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 import java.util.Objects; 30 31 /** 32 * A token representing a lock on a region of a file. 33 * 34 * <p> A file-lock object is created each time a lock is acquired on a file via 35 * one of the {@link FileChannel#lock(long,long,boolean) lock} or {@link 36 * FileChannel#tryLock(long,long,boolean) tryLock} methods of the 37 * {@link FileChannel} class, or the {@link 38 * AsynchronousFileChannel#lock(long,long,boolean,Object,CompletionHandler) lock} 39 * or {@link AsynchronousFileChannel#tryLock(long,long,boolean) tryLock} 40 * methods of the {@link AsynchronousFileChannel} class. 41 * 42 * <p> A file-lock object is initially valid. It remains valid until the lock 43 * is released by invoking the {@link #release release} method, by closing the 44 * channel that was used to acquire it, or by the termination of the Java 45 * virtual machine, whichever comes first. The validity of a lock may be 46 * tested by invoking its {@link #isValid isValid} method. 47 * 48 * <p> A file lock is either <i>exclusive</i> or <i>shared</i>. A shared lock 49 * prevents other concurrently-running programs from acquiring an overlapping 50 * exclusive lock, but does allow them to acquire overlapping shared locks. An 51 * exclusive lock prevents other programs from acquiring an overlapping lock of 52 * either type. Once it is released, a lock has no further effect on the locks 53 * that may be acquired by other programs. 54 * 55 * <p> Whether a lock is exclusive or shared may be determined by invoking its 56 * {@link #isShared isShared} method. Some platforms do not support shared 57 * locks, in which case a request for a shared lock is automatically converted 58 * into a request for an exclusive lock. 59 * 60 * <p> The locks held on a particular file by a single Java virtual machine do 61 * not overlap. The {@link #overlaps overlaps} method may be used to test 62 * whether a candidate lock range overlaps an existing lock. 63 * 64 * <p> A file-lock object records the file channel upon whose file the lock is 65 * held, the type and validity of the lock, and the position and size of the 66 * locked region. Only the validity of a lock is subject to change over time; 67 * all other aspects of a lock's state are immutable. 68 * 69 * <p> File locks are held on behalf of the entire Java virtual machine. 70 * They are not suitable for controlling access to a file by multiple 71 * threads within the same virtual machine. 72 * 73 * <p> File-lock objects are safe for use by multiple concurrent threads. 74 * 75 * 76 * <a id="pdep"></a><h2> Platform dependencies </h2> 77 * 78 * <p> This file-locking API is intended to map directly to the native locking 79 * facility of the underlying operating system. Thus the locks held on a file 80 * should be visible to all programs that have access to the file, regardless 81 * of the language in which those programs are written. 82 * 83 * <p> Whether or not a lock actually prevents another program from accessing 84 * the content of the locked region is system-dependent and therefore 85 * unspecified. The native file-locking facilities of some systems are merely 86 * <i>advisory</i>, meaning that programs must cooperatively observe a known 87 * locking protocol in order to guarantee data integrity. On other systems 88 * native file locks are <i>mandatory</i>, meaning that if one program locks a 89 * region of a file then other programs are actually prevented from accessing 90 * that region in a way that would violate the lock. On yet other systems, 91 * whether native file locks are advisory or mandatory is configurable on a 92 * per-file basis. To ensure consistent and correct behavior across platforms, 93 * it is strongly recommended that the locks provided by this API be used as if 94 * they were advisory locks. 95 * 96 * <p> On some systems, acquiring a mandatory lock on a region of a file 97 * prevents that region from being {@link java.nio.channels.FileChannel#map 98 * <i>mapped into memory</i>}, and vice versa. Programs that combine 99 * locking and mapping should be prepared for this combination to fail. 100 * 101 * <p> On some systems, closing a channel releases all locks held by the Java 102 * virtual machine on the underlying file regardless of whether the locks were 103 * acquired via that channel or via another channel open on the same file. It 104 * is strongly recommended that, within a program, a unique channel be used to 105 * acquire all locks on any given file. 106 * 107 * <p> Some network filesystems permit file locking to be used with 108 * memory-mapped files only when the locked regions are page-aligned and a 109 * whole multiple of the underlying hardware's page size. Some network 110 * filesystems do not implement file locks on regions that extend past a 111 * certain position, often 2<sup>30</sup> or 2<sup>31</sup>. In general, great 112 * care should be taken when locking files that reside on network filesystems. 113 * 114 * 115 * @author Mark Reinhold 116 * @author JSR-51 Expert Group 117 * @since 1.4 118 */ 119 120 public abstract class FileLock implements AutoCloseable { 121 122 private final Channel channel; 123 private final long position; 124 private final long size; 125 private final boolean shared; 126 127 /** 128 * Initializes a new instance of this class. 129 * 130 * @param channel 131 * The file channel upon whose file this lock is held 132 * 133 * @param position 134 * The position within the file at which the locked region starts; 135 * must be non-negative 136 * 137 * @param size 138 * The size of the locked region; must be non-negative, and the sum 139 * {@code position} + {@code size} must be non-negative 140 * 141 * @param shared 142 * {@code true} if this lock is shared, 143 * {@code false} if it is exclusive 144 * 145 * @throws IllegalArgumentException 146 * If the preconditions on the parameters do not hold 147 */ FileLock(FileChannel channel, long position, long size, boolean shared)148 protected FileLock(FileChannel channel, 149 long position, long size, boolean shared) 150 { 151 Objects.requireNonNull(channel, "Null channel"); 152 if (position < 0) 153 throw new IllegalArgumentException("Negative position"); 154 if (size < 0) 155 throw new IllegalArgumentException("Negative size"); 156 if (position + size < 0) 157 throw new IllegalArgumentException("Negative position + size"); 158 this.channel = channel; 159 this.position = position; 160 this.size = size; 161 this.shared = shared; 162 } 163 164 /** 165 * Initializes a new instance of this class. 166 * 167 * @param channel 168 * The channel upon whose file this lock is held 169 * 170 * @param position 171 * The position within the file at which the locked region starts; 172 * must be non-negative 173 * 174 * @param size 175 * The size of the locked region; must be non-negative, and the sum 176 * {@code position} + {@code size} must be non-negative 177 * 178 * @param shared 179 * {@code true} if this lock is shared, 180 * {@code false} if it is exclusive 181 * 182 * @throws IllegalArgumentException 183 * If the preconditions on the parameters do not hold 184 * 185 * @since 1.7 186 */ FileLock(AsynchronousFileChannel channel, long position, long size, boolean shared)187 protected FileLock(AsynchronousFileChannel channel, 188 long position, long size, boolean shared) 189 { 190 Objects.requireNonNull(channel, "Null channel"); 191 if (position < 0) 192 throw new IllegalArgumentException("Negative position"); 193 if (size < 0) 194 throw new IllegalArgumentException("Negative size"); 195 if (position + size < 0) 196 throw new IllegalArgumentException("Negative position + size"); 197 this.channel = channel; 198 this.position = position; 199 this.size = size; 200 this.shared = shared; 201 } 202 203 /** 204 * Returns the file channel upon whose file this lock was acquired. 205 * 206 * <p> This method has been superseded by the {@link #acquiredBy acquiredBy} 207 * method. 208 * 209 * @return The file channel, or {@code null} if the file lock was not 210 * acquired by a file channel. 211 */ channel()212 public final FileChannel channel() { 213 return (channel instanceof FileChannel) ? (FileChannel)channel : null; 214 } 215 216 /** 217 * Returns the channel upon whose file this lock was acquired. 218 * 219 * @return The channel upon whose file this lock was acquired. 220 * 221 * @since 1.7 222 */ acquiredBy()223 public Channel acquiredBy() { 224 return channel; 225 } 226 227 /** 228 * Returns the position within the file of the first byte of the locked 229 * region. 230 * 231 * <p> A locked region need not be contained within, or even overlap, the 232 * actual underlying file, so the value returned by this method may exceed 233 * the file's current size. </p> 234 * 235 * @return The position 236 */ position()237 public final long position() { 238 return position; 239 } 240 241 /** 242 * Returns the size of the locked region in bytes. 243 * 244 * <p> A locked region need not be contained within, or even overlap, the 245 * actual underlying file, so the value returned by this method may exceed 246 * the file's current size. </p> 247 * 248 * @return The size of the locked region 249 */ size()250 public final long size() { 251 return size; 252 } 253 254 /** 255 * Tells whether this lock is shared. 256 * 257 * @return {@code true} if lock is shared, 258 * {@code false} if it is exclusive 259 */ isShared()260 public final boolean isShared() { 261 return shared; 262 } 263 264 /** 265 * Tells whether or not this lock overlaps the given lock range. 266 * 267 * @param position 268 * The starting position of the lock range 269 * @param size 270 * The size of the lock range 271 * 272 * @return {@code true} if, and only if, this lock and the given lock 273 * range overlap by at least one byte 274 */ overlaps(long position, long size)275 public final boolean overlaps(long position, long size) { 276 if (position + size <= this.position) 277 return false; // That is below this 278 if (this.position + this.size <= position) 279 return false; // This is below that 280 return true; 281 } 282 283 /** 284 * Tells whether or not this lock is valid. 285 * 286 * <p> A lock object remains valid until it is released or the associated 287 * file channel is closed, whichever comes first. </p> 288 * 289 * @return {@code true} if, and only if, this lock is valid 290 */ isValid()291 public abstract boolean isValid(); 292 293 /** 294 * Releases this lock. 295 * 296 * <p> If this lock object is valid then invoking this method releases the 297 * lock and renders the object invalid. If this lock object is invalid 298 * then invoking this method has no effect. </p> 299 * 300 * @throws ClosedChannelException 301 * If the channel that was used to acquire this lock 302 * is no longer open 303 * 304 * @throws IOException 305 * If an I/O error occurs 306 */ release()307 public abstract void release() throws IOException; 308 309 /** 310 * This method invokes the {@link #release} method. It was added 311 * to the class so that it could be used in conjunction with the 312 * automatic resource management block construct. 313 * 314 * @since 1.7 315 */ close()316 public final void close() throws IOException { 317 release(); 318 } 319 320 /** 321 * Returns a string describing the range, type, and validity of this lock. 322 * 323 * @return A descriptive string 324 */ toString()325 public final String toString() { 326 return (this.getClass().getName() 327 + "[" + position 328 + ":" + size 329 + " " + (shared ? "shared" : "exclusive") 330 + " " + (isValid() ? "valid" : "invalid") 331 + "]"); 332 } 333 334 } 335