1 /* 2 * Copyright (c) 2000, 2013, 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"></a><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 {@code true} 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 {@code false} 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 {@code true} 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 // not used, but a potential target for a store, see load() for details. 143 private static byte unused; 144 145 /** 146 * Loads this buffer's content into physical memory. 147 * 148 * <p> This method makes a best effort to ensure that, when it returns, 149 * this buffer's content is resident in physical memory. Invoking this 150 * method may cause some number of page faults and I/O operations to 151 * occur. </p> 152 * 153 * @return This buffer 154 */ 155 public final MappedByteBuffer load() { 156 checkMapped(); 157 if ((address == 0) || (capacity() == 0)) 158 return this; 159 long offset = mappingOffset(); 160 long length = mappingLength(offset); 161 load0(mappingAddress(offset), length); 162 163 // Read a byte from each page to bring it into memory. A checksum 164 // is computed as we go along to prevent the compiler from otherwise 165 // considering the loop as dead code. 166 Unsafe unsafe = Unsafe.getUnsafe(); 167 int ps = Bits.pageSize(); 168 int count = Bits.pageCount(length); 169 long a = mappingAddress(offset); 170 byte x = 0; 171 for (int i=0; i<count; i++) { 172 x ^= unsafe.getByte(a); 173 a += ps; 174 } 175 if (unused != 0) 176 unused = x; 177 178 return this; 179 } 180 181 /** 182 * Forces any changes made to this buffer's content to be written to the 183 * storage device containing the mapped file. 184 * 185 * <p> If the file mapped into this buffer resides on a local storage 186 * device then when this method returns it is guaranteed that all changes 187 * made to the buffer since it was created, or since this method was last 188 * invoked, will have been written to that device. 189 * 190 * <p> If the file does not reside on a local device then no such guarantee 191 * is made. 192 * 193 * <p> If this buffer was not mapped in read/write mode ({@link 194 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this 195 * method has no effect. </p> 196 * 197 * @return This buffer 198 */ 199 public final MappedByteBuffer force() { 200 checkMapped(); 201 if ((address != 0) && (capacity() != 0)) { 202 long offset = mappingOffset(); 203 force0(fd, mappingAddress(offset), mappingLength(offset)); 204 } 205 return this; 206 } 207 208 private native boolean isLoaded0(long address, long length, int pageCount); 209 private native void load0(long address, long length); 210 private native void force0(FileDescriptor fd, long address, long length); 211 212 // -- Covariant return type overrides 213 214 /** 215 * {@inheritDoc} 216 * @since 1.9 217 */ 218 @Override 219 public final MappedByteBuffer position(int newPosition) { 220 super.position(newPosition); 221 return this; 222 } 223 224 /** 225 * {@inheritDoc} 226 * @since 1.9 227 */ 228 @Override 229 public final MappedByteBuffer limit(int newLimit) { 230 super.limit(newLimit); 231 return this; 232 } 233 234 /** 235 * {@inheritDoc} 236 * @since 1.9 237 */ 238 @Override 239 public final MappedByteBuffer mark() { 240 super.mark(); 241 return this; 242 } 243 244 /** 245 * {@inheritDoc} 246 * @since 1.9 247 */ 248 @Override 249 public final MappedByteBuffer reset() { 250 super.reset(); 251 return this; 252 } 253 254 /** 255 * {@inheritDoc} 256 * @since 1.9 257 */ 258 @Override 259 public final MappedByteBuffer clear() { 260 super.clear(); 261 return this; 262 } 263 264 /** 265 * {@inheritDoc} 266 * @since 1.9 267 */ 268 @Override 269 public final MappedByteBuffer flip() { 270 super.flip(); 271 return this; 272 } 273 274 /** 275 * {@inheritDoc} 276 * @since 1.9 277 */ 278 @Override 279 public final MappedByteBuffer rewind() { 280 super.rewind(); 281 return this; 282 } 283 }