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 // via a call to FileChannel.map_persistent and false if it is
82 // some other sort of file mapped via a call to FileChannel.map.
83 // this flag only determines the behaviour of force operations.
84 private final boolean isPersistent;
85
86 // This should only be invoked by the DirectByteBuffer constructors
87 //
88 MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
89 FileDescriptor fd, boolean isPersistent) {
90 super(mark, pos, lim, cap);
91 this.fd = fd;
92 this.isPersistent = isPersistent;
93 }
94
95 MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
96 boolean isPersistent) {
97 super(mark, pos, lim, cap);
98 this.fd = null;
99 this.isPersistent = isPersistent;
100 }
101
102 MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private
103 super(mark, pos, lim, cap);
104 this.fd = null;
105 this.isPersistent = false;
106 }
107
108 // Returns the distance (in bytes) of the buffer from the page aligned address
109 // of the mapping. Computed each time to avoid storing in every direct buffer.
110 private long mappingOffset() {
111 int ps = Bits.pageSize();
112 long offset = address % ps;
113 return (offset >= 0) ? offset : (ps + offset);
114 }
115
116 private long mappingAddress(long mappingOffset) {
117 return address - mappingOffset;
118 }
119
120 private long mappingLength(long mappingOffset) {
121 return (long)capacity() + mappingOffset;
122 }
123
124 /**
125 * Tells whether this buffer was mapped against persistent memory
126 * by calling {@link java.nio.channels.FileChannel#map_persistent
127 * FileChannel.map_persistent} or mapped against some other form
128 * of device file using {@link java.nio.channels.FileChannel#map
129 * FileChannel.map}.
130 *
131 * @return true if the file was mapped using {@link
132 * java.nio.channels.FileChannel#map_persistent
133 * FileChannel.map_persistent} otherwise false.
134 */
135 public boolean isPersistent() {
136 return isPersistent;
137 }
138
139 /**
140 * Tells whether or not this buffer's content is resident in physical
141 * memory.
142 *
143 * <p> A return value of {@code true} implies that it is highly likely
144 * that all of the data in this buffer is resident in physical memory and
145 * may therefore be accessed without incurring any virtual-memory page
146 * faults or I/O operations. A return value of {@code false} does not
147 * necessarily imply that the buffer's content is not resident in physical
148 * memory.
149 *
150 * <p> The returned value is a hint, rather than a guarantee, because the
151 * underlying operating system may have paged out some of the buffer's data
152 * by the time that an invocation of this method returns. </p>
153 *
154 * @return {@code true} if it is likely that this buffer's content
155 * is resident in physical memory
156 */
157 public final boolean isLoaded() {
158 if (fd == null) {
159 return true;
160 }
161 // a persistent mapped buffer is always loaded
162 if (isPersistent()) {
163 return true;
164 }
165 if ((address == 0) || (capacity() == 0))
166 return true;
167 long offset = mappingOffset();
168 long length = mappingLength(offset);
169 return isLoaded0(mappingAddress(offset), length, Bits.pageCount(length));
170 }
171
172 // not used, but a potential target for a store, see load() for details.
173 private static byte unused;
174
175 /**
176 * Loads this buffer's content into physical memory.
177 *
178 * <p> This method makes a best effort to ensure that, when it returns,
179 * this buffer's content is resident in physical memory. Invoking this
180 * method may cause some number of page faults and I/O operations to
181 * occur. </p>
182 *
183 * @return This buffer
184 */
185 public final MappedByteBuffer load() {
186 if (fd == null) {
187 return this;
188 }
189 // no need to load a persistent mapped buffer
190 if (isPersistent()) {
191 return this;
192 }
193 if ((address == 0) || (capacity() == 0))
194 return this;
195 long offset = mappingOffset();
196 long length = mappingLength(offset);
197 load0(mappingAddress(offset), length);
198
199 // Read a byte from each page to bring it into memory. A checksum
200 // is computed as we go along to prevent the compiler from otherwise
201 // considering the loop as dead code.
202 Unsafe unsafe = Unsafe.getUnsafe();
203 int ps = Bits.pageSize();
204 int count = Bits.pageCount(length);
205 long a = mappingAddress(offset);
206 byte x = 0;
207 try {
208 for (int i=0; i<count; i++) {
209 // TODO consider changing to getByteOpaque thus avoiding
210 // dead code elimination and the need to calculate a checksum
211 x ^= unsafe.getByte(a);
212 a += ps;
213 }
214 } finally {
215 Reference.reachabilityFence(this);
216 }
217 if (unused != 0)
218 unused = x;
219
220 return this;
221 }
222
223 /**
224 * Forces any changes made to this buffer's content to be written to the
225 * storage device containing the mapped file.
226 *
227 * <p> If the file mapped into this buffer resides on a local storage
228 * device then when this method returns it is guaranteed that all changes
229 * made to the buffer since it was created, or since this method was last
230 * invoked, will have been written to that device.
231 *
232 * <p> If the file does not reside on a local device then no such guarantee
233 * is made.
234 *
235 * <p> If this buffer was not mapped in read/write mode ({@link
236 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
237 * method has no effect. </p>
238 *
239 * @return This buffer
240 */
241 public final MappedByteBuffer force() {
242 return force(0, capacity());
243 }
244
245 /**
246 * Forces any changes made to some subregion of this buffer's
247 * content to be written to the storage device containing the
248 * mapped file.
249 *
250 * <p> If the file mapped into this buffer resides on a local storage
251 * device then when this method returns it is guaranteed that all changes
252 * made to the buffer since it was created, or since this method was last
253 * invoked, will have been written to that device.
254 *
255 * <p> If the file does not reside on a local device then no such guarantee
256 * is made.
257 *
258 * <p> If this buffer was not mapped in read/write mode ({@link
259 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
260 * method has no effect. </p>
261 *
262 * @param from
263 * The offset to the first byte in the buffer subregion
264 * that is to be written back to storage
265 *
266 * @param to
267 * The offset to the first byte beyond the buffer subregion
268 * that is to be written back to storage
269 *
270 * @return This buffer
271 */
272 public final MappedByteBuffer force(long from, long to) {
273 if (fd == null) {
274 return this;
275 }
276 if ((address != 0) && (capacity() != 0)) {
277 // check inputs
278 if (from < 0 || from >= capacity()) {
279 throw new IllegalArgumentException();
280 }
281 if (to < from || to > capacity()) {
282 throw new IllegalArgumentException();
283 }
284
285 long offset = mappingOffset();
286 long a = mappingAddress(offset) + from;
287 long length = offset + to;
288 if (isPersistent) {
289 // simply force writeback of associated cache lines
290 Unsafe unsafe = Unsafe.getUnsafe();
291 unsafe.writebackMemory(a, length);
292 } else {
293 // writeback using device associated with fd
294 force0(fd, a, length);
295 }
296 }
297 return this;
298 }
299
300 private native boolean isLoaded0(long address, long length, int pageCount);
301 private native void load0(long address, long length);
302 private native void force0(FileDescriptor fd, long address, long length);
303
304 // -- Covariant return type overrides
305
306 /**
307 * {@inheritDoc}
308 */
309 @Override
310 public final MappedByteBuffer position(int newPosition) {
311 super.position(newPosition);
312 return this;
313 }
314
315 /**
316 * {@inheritDoc}
317 */
318 @Override
319 public final MappedByteBuffer limit(int newLimit) {
320 super.limit(newLimit);
321 return this;
322 }
323
324 /**
325 * {@inheritDoc}
326 */
327 @Override
328 public final MappedByteBuffer mark() {
329 super.mark();
330 return this;
331 }
332
333 /**
334 * {@inheritDoc}
335 */
336 @Override
337 public final MappedByteBuffer reset() {
338 super.reset();
339 return this;
340 }
341
342 /**
343 * {@inheritDoc}
344 */
345 @Override
346 public final MappedByteBuffer clear() {
347 super.clear();
348 return this;
349 }
350
351 /**
352 * {@inheritDoc}
353 */
354 @Override
355 public final MappedByteBuffer flip() {
356 super.flip();
357 return this;
358 }
359
360 /**
361 * {@inheritDoc}
362 */
363 @Override
364 public final MappedByteBuffer rewind() {
365 super.rewind();
366 return this;
367 }
368 }