1 /*
2 * Copyright (c) 2000, 2010, 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 sun.misc.Unsafe;
30
31
32 /**
33 * A direct byte buffer whose content is a memory-mapped region of a file.
34 *
35 * <p> Mapped byte buffers are created via the {@link
36 * java.nio.channels.FileChannel#map FileChannel.map} method. This class
37 * extends the {@link ByteBuffer} class with operations that are specific to
38 * memory-mapped file regions.
39 *
40 * <p> A mapped byte buffer and the file mapping that it represents remain
41 * valid until the buffer itself is garbage-collected.
42 *
43 * <p> The content of a mapped byte buffer can change at any time, for example
44 * if the content of the corresponding region of the mapped file is changed by
45 * this program or another. Whether or not such changes occur, and when they
46 * occur, is operating-system dependent and therefore unspecified.
47 *
48 * <a name="inaccess"><p> All or part of a mapped byte buffer may become
49 * inaccessible at any time, for example if the mapped file is truncated. An
50 * attempt to access an inaccessible region of a mapped byte buffer will not
51 * change the buffer's content and will cause an unspecified exception to be
52 * thrown either at the time of the access or at some later time. It is
53 * therefore strongly recommended that appropriate precautions be taken to
54 * avoid the manipulation of a mapped file by this program, or by a
55 * concurrently running program, except to read or write the file's content.
56 *
57 * <p> Mapped byte buffers otherwise behave no differently than ordinary direct
58 * byte buffers. </p>
59 *
60 *
61 * @author Mark Reinhold
62 * @author JSR-51 Expert Group
63 * @since 1.4
64 */
65
66 public abstract class MappedByteBuffer
67 extends ByteBuffer
68 {
69
70 // This is a little bit backwards: By rights MappedByteBuffer should be a
71 // subclass of DirectByteBuffer, but to keep the spec clear and simple, and
72 // for optimization purposes, it's easier to do it the other way around.
73 // This works because DirectByteBuffer is a package-private class.
74
75 // For mapped buffers, a FileDescriptor that may be used for mapping
76 // operations if valid; null if the buffer is not mapped.
77 private final FileDescriptor fd;
78
79 // This should only be invoked by the DirectByteBuffer constructors
80 //
81 MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
82 FileDescriptor fd)
83 {
84 super(mark, pos, lim, cap);
85 this.fd = fd;
86 }
87
88 MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private
89 super(mark, pos, lim, cap);
90 this.fd = null;
91 }
92
93 private void checkMapped() {
94 if (fd == null)
95 // Can only happen if a luser explicitly casts a direct byte buffer
96 throw new UnsupportedOperationException();
97 }
98
99 // Returns the distance (in bytes) of the buffer from the page aligned address
100 // of the mapping. Computed each time to avoid storing in every direct buffer.
101 private long mappingOffset() {
102 int ps = Bits.pageSize();
103 long offset = address % ps;
104 return (offset >= 0) ? offset : (ps + offset);
105 }
106
107 private long mappingAddress(long mappingOffset) {
108 return address - mappingOffset;
109 }
110
111 private long mappingLength(long mappingOffset) {
112 return (long)capacity() + mappingOffset;
113 }
114
115 /**
116 * Tells whether or not this buffer's content is resident in physical
117 * memory.
118 *
119 * <p> A return value of <tt>true</tt> implies that it is highly likely
120 * that all of the data in this buffer is resident in physical memory and
121 * may therefore be accessed without incurring any virtual-memory page
122 * faults or I/O operations. A return value of <tt>false</tt> does not
123 * necessarily imply that the buffer's content is not resident in physical
124 * memory.
125 *
126 * <p> The returned value is a hint, rather than a guarantee, because the
127 * underlying operating system may have paged out some of the buffer's data
128 * by the time that an invocation of this method returns. </p>
129 *
130 * @return <tt>true</tt> if it is likely that this buffer's content
131 * is resident in physical memory
132 */
133 public final boolean isLoaded() {
134 checkMapped();
135 if ((address == 0) || (capacity() == 0))
136 return true;
137 long offset = mappingOffset();
138 long length = mappingLength(offset);
139 return isLoaded0(mappingAddress(offset), length, Bits.pageCount(length));
140 }
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 checkMapped();
154 if ((address == 0) || (capacity() == 0))
155 return this;
156 long offset = mappingOffset();
157 long length = mappingLength(offset);
158 load0(mappingAddress(offset), length);
159
160 // touch each page
161 Unsafe unsafe = Unsafe.getUnsafe();
162 int ps = Bits.pageSize();
163 int count = Bits.pageCount(length);
164 long a = mappingAddress(offset);
165 for (int i=0; i<count; i++) {
166 unsafe.getByte(a);
167 a += ps;
168 }
169
170 return this;
171 }
172
173 /**
174 * Forces any changes made to this buffer's content to be written to the
175 * storage device containing the mapped file.
176 *
177 * <p> If the file mapped into this buffer resides on a local storage
178 * device then when this method returns it is guaranteed that all changes
179 * made to the buffer since it was created, or since this method was last
180 * invoked, will have been written to that device.
181 *
182 * <p> If the file does not reside on a local device then no such guarantee
183 * is made.
184 *
185 * <p> If this buffer was not mapped in read/write mode ({@link
186 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
187 * method has no effect. </p>
188 *
189 * @return This buffer
190 */
191 public final MappedByteBuffer force() {
192 checkMapped();
193 if ((address != 0) && (capacity() != 0)) {
194 long offset = mappingOffset();
195 force0(fd, mappingAddress(offset), mappingLength(offset));
196 }
197 return this;
198 }
199
200 private native boolean isLoaded0(long address, long length, int pageCount);
201 private native void load0(long address, long length);
202 private native void force0(FileDescriptor fd, long address, long length);
203 }
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