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