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 // This should only be invoked by the DirectByteBuffer constructors
81 //
82 MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
83 FileDescriptor fd)
84 {
85 super(mark, pos, lim, cap);
86 this.fd = fd;
87 }
88
89 MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private
90 super(mark, pos, lim, cap);
91 this.fd = null;
92 }
93
94 // Returns the distance (in bytes) of the buffer from the page aligned address
95 // of the mapping. Computed each time to avoid storing in every direct buffer.
96 private long mappingOffset() {
97 int ps = Bits.pageSize();
98 long offset = address % ps;
99 return (offset >= 0) ? offset : (ps + offset);
100 }
101
102 private long mappingAddress(long mappingOffset) {
103 return address - mappingOffset;
104 }
105
106 private long mappingLength(long mappingOffset) {
107 return (long)capacity() + mappingOffset;
108 }
109
110 /**
111 * Tells whether or not this buffer's content is resident in physical
112 * memory.
113 *
114 * <p> A return value of {@code true} implies that it is highly likely
115 * that all of the data in this buffer is resident in physical memory and
116 * may therefore be accessed without incurring any virtual-memory page
117 * faults or I/O operations. A return value of {@code false} does not
118 * necessarily imply that the buffer's content is not resident in physical
119 * memory.
120 *
121 * <p> The returned value is a hint, rather than a guarantee, because the
122 * underlying operating system may have paged out some of the buffer's data
123 * by the time that an invocation of this method returns. </p>
124 *
125 * @return {@code true} if it is likely that this buffer's content
126 * is resident in physical memory
127 */
128 public final boolean isLoaded() {
129 if (fd == null) {
130 return true;
131 }
132 if ((address == 0) || (capacity() == 0))
133 return true;
134 long offset = mappingOffset();
135 long length = mappingLength(offset);
136 return isLoaded0(mappingAddress(offset), length, Bits.pageCount(length));
137 }
138
139 // not used, but a potential target for a store, see load() for details.
140 private static byte unused;
141
142 /**
143 * Loads this buffer's content into physical memory.
144 *
145 * <p> This method makes a best effort to ensure that, when it returns,
146 * this buffer's content is resident in physical memory. Invoking this
147 * method may cause some number of page faults and I/O operations to
148 * occur. </p>
149 *
150 * @return This buffer
151 */
152 public final MappedByteBuffer load() {
153 if (fd == null) {
154 return this;
155 }
156 if ((address == 0) || (capacity() == 0))
157 return this;
158 long offset = mappingOffset();
159 long length = mappingLength(offset);
160 load0(mappingAddress(offset), length);
161
162 // Read a byte from each page to bring it into memory. A checksum
163 // is computed as we go along to prevent the compiler from otherwise
164 // considering the loop as dead code.
165 Unsafe unsafe = Unsafe.getUnsafe();
166 int ps = Bits.pageSize();
167 int count = Bits.pageCount(length);
168 long a = mappingAddress(offset);
169 byte x = 0;
170 try {
171 for (int i=0; i<count; i++) {
172 // TODO consider changing to getByteOpaque thus avoiding
173 // dead code elimination and the need to calculate a checksum
174 x ^= unsafe.getByte(a);
175 a += ps;
176 }
177 } finally {
178 Reference.reachabilityFence(this);
179 }
180 if (unused != 0)
181 unused = x;
182
183 return this;
184 }
185
186 /**
187 * Forces any changes made to this buffer's content to be written to the
188 * storage device containing the mapped file.
189 *
190 * <p> If the file mapped into this buffer resides on a local storage
191 * device then when this method returns it is guaranteed that all changes
192 * made to the buffer since it was created, or since this method was last
193 * invoked, will have been written to that device.
194 *
195 * <p> If the file does not reside on a local device then no such guarantee
196 * is made.
197 *
198 * <p> If this buffer was not mapped in read/write mode ({@link
199 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
200 * method has no effect. </p>
201 *
202 * @return This buffer
203 */
204 public final MappedByteBuffer force() {
205 if (fd == null) {
206 return this;
207 }
208 if ((address != 0) && (capacity() != 0)) {
209 long offset = mappingOffset();
210 force0(fd, mappingAddress(offset), mappingLength(offset));
211 }
212 return this;
213 }
214
215 private native boolean isLoaded0(long address, long length, int pageCount);
216 private native void load0(long address, long length);
217 private native void force0(FileDescriptor fd, long address, long length);
218
219 // -- Covariant return type overrides
220
221 /**
222 * {@inheritDoc}
223 */
224 @Override
225 public final MappedByteBuffer position(int newPosition) {
226 super.position(newPosition);
227 return this;
228 }
229
230 /**
231 * {@inheritDoc}
232 */
233 @Override
234 public final MappedByteBuffer limit(int newLimit) {
235 super.limit(newLimit);
236 return this;
237 }
238
239 /**
240 * {@inheritDoc}
241 */
242 @Override
243 public final MappedByteBuffer mark() {
244 super.mark();
245 return this;
246 }
247
248 /**
249 * {@inheritDoc}
250 */
251 @Override
252 public final MappedByteBuffer reset() {
253 super.reset();
254 return this;
255 }
256
257 /**
258 * {@inheritDoc}
259 */
260 @Override
261 public final MappedByteBuffer clear() {
262 super.clear();
263 return this;
264 }
265
266 /**
267 * {@inheritDoc}
268 */
269 @Override
270 public final MappedByteBuffer flip() {
271 super.flip();
272 return this;
273 }
274
275 /**
276 * {@inheritDoc}
277 */
278 @Override
279 public final MappedByteBuffer rewind() {
280 super.rewind();
281 return this;
282 }
283 }