1 /*
2 * Copyright (c) 2000, 2018, 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 jdk.internal.misc.Unsafe;
31
32
33 /**
34 * A direct byte buffer whose content is a memory-mapped region of a file.
35 *
36 * <p> Mapped byte buffers are created via the {@link
37 * java.nio.channels.FileChannel#map FileChannel.map} method. This class
38 * extends the {@link ByteBuffer} class with operations that are specific to
39 * memory-mapped file regions.
40 *
41 * <p> A mapped byte buffer and the file mapping that it represents remain
42 * valid until the buffer itself is garbage-collected.
43 *
44 * <p> The content of a mapped byte buffer can change at any time, for example
45 * if the content of the corresponding region of the mapped file is changed by
46 * this program or another. Whether or not such changes occur, and when they
47 * occur, is operating-system dependent and therefore unspecified.
48 *
49 * <a id="inaccess"></a><p> All or part of a mapped byte buffer may become
50 * inaccessible at any time, for example if the mapped file is truncated. An
51 * attempt to access an inaccessible region of a mapped byte buffer will not
52 * change the buffer's content and will cause an unspecified exception to be
53 * thrown either at the time of the access or at some later time. It is
54 * therefore strongly recommended that appropriate precautions be taken to
55 * avoid the manipulation of a mapped file by this program, or by a
56 * concurrently running program, except to read or write the file's content.
57 *
58 * <p> Mapped byte buffers otherwise behave no differently than ordinary direct
59 * byte buffers. </p>
60 *
61 *
62 * @author Mark Reinhold
63 * @author JSR-51 Expert Group
64 * @since 1.4
65 */
66
67 public abstract class MappedByteBuffer
68 extends ByteBuffer
69 {
70
71 // This is a little bit backwards: By rights MappedByteBuffer should be a
72 // subclass of DirectByteBuffer, but to keep the spec clear and simple, and
73 // for optimization purposes, it's easier to do it the other way around.
74 // This works because DirectByteBuffer is a package-private class.
75
76 // For mapped buffers, a FileDescriptor that may be used for mapping
77 // operations if valid; null if the buffer is not mapped.
78 private final FileDescriptor fd;
79
80 // A flag true if this buffer is mapped against persistent memory
81 // using one of the persistent FileChannel.MapMode modes,
82 // MapMode.READ_ONLY_PERSISTENT or MapMode.READ_WRITE_PERSISTENT
83 // and false if it is mapped using any of other modes. this flag
84 // only determines the behaviour of force operations.
85 private final boolean isPersistent;
86
87 // This should only be invoked by the DirectByteBuffer constructors
88 //
89 MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
90 FileDescriptor fd, boolean isPersistent) {
91 super(mark, pos, lim, cap);
92 this.fd = fd;
93 this.isPersistent = isPersistent;
94 }
95
96 MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
97 boolean isPersistent) {
98 super(mark, pos, lim, cap);
99 this.fd = null;
100 this.isPersistent = isPersistent;
101 }
102
103 MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private
104 super(mark, pos, lim, cap);
105 this.fd = null;
106 this.isPersistent = false;
107 }
108
109 // Returns the distance (in bytes) of the buffer from the page aligned address
110 // of the mapping. Computed each time to avoid storing in every direct buffer.
111 private long mappingOffset() {
112 int ps = Bits.pageSize();
113 long offset = address % ps;
114 return (offset >= 0) ? offset : (ps + offset);
115 }
116
117 private long mappingAddress(long mappingOffset) {
118 return address - mappingOffset;
119 }
120
121 private long mappingLength(long mappingOffset) {
122 return (long)capacity() + mappingOffset;
123 }
124
125 /**
126 * Tells whether this buffer was mapped against a non-volatile
127 * memory device by passing one of the persistent map modes {@link
128 * java.nio.channels.FileChannel.MapMode#READ_ONLY_PERSISTENT
129 * MapMode#READ_ONLY_PERSISTENT} or {@link
130 * java.nio.channels.FileChannel.MapMode#READ_ONLY_PERSISTENT
131 * MapMode#READ_WRITE_PERSISTENT} in the call to {@link
132 * java.nio.channels.FileChannel#map FileChannel.map} or mapped
133 * against some other form of device file by pasing one of the
134 * other map modes.
135 *
136 * @return true if the file was mapped using against a
137 * non-volatile memory device by passing one of the persistent map
138 * modes {@link
139 * java.nio.channels.FileChannel.MapMode#READ_ONLY_PERSISTENT
140 * MapMode#READ_ONLY_PERSISTENT} or {@link
141 * java.nio.channels.FileChannel.MapMode#READ_ONLY_PERSISTENT
142 * MapMode#READ_WRITE_PERSISTENT} in the call to {@link
143 * java.nio.channels.FileChannel#map FileChannel.map} otherwise
144 * false.
145 */
146 public boolean isPersistent() {
147 return isPersistent;
148 }
149
150 /**
151 * Tells whether or not this buffer's content is resident in physical
152 * memory.
153 *
154 * <p> A return value of {@code true} implies that it is highly likely
155 * that all of the data in this buffer is resident in physical memory and
156 * may therefore be accessed without incurring any virtual-memory page
157 * faults or I/O operations. A return value of {@code false} does not
158 * necessarily imply that the buffer's content is not resident in physical
159 * memory.
160 *
161 * <p> The returned value is a hint, rather than a guarantee, because the
162 * underlying operating system may have paged out some of the buffer's data
163 * by the time that an invocation of this method returns. </p>
164 *
165 * @return {@code true} if it is likely that this buffer's content
166 * is resident in physical memory
167 */
168 public final boolean isLoaded() {
169 if (fd == null) {
170 return true;
171 }
172 // a persistent mapped buffer is always loaded
173 if (isPersistent()) {
174 return true;
175 }
176 if ((address == 0) || (capacity() == 0))
177 return true;
178 long offset = mappingOffset();
179 long length = mappingLength(offset);
180 return isLoaded0(mappingAddress(offset), length, Bits.pageCount(length));
181 }
182
183 // not used, but a potential target for a store, see load() for details.
184 private static byte unused;
185
186 /**
187 * Loads this buffer's content into physical memory.
188 *
189 * <p> This method makes a best effort to ensure that, when it returns,
190 * this buffer's content is resident in physical memory. Invoking this
191 * method may cause some number of page faults and I/O operations to
192 * occur. </p>
193 *
194 * @return This buffer
195 */
196 public final MappedByteBuffer load() {
197 if (fd == null) {
198 return this;
199 }
200 // no need to load a persistent mapped buffer
201 if (isPersistent()) {
202 return this;
203 }
204 if ((address == 0) || (capacity() == 0))
205 return this;
206 long offset = mappingOffset();
207 long length = mappingLength(offset);
208 load0(mappingAddress(offset), length);
209
210 // Read a byte from each page to bring it into memory. A checksum
211 // is computed as we go along to prevent the compiler from otherwise
212 // considering the loop as dead code.
213 Unsafe unsafe = Unsafe.getUnsafe();
214 int ps = Bits.pageSize();
215 int count = Bits.pageCount(length);
216 long a = mappingAddress(offset);
217 byte x = 0;
218 try {
219 for (int i=0; i<count; i++) {
220 // TODO consider changing to getByteOpaque thus avoiding
221 // dead code elimination and the need to calculate a checksum
222 x ^= unsafe.getByte(a);
223 a += ps;
224 }
225 } finally {
226 Reference.reachabilityFence(this);
227 }
228 if (unused != 0)
229 unused = x;
230
231 return this;
232 }
233
234 /**
235 * Forces any changes made to this buffer's content to be written to the
236 * storage device containing the mapped file.
237 *
238 * <p> If the file mapped into this buffer resides on a local storage
239 * device then when this method returns it is guaranteed that all changes
240 * made to the buffer since it was created, or since this method was last
241 * invoked, will have been written to that device.
242 *
243 * <p> If the file does not reside on a local device then no such guarantee
244 * is made.
245 *
246 * <p> If this buffer was not mapped in read/write mode ({@link
247 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
248 * method has no effect. </p>
249 *
250 * @return This buffer
251 */
252 public final MappedByteBuffer force() {
253 return force(0, capacity());
254 }
255
256 /**
257 * Forces any changes made to some region of this buffer's content
258 * to be written to the storage device containing the mapped file.
259 *
260 * <p> If the file mapped into this buffer resides on a local storage
261 * device then when this method returns it is guaranteed that all changes
262 * made to the buffer since it was created, or since this method was last
263 * invoked, will have been written to that device.
264 *
265 * <p> If the file does not reside on a local device then no such guarantee
266 * is made.
267 *
268 * <p> If this buffer was not mapped in read/write mode ({@link
269 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
270 * method has no effect. </p>
271 *
272 * @param from
273 * The offset to the first byte in the buffer region that
274 * is to be written back to storage
275 *
276 * @param to
277 * The offset to the first byte beyond the buffer region
278 * that is to be written back to storage
279 *
280 * @return This buffer
281 *
282 * @since 12
283 */
284 public final MappedByteBuffer force(long from, long to) {
285 if (fd == null) {
286 return this;
287 }
288 if ((address != 0) && (capacity() != 0)) {
289 // check inputs
290 if (from < 0 || from >= capacity()) {
291 throw new IllegalArgumentException();
292 }
293 if (to < from || to > capacity()) {
294 throw new IllegalArgumentException();
295 }
296
297 long offset = mappingOffset();
298 long a = mappingAddress(offset) + from;
299 long length = to - from;
300 if (isPersistent) {
301 // simply force writeback of associated cache lines
302 Unsafe unsafe = Unsafe.getUnsafe();
303 unsafe.writebackMemory(a, length);
304 } else {
305 // writeback using device associated with fd
306 force0(fd, a, length);
307 }
308 }
309 return this;
310 }
311
312 private native boolean isLoaded0(long address, long length, int pageCount);
313 private native void load0(long address, long length);
314 private native void force0(FileDescriptor fd, long address, long length);
315
316 // -- Covariant return type overrides
317
318 /**
319 * {@inheritDoc}
320 */
321 @Override
322 public final MappedByteBuffer position(int newPosition) {
323 super.position(newPosition);
324 return this;
325 }
326
327 /**
328 * {@inheritDoc}
329 */
330 @Override
331 public final MappedByteBuffer limit(int newLimit) {
332 super.limit(newLimit);
333 return this;
334 }
335
336 /**
337 * {@inheritDoc}
338 */
339 @Override
340 public final MappedByteBuffer mark() {
341 super.mark();
342 return this;
343 }
344
345 /**
346 * {@inheritDoc}
347 */
348 @Override
349 public final MappedByteBuffer reset() {
350 super.reset();
351 return this;
352 }
353
354 /**
355 * {@inheritDoc}
356 */
357 @Override
358 public final MappedByteBuffer clear() {
359 super.clear();
360 return this;
361 }
362
363 /**
364 * {@inheritDoc}
365 */
366 @Override
367 public final MappedByteBuffer flip() {
368 super.flip();
369 return this;
370 }
371
372 /**
373 * {@inheritDoc}
374 */
375 @Override
376 public final MappedByteBuffer rewind() {
377 super.rewind();
378 return this;
379 }
380 }