111
112 // Returns the distance (in bytes) of the buffer start from the
113 // largest page aligned address of the mapping less than or equal
114 // to the start address.
115 private long mappingOffset() {
116 return mappingOffset(0);
117 }
118
119 // Returns the distance (in bytes) of the buffer element
120 // identified by index from the largest page aligned address of
121 // the mapping less than or equal to the element address. Computed
122 // each time to avoid storing in every direct buffer.
123 private long mappingOffset(int index) {
124 int ps = Bits.pageSize();
125 long indexAddress = address + index;
126 long baseAddress = alignDown(indexAddress, ps);
127 return indexAddress - baseAddress;
128 }
129
130 // Given an offset previously obtained from calling
131 // mappingOffset() returns the largest page aligned address of the
132 // mapping less than or equal to the buffer start address.
133 private long mappingAddress(long mappingOffset) {
134 return mappingAddress(mappingOffset, 0);
135 }
136
137 // Given an offset previously otained from calling
138 // mappingOffset(index) returns the largest page aligned address
139 // of the mapping less than or equal to the address of the buffer
140 // element identified by index.
141 private long mappingAddress(long mappingOffset, long index) {
142 long indexAddress = address + index;
143 return indexAddress - mappingOffset;
144 }
145
146 // given a mappingOffset previously otained from calling
147 // mappingOffset() return that offset added to the buffer
148 // capacity.
149 private long mappingLength(long mappingOffset) {
150 return mappingLength(mappingOffset, (long)capacity());
151 }
152
153 // given a mappingOffset previously otained from calling
154 // mappingOffset(index) return that offset added to the supplied
155 // length.
156 private long mappingLength(long mappingOffset, long length) {
157 return length + mappingOffset;
158 }
159
160 // align address down to page size
161 private static long alignDown(long address, int pageSize) {
162 // pageSize must be a power of 2
163 return address & ~(pageSize - 1);
164 }
165
166 /**
167 * Tells whether this buffer was mapped against a non-volatile
168 * memory device by passing one of the sync map modes {@link
169 * jdk.nio.mapmode.ExtendedMapMode#READ_ONLY_SYNC
170 * ExtendedMapModeMapMode#READ_ONLY_SYNC} or {@link
171 * jdk.nio.mapmode.ExtendedMapMode#READ_ONLY_SYNC
172 * ExtendedMapMode#READ_WRITE_SYNC} in the call to {@link
173 * java.nio.channels.FileChannel#map FileChannel.map} or was
174 * mapped by passing one of the other map modes.
175 *
176 * @return true if the file was mapped using one of the sync map
177 * modes, otherwise false.
178 */
179 private boolean isSync() {
180 return isSync;
181 }
182
183 /**
184 * Tells whether or not this buffer's content is resident in physical
185 * memory.
186 *
187 * <p> A return value of {@code true} implies that it is highly likely
188 * that all of the data in this buffer is resident in physical memory and
189 * may therefore be accessed without incurring any virtual-memory page
190 * faults or I/O operations. A return value of {@code false} does not
191 * necessarily imply that the buffer's content is not resident in physical
192 * memory.
193 *
194 * <p> The returned value is a hint, rather than a guarantee, because the
195 * underlying operating system may have paged out some of the buffer's data
196 * by the time that an invocation of this method returns. </p>
197 *
198 * @return {@code true} if it is likely that this buffer's content
199 * is resident in physical memory
200 */
201 public final boolean isLoaded() {
202 if (fd == null) {
203 return true;
204 }
205 // a sync mapped buffer is always loaded
206 if (isSync()) {
207 return true;
208 }
209 if ((address == 0) || (capacity() == 0))
210 return true;
211 long offset = mappingOffset();
212 long length = mappingLength(offset);
213 return isLoaded0(mappingAddress(offset), length, Bits.pageCount(length));
214 }
215
216 // not used, but a potential target for a store, see load() for details.
217 private static byte unused;
218
219 /**
220 * Loads this buffer's content into physical memory.
221 *
222 * <p> This method makes a best effort to ensure that, when it returns,
223 * this buffer's content is resident in physical memory. Invoking this
224 * method may cause some number of page faults and I/O operations to
225 * occur. </p>
226 *
227 * @return This buffer
228 */
229 public final MappedByteBuffer load() {
230 if (fd == null) {
231 return this;
232 }
233 // no need to load a sync mapped buffer
234 if (isSync()) {
235 return this;
236 }
237 if ((address == 0) || (capacity() == 0))
238 return this;
239 long offset = mappingOffset();
240 long length = mappingLength(offset);
241 load0(mappingAddress(offset), length);
242
243 // Read a byte from each page to bring it into memory. A checksum
244 // is computed as we go along to prevent the compiler from otherwise
245 // considering the loop as dead code.
246 Unsafe unsafe = Unsafe.getUnsafe();
247 int ps = Bits.pageSize();
248 int count = Bits.pageCount(length);
249 long a = mappingAddress(offset);
250 byte x = 0;
251 try {
252 for (int i=0; i<count; i++) {
253 // TODO consider changing to getByteOpaque thus avoiding
254 // dead code elimination and the need to calculate a checksum
255 x ^= unsafe.getByte(a);
256 a += ps;
257 }
258 } finally {
259 Reference.reachabilityFence(this);
260 }
261 if (unused != 0)
262 unused = x;
263
264 return this;
265 }
266
267 /**
268 * Forces any changes made to this buffer's content to be written to the
269 * storage device containing the mapped file.
270 *
271 * <p> If the file mapped into this buffer resides on a local storage
272 * device then when this method returns it is guaranteed that all changes
273 * made to the buffer since it was created, or since this method was last
274 * invoked, will have been written to that device.
275 *
276 * <p> If the file does not reside on a local device then no such guarantee
277 * is made.
278 *
279 * <p> If this buffer was not mapped in read/write mode ({@link
280 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then
281 * invoking this method may have no effect. In particular, the
282 * method has no effect for buffers mapped in read-only or private
283 * mapping modes. This method may or may not have an effect for
284 * implementation-specific mapping modes. </p>
285 *
286 * @return This buffer
287 */
288 public final MappedByteBuffer force() {
289 if (fd == null) {
290 return this;
291 }
292 if (isSync) {
293 return force(0, limit());
294 }
295 if ((address != 0) && (capacity() != 0)) {
296 long offset = mappingOffset();
297 force0(fd, mappingAddress(offset), mappingLength(offset));
298 }
299 return this;
300 }
301
302 /**
303 * Forces any changes made to a region of this buffer's content to
304 * be written to the storage device containing the mapped
305 * file. The region starts at the given {@code index} in this
306 * buffer and is {@code length} bytes.
307 *
308 * <p> If the file mapped into this buffer resides on a local
309 * storage device then when this method returns it is guaranteed
310 * that all changes made to the selected region buffer since it
311 * was created, or since this method was last invoked, will have
312 * been written to that device. The force operation is free to
313 * write bytes that lie outside the specified region, for example
314 * to ensure that data blocks of some device-specific granularity
315 * are transferred in their entirety.
316 *
317 * <p> If the file does not reside on a local device then no such
318 * guarantee is made.
319 *
320 * <p> If this buffer was not mapped in read/write mode ({@link
321 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then
322 * invoking this method may have no effect. In particular, the
323 * method has no effect for buffers mapped in read-only or private
324 * mapping modes. This method may or may not have an effect for
325 * implementation-specific mapping modes. </p>
326 *
327 * @param index
328 * The index of the first byte in the buffer region that is
329 * to be written back to storage; must be non-negative
330 * and less than limit()
331 *
332 * @param length
333 * The length of the region in bytes; must be non-negative
334 * and no larger than limit() - index
335 *
336 * @throws IndexOutOfBoundsException
337 * if the preconditions on the index and length do not
338 * hold.
339 *
340 * @return This buffer
341 *
342 * @since 13
343 */
344 public final MappedByteBuffer force(int index, int length) {
345 if (fd == null) {
346 return this;
347 }
348 if ((address != 0) && (limit() != 0)) {
349 // check inputs
350 Objects.checkFromIndexSize(index, length, limit());
351 if (isSync) {
352 // simply force writeback of associated cache lines
353 Unsafe.getUnsafe().writebackMemory(address + index, length);
354 } else {
355 // force writeback via file descriptor
356 long offset = mappingOffset(index);
357 force0(fd, mappingAddress(offset, index), mappingLength(offset, length));
358 }
359 }
360 return this;
361 }
362
363 private native boolean isLoaded0(long address, long length, int pageCount);
364 private native void load0(long address, long length);
365 private native void force0(FileDescriptor fd, long address, long length);
366
367 // -- Covariant return type overrides
368
369 /**
370 * {@inheritDoc}
371 */
372 @Override
373 public final MappedByteBuffer position(int newPosition) {
374 super.position(newPosition);
375 return this;
376 }
377
378 /**
379 * {@inheritDoc}
380 */
381 @Override
382 public final MappedByteBuffer limit(int newLimit) {
383 super.limit(newLimit);
384 return this;
385 }
386
387 /**
388 * {@inheritDoc}
389 */
390 @Override
391 public final MappedByteBuffer mark() {
392 super.mark();
393 return this;
394 }
395
396 /**
397 * {@inheritDoc}
398 */
399 @Override
400 public final MappedByteBuffer reset() {
401 super.reset();
402 return this;
403 }
404
405 /**
406 * {@inheritDoc}
407 */
408 @Override
409 public final MappedByteBuffer clear() {
410 super.clear();
411 return this;
412 }
413
414 /**
415 * {@inheritDoc}
416 */
417 @Override
418 public final MappedByteBuffer flip() {
419 super.flip();
420 return this;
421 }
422
423 /**
424 * {@inheritDoc}
425 */
426 @Override
427 public final MappedByteBuffer rewind() {
428 super.rewind();
429 return this;
430 }
431 }