1 /* 2 * Copyright (c) 2001, 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.channels; 27 28 import java.io.IOException; 29 30 /** 31 * A token representing a lock on a region of a file. 32 * 33 * <p> A file-lock object is created each time a lock is acquired on a file via 34 * one of the {@link FileChannel#lock(long,long,boolean) lock} or {@link 35 * FileChannel#tryLock(long,long,boolean) tryLock} methods of the 36 * {@link FileChannel} class, or the {@link 37 * AsynchronousFileChannel#lock(long,long,boolean,Object,CompletionHandler) lock} 38 * or {@link AsynchronousFileChannel#tryLock(long,long,boolean) tryLock} 39 * methods of the {@link AsynchronousFileChannel} class. 40 * 41 * <p> A file-lock object is initially valid. It remains valid until the lock 42 * is released by invoking the {@link #release release} method, by closing the 43 * channel that was used to acquire it, or by the termination of the Java 44 * virtual machine, whichever comes first. The validity of a lock may be 45 * tested by invoking its {@link #isValid isValid} method. 46 * 47 * <p> A file lock is either <i>exclusive</i> or <i>shared</i>. A shared lock 48 * prevents other concurrently-running programs from acquiring an overlapping 49 * exclusive lock, but does allow them to acquire overlapping shared locks. An 50 * exclusive lock prevents other programs from acquiring an overlapping lock of 51 * either type. Once it is released, a lock has no further effect on the locks 52 * that may be acquired by other programs. 53 * 54 * <p> Whether a lock is exclusive or shared may be determined by invoking its 55 * {@link #isShared isShared} method. Some platforms do not support shared 56 * locks, in which case a request for a shared lock is automatically converted 57 * into a request for an exclusive lock. 58 * 59 * <p> The locks held on a particular file by a single Java virtual machine do 60 * not overlap. The {@link #overlaps overlaps} method may be used to test 61 * whether a candidate lock range overlaps an existing lock. 62 * 63 * <p> A file-lock object records the file channel upon whose file the lock is 64 * held, the type and validity of the lock, and the position and size of the 65 * locked region. Only the validity of a lock is subject to change over time; 66 * all other aspects of a lock's state are immutable. 67 * 68 * <p> File locks are held on behalf of the entire Java virtual machine. 69 * They are not suitable for controlling access to a file by multiple 70 * threads within the same virtual machine. 71 * 72 * <p> File-lock objects are safe for use by multiple concurrent threads. 73 * 74 * 75 * <a name="pdep"></a><h2> Platform dependencies </h2> 76 * 77 * <p> This file-locking API is intended to map directly to the native locking 78 * facility of the underlying operating system. Thus the locks held on a file 79 * should be visible to all programs that have access to the file, regardless 80 * of the language in which those programs are written. 81 * 82 * <p> Whether or not a lock actually prevents another program from accessing 83 * the content of the locked region is system-dependent and therefore 84 * unspecified. The native file-locking facilities of some systems are merely 85 * <i>advisory</i>, meaning that programs must cooperatively observe a known 86 * locking protocol in order to guarantee data integrity. On other systems 87 * native file locks are <i>mandatory</i>, meaning that if one program locks a 88 * region of a file then other programs are actually prevented from accessing 89 * that region in a way that would violate the lock. On yet other systems, 90 * whether native file locks are advisory or mandatory is configurable on a 91 * per-file basis. To ensure consistent and correct behavior across platforms, 92 * it is strongly recommended that the locks provided by this API be used as if 93 * they were advisory locks. 94 * 95 * <p> On some systems, acquiring a mandatory lock on a region of a file 96 * prevents that region from being {@link java.nio.channels.FileChannel#map 97 * <i>mapped into memory</i>}, and vice versa. Programs that combine 98 * locking and mapping should be prepared for this combination to fail. 99 * 100 * <p> On some systems, closing a channel releases all locks held by the Java 101 * virtual machine on the underlying file regardless of whether the locks were 102 * acquired via that channel or via another channel open on the same file. It 103 * is strongly recommended that, within a program, a unique channel be used to 104 * acquire all locks on any given file. 105 * 106 * <p> Some network filesystems permit file locking to be used with 107 * memory-mapped files only when the locked regions are page-aligned and a 108 * whole multiple of the underlying hardware's page size. Some network 109 * filesystems do not implement file locks on regions that extend past a 110 * certain position, often 2<sup>30</sup> or 2<sup>31</sup>. In general, great 111 * care should be taken when locking files that reside on network filesystems. 112 * 113 * 114 * @author Mark Reinhold 115 * @author JSR-51 Expert Group 116 * @since 1.4 117 */ 118 119 public abstract class FileLock implements AutoCloseable { 120 121 private final Channel channel; 122 private final long position; 123 private final long size; 124 private final boolean shared; 125 126 /** 127 * Initializes a new instance of this class. 128 * 129 * @param channel 130 * The file channel upon whose file this lock is held 131 * 132 * @param position 133 * The position within the file at which the locked region starts; 134 * must be non-negative 135 * 136 * @param size 137 * The size of the locked region; must be non-negative, and the sum 138 * <tt>position</tt> + <tt>size</tt> must be non-negative 139 * 140 * @param shared 141 * <tt>true</tt> if this lock is shared, 142 * <tt>false</tt> if it is exclusive 143 * 144 * @throws IllegalArgumentException 145 * If the preconditions on the parameters do not hold 146 */ FileLock(FileChannel channel, long position, long size, boolean shared)147 protected FileLock(FileChannel channel, 148 long position, long size, boolean shared) 149 { 150 if (position < 0) 151 throw new IllegalArgumentException("Negative position"); 152 if (size < 0) 153 throw new IllegalArgumentException("Negative size"); 154 if (position + size < 0) 155 throw new IllegalArgumentException("Negative position + size"); 156 this.channel = channel; 157 this.position = position; 158 this.size = size; 159 this.shared = shared; 160 } 161 162 /** 163 * Initializes a new instance of this class. 164 * 165 * @param channel 166 * The channel upon whose file this lock is held 167 * 168 * @param position 169 * The position within the file at which the locked region starts; 170 * must be non-negative 171 * 172 * @param size 173 * The size of the locked region; must be non-negative, and the sum 174 * <tt>position</tt> + <tt>size</tt> must be non-negative 175 * 176 * @param shared 177 * <tt>true</tt> if this lock is shared, 178 * <tt>false</tt> if it is exclusive 179 * 180 * @throws IllegalArgumentException 181 * If the preconditions on the parameters do not hold 182 * 183 * @since 1.7 184 */ FileLock(AsynchronousFileChannel channel, long position, long size, boolean shared)185 protected FileLock(AsynchronousFileChannel channel, 186 long position, long size, boolean shared) 187 { 188 if (position < 0) 189 throw new IllegalArgumentException("Negative position"); 190 if (size < 0) 191 throw new IllegalArgumentException("Negative size"); 192 if (position + size < 0) 193 throw new IllegalArgumentException("Negative position + size"); 194 this.channel = channel; 195 this.position = position; 196 this.size = size; 197 this.shared = shared; 198 } 199 200 /** 201 * Returns the file channel upon whose file this lock was acquired. 202 * 203 * <p> This method has been superseded by the {@link #acquiredBy acquiredBy} 204 * method. 205 * 206 * @return The file channel, or {@code null} if the file lock was not 207 * acquired by a file channel. 208 */ channel()209 public final FileChannel channel() { 210 return (channel instanceof FileChannel) ? (FileChannel)channel : null; 211 } 212 213 /** 214 * Returns the channel upon whose file this lock was acquired. 215 * 216 * @return The channel upon whose file this lock was acquired. 217 * 218 * @since 1.7 219 */ acquiredBy()220 public Channel acquiredBy() { 221 return channel; 222 } 223 224 /** 225 * Returns the position within the file of the first byte of the locked 226 * region. 227 * 228 * <p> A locked region need not be contained within, or even overlap, the 229 * actual underlying file, so the value returned by this method may exceed 230 * the file's current size. </p> 231 * 232 * @return The position 233 */ position()234 public final long position() { 235 return position; 236 } 237 238 /** 239 * Returns the size of the locked region in bytes. 240 * 241 * <p> A locked region need not be contained within, or even overlap, the 242 * actual underlying file, so the value returned by this method may exceed 243 * the file's current size. </p> 244 * 245 * @return The size of the locked region 246 */ size()247 public final long size() { 248 return size; 249 } 250 251 /** 252 * Tells whether this lock is shared. 253 * 254 * @return <tt>true</tt> if lock is shared, 255 * <tt>false</tt> if it is exclusive 256 */ isShared()257 public final boolean isShared() { 258 return shared; 259 } 260 261 /** 262 * Tells whether or not this lock overlaps the given lock range. 263 * 264 * @param position 265 * The starting position of the lock range 266 * @param size 267 * The size of the lock range 268 * 269 * @return <tt>true</tt> if, and only if, this lock and the given lock 270 * range overlap by at least one byte 271 */ overlaps(long position, long size)272 public final boolean overlaps(long position, long size) { 273 if (position + size <= this.position) 274 return false; // That is below this 275 if (this.position + this.size <= position) 276 return false; // This is below that 277 return true; 278 } 279 280 /** 281 * Tells whether or not this lock is valid. 282 * 283 * <p> A lock object remains valid until it is released or the associated 284 * file channel is closed, whichever comes first. </p> 285 * 286 * @return <tt>true</tt> if, and only if, this lock is valid 287 */ isValid()288 public abstract boolean isValid(); 289 290 /** 291 * Releases this lock. 292 * 293 * <p> If this lock object is valid then invoking this method releases the 294 * lock and renders the object invalid. If this lock object is invalid 295 * then invoking this method has no effect. </p> 296 * 297 * @throws ClosedChannelException 298 * If the channel that was used to acquire this lock 299 * is no longer open 300 * 301 * @throws IOException 302 * If an I/O error occurs 303 */ release()304 public abstract void release() throws IOException; 305 306 /** 307 * This method invokes the {@link #release} method. It was added 308 * to the class so that it could be used in conjunction with the 309 * automatic resource management block construct. 310 * 311 * @since 1.7 312 */ close()313 public final void close() throws IOException { 314 release(); 315 } 316 317 /** 318 * Returns a string describing the range, type, and validity of this lock. 319 * 320 * @return A descriptive string 321 */ toString()322 public final String toString() { 323 return (this.getClass().getName() 324 + "[" + position 325 + ":" + size 326 + " " + (shared ? "shared" : "exclusive") 327 + " " + (isValid() ? "valid" : "invalid") 328 + "]"); 329 } 330 331 } 332