Java多线程系列--“JUC锁”07之 LockSupport
概述
本章介绍JUC(java.util.concurrent)包中的LockSupport。内容包括：
LockSupport介绍
LockSupport函数列表
LockSupport参考代码(基于JDK1.7.0_40)
LockSupport示例
转载请注明出处：http://www.cnblogs.com/skywang12345/p/3505784.html
LockSupport介绍
LockSupport是用来创建锁和其他同步类的基本线程阻塞原语。
LockSupport中的park() 和 unpark() 的作用分别是阻塞线程和解除阻塞线程，而且park()和unpark()不会遇到“Thread.suspend 和 Thread.resume所可能引发的死锁”问题。
因为park() 和 unpark()有许可的存在；调用 park() 的线程和另一个试图将其 unpark() 的线程之间的竞争将保持活性。
LockSupport函数列表
// 返回提供给最近一次尚未解除阻塞的 park 方法调用的 blocker 对象，如果该调用不受阻塞，则返回 null。 static Object getBlocker(Thread t) // 为了线程调度，禁用当前线程，除非许可可用。 static void park() // 为了线程调度，在许可可用之前禁用当前线程。 static void park(Object blocker) // 为了线程调度禁用当前线程，最多等待指定的等待时间，除非许可可用。 static void parkNanos(long nanos) // 为了线程调度，在许可可用前禁用当前线程，并最多等待指定的等待时间。 static void parkNanos(Object blocker, long nanos) // 为了线程调度，在指定的时限前禁用当前线程，除非许可可用。 static void parkUntil(long deadline) // 为了线程调度，在指定的时限前禁用当前线程，除非许可可用。 static void parkUntil(Object blocker, long deadline) // 如果给定线程的许可尚不可用，则使其可用。 static void unpark(Thread thread)
LockSupport参考代码(基于JDK1.7.0_40)
LockSupport.java的源码如下：
1 /* 2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10 * 11 * 12 * 13 * 14 * 15 * 16 * 17 * 18 * 19 * 20 * 21 * 22 * 23 */ 24 25 /* 26 * 27 * 28 * 29 * 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36 package java.util.concurrent.locks; 37 import java.util.concurrent.*; 38 import sun.misc.Unsafe; 39 40 41 /** 42 * Basic thread blocking primitives for creating locks and other 43 * synchronization classes. 44 * 45 * <p>This class associates, with each thread that uses it, a permit 46 * (in the sense of the {@link java.util.concurrent.Semaphore 47 * Semaphore} class). A call to {@code park} will return immediately 48 * if the permit is available, consuming it in the process; otherwise 49 * it <em>may</em> block. A call to {@code unpark} makes the permit 50 * available, if it was not already available. (Unlike with Semaphores 51 * though, permits do not accumulate. There is at most one.) 52 * 53 * <p>Methods {@code park} and {@code unpark} provide efficient 54 * means of blocking and unblocking threads that do not encounter the 55 * problems that cause the deprecated methods {@code Thread.suspend} 56 * and {@code Thread.resume} to be unusable for such purposes: Races 57 * between one thread invoking {@code park} and another thread trying 58 * to {@code unpark} it will preserve liveness, due to the 59 * permit. Additionally, {@code park} will return if the caller's 60 * thread was interrupted, and timeout versions are supported. The 61 * {@code park} method may also return at any other time, for "no 62 * reason", so in general must be invoked within a loop that rechecks 63 * conditions upon return. In this sense {@code park} serves as an 64 * optimization of a "busy wait" that does not waste as much time 65 * spinning, but must be paired with an {@code unpark} to be 66 * effective. 67 * 68 * <p>The three forms of {@code park} each also support a 69 * {@code blocker} object parameter. This object is recorded while 70 * the thread is blocked to permit monitoring and diagnostic tools to 71 * identify the reasons that threads are blocked. (Such tools may 72 * access blockers using method {@link #getBlocker}.) The use of these 73 * forms rather than the original forms without this parameter is 74 * strongly encouraged. The normal argument to supply as a 75 * {@code blocker} within a lock implementation is {@code this}. 76 * 77 * <p>These methods are designed to be used as tools for creating 78 * higher-level synchronization utilities, and are not in themselves 79 * useful for most concurrency control applications. The {@code park} 80 * method is designed for use only in constructions of the form: 81 * <pre>while (!canProceed()) { ... LockSupport.park(this); }</pre> 82 * where neither {@code canProceed} nor any other actions prior to the 83 * call to {@code park} entail locking or blocking. Because only one 84 * permit is associated with each thread, any intermediary uses of 85 * {@code park} could interfere with its intended effects. 86 * 87 * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out 88 * non-reentrant lock class: 89 * <pre>{@code 90 * class FIFOMutex { 91 * private final AtomicBoolean locked = new AtomicBoolean(false); 92 * private final Queue<Thread> waiters 93 * = new ConcurrentLinkedQueue<Thread>(); 94 * 95 * public void lock() { 96 * boolean wasInterrupted = false; 97 * Thread current = Thread.currentThread(); 98 * waiters.add(current); 99 * 100 * // Block while not first in queue or cannot acquire lock 101 * while (waiters.peek() != current || 102 * !locked.compareAndSet(false, true)) { 103 * LockSupport.park(this); 104 * if (Thread.interrupted()) // ignore interrupts while waiting 105 * wasInterrupted = true; 106 * } 107 * 108 * waiters.remove(); 109 * if (wasInterrupted) // reassert interrupt status on exit 110 * current.interrupt(); 111 * } 112 * 113 * public void unlock() { 114 * locked.set(false); 115 * LockSupport.unpark(waiters.peek()); 116 * } 117 * }}</pre> 118 */ 119 120 public class LockSupport { 121 private LockSupport() {} // Cannot be instantiated. 122 123 // Hotspot implementation via intrinsics API 124 private static final Unsafe unsafe = Unsafe.getUnsafe(); 125 private static final long parkBlockerOffset; 126 127 static { 128 try { 129 parkBlockerOffset = unsafe.objectFieldOffset 130 (java.lang.Thread.class.getDeclaredField("parkBlocker")); 131 } catch (Exception ex) { throw new Error(ex); } 132 } 133 134 private static void setBlocker(Thread t, Object arg) { 135 // Even though volatile, hotspot doesn't need a write barrier here. 136 unsafe.putObject(t, parkBlockerOffset, arg); 137 } 138 139 /** 140 * Makes available the permit for the given thread, if it 141 * was not already available. If the thread was blocked on 142 * {@code park} then it will unblock. Otherwise, its next call 143 * to {@code park} is guaranteed not to block. This operation 144 * is not guaranteed to have any effect at all if the given 145 * thread has not been started. 146 * 147 * @param thread the thread to unpark, or {@code null}, in which case 148 * this operation has no effect 149 */ 150 public static void unpark(Thread thread) { 151 if (thread != null) 152 unsafe.unpark(thread); 153 } 154 155 /** 156 * Disables the current thread for thread scheduling purposes unless the 157 * permit is available. 158 * 159 * <p>If the permit is available then it is consumed and the call returns 160 * immediately; otherwise 161 * the current thread becomes disabled for thread scheduling 162 * purposes and lies dormant until one of three things happens: 163 * 164 * <ul> 165 * <li>Some other thread invokes {@link #unpark unpark} with the 166 * current thread as the target; or 167 * 168 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 169 * the current thread; or 170 * 171 * <li>The call spuriously (that is, for no reason) returns. 172 * </ul> 173 * 174 * <p>This method does <em>not</em> report which of these caused the 175 * method to return. Callers should re-check the conditions which caused 176 * the thread to park in the first place. Callers may also determine, 177 * for example, the interrupt status of the thread upon return. 178 * 179 * @param blocker the synchronization object responsible for this 180 * thread parking 181 * @since 1.6 182 */ 183 public static void park(Object blocker) { 184 Thread t = Thread.currentThread(); 185 setBlocker(t, blocker); 186 unsafe.park(false, 0L); 187 setBlocker(t, null); 188 } 189 190 /** 191 * Disables the current thread for thread scheduling purposes, for up to 192 * the specified waiting time, unless the permit is available. 193 * 194 * <p>If the permit is available then it is consumed and the call 195 * returns immediately; otherwise the current thread becomes disabled 196 * for thread scheduling purposes and lies dormant until one of four 197 * things happens: 198 * 199 * <ul> 200 * <li>Some other thread invokes {@link #unpark unpark} with the 201 * current thread as the target; or 202 * 203 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 204 * the current thread; or 205 * 206 * <li>The specified waiting time elapses; or 207 * 208 * <li>The call spuriously (that is, for no reason) returns. 209 * </ul> 210 * 211 * <p>This method does <em>not</em> report which of these caused the 212 * method to return. Callers should re-check the conditions which caused 213 * the thread to park in the first place. Callers may also determine, 214 * for example, the interrupt status of the thread, or the elapsed time 215 * upon return. 216 * 217 * @param blocker the synchronization object responsible for this 218 * thread parking 219 * @param nanos the maximum number of nanoseconds to wait 220 * @since 1.6 221 */ 222 public static void parkNanos(Object blocker, long nanos) { 223 if (nanos > 0) { 224 Thread t = Thread.currentThread(); 225 setBlocker(t, blocker); 226 unsafe.park(false, nanos); 227 setBlocker(t, null); 228 } 229 } 230 231 /** 232 * Disables the current thread for thread scheduling purposes, until 233 * the specified deadline, unless the permit is available. 234 * 235 * <p>If the permit is available then it is consumed and the call 236 * returns immediately; otherwise the current thread becomes disabled 237 * for thread scheduling purposes and lies dormant until one of four 238 * things happens: 239 * 240 * <ul> 241 * <li>Some other thread invokes {@link #unpark unpark} with the 242 * current thread as the target; or 243 * 244 * <li>Some other thread {@linkplain Thread#interrupt interrupts} the 245 * current thread; or 246 * 247 * <li>The specified deadline passes; or 248 * 249 * <li>The call spuriously (that is, for no reason) returns. 250 * </ul> 251 * 252 * <p>This method does <em>not</em> report which of these caused the 253 * method to return. Callers should re-check the conditions which caused 254 * the thread to park in the first place. Callers may also determine, 255 * for example, the interrupt status of the thread, or the current time 256 * upon return. 257 * 258 * @param blocker the synchronization object responsible for this 259 * thread parking 260 * @param deadline the absolute time, in milliseconds from the Epoch, 261 * to wait until 262 * @since 1.6 263 */ 264 public static void parkUntil(Object blocker, long deadline) { 265 Thread t = Thread.currentThread(); 266 setBlocker(t, blocker); 267 unsafe.park(true, deadline); 268 setBlocker(t, null); 269 } 270 271 /** 272 * Returns the blocker object supplied to the most recent 273 * invocation of a park method that has not yet unblocked, or null 274 * if not blocked. The value returned is just a momentary 275 * snapshot -- the thread may have since unblocked or blocked on a 276 * different blocker object. 277 * 278 * @param t the thread 279 * @return the blocker 280 * @throws NullPointerException if argument is null 281 * @since 1.6 282 */ 283 public static Object getBlocker(Thread t) { 284 if (t == null) 285 throw new NullPointerException(); 286 return unsafe.getObjectVolatile(t, parkBlockerOffset); 287 } 288 289 /** 290 * Disables the current thread for thread scheduling purposes unless the 291 * permit is available. 292 * 293 * <p>If the permit is available then it is consumed and the call 294 * returns immediately; otherwise the current thread becomes disabled 295 * for thread scheduling purposes and lies dormant until one of three 296 * things happens: 297 * 298 * <ul> 299 * 300 * <li>Some other thread invokes {@link #unpark unpark} with the 301 * current thread as the target; or 302 * 303 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 304 * the current thread; or 305 * 306 * <li>The call spuriously (that is, for no reason) returns. 307 * </ul> 308 * 309 * <p>This method does <em>not</em> report which of these caused the 310 * method to return. Callers should re-check the conditions which caused 311 * the thread to park in the first place. Callers may also determine, 312 * for example, the interrupt status of the thread upon return. 313 */ 314 public static void park() { 315 unsafe.park(false, 0L); 316 } 317 318 /** 319 * Disables the current thread for thread scheduling purposes, for up to 320 * the specified waiting time, unless the permit is available. 321 * 322 * <p>If the permit is available then it is consumed and the call 323 * returns immediately; otherwise the current thread becomes disabled 324 * for thread scheduling purposes and lies dormant until one of four 325 * things happens: 326 * 327 * <ul> 328 * <li>Some other thread invokes {@link #unpark unpark} with the 329 * current thread as the target; or 330 * 331 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 332 * the current thread; or 333 * 334 * <li>The specified waiting time elapses; or 335 * 336 * <li>The call spuriously (that is, for no reason) returns. 337 * </ul> 338 * 339 * <p>This method does <em>not</em> report which of these caused the 340 * method to return. Callers should re-check the conditions which caused 341 * the thread to park in the first place. Callers may also determine, 342 * for example, the interrupt status of the thread, or the elapsed time 343 * upon return. 344 * 345 * @param nanos the maximum number of nanoseconds to wait 346 */ 347 public static void parkNanos(long nanos) { 348 if (nanos > 0) 349 unsafe.park(false, nanos); 350 } 351 352 /** 353 * Disables the current thread for thread scheduling purposes, until 354 * the specified deadline, unless the permit is available. 355 * 356 * <p>If the permit is available then it is consumed and the call 357 * returns immediately; otherwise the current thread becomes disabled 358 * for thread scheduling purposes and lies dormant until one of four 359 * things happens: 360 * 361 * <ul> 362 * <li>Some other thread invokes {@link #unpark unpark} with the 363 * current thread as the target; or 364 * 365 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 366 * the current thread; or 367 * 368 * <li>The specified deadline passes; or 369 * 370 * <li>The call spuriously (that is, for no reason) returns. 371 * </ul> 372 * 373 * <p>This method does <em>not</em> report which of these caused the 374 * method to return. Callers should re-check the conditions which caused 375 * the thread to park in the first place. Callers may also determine, 376 * for example, the interrupt status of the thread, or the current time 377 * upon return. 378 * 379 * @param deadline the absolute time, in milliseconds from the Epoch, 380 * to wait until 381 */ 382 public static void parkUntil(long deadline) { 383 unsafe.park(true, deadline); 384 } 385 }
说明：LockSupport是通过调用Unsafe函数中的接口实现阻塞和解除阻塞的。
LockSupport示例
对比下面的“示例1”和“示例2”可以更清晰的了解LockSupport的用法。
示例1
1 public class WaitTest1 { 2 3 public static void main(String[] args) { 4 5 ThreadA ta = new ThreadA("ta"); 6 7 synchronized(ta) { // 通过synchronized(ta)获取“对象ta的同步锁” 8 try { 9 System.out.println(Thread.currentThread().getName()+" start ta"); 10 ta.start(); 11 12 System.out.println(Thread.currentThread().getName()+" block"); 13 // 主线程等待 14 ta.wait(); 15 16 System.out.println(Thread.currentThread().getName()+" continue"); 17 } catch (InterruptedException e) { 18 e.printStackTrace(); 19 } 20 } 21 } 22 23 static class ThreadA extends Thread{ 24 25 public ThreadA(String name) { 26 super(name); 27 } 28 29 public void run() { 30 synchronized (this) { // 通过synchronized(this)获取“当前对象的同步锁” 31 System.out.println(Thread.currentThread().getName()+" wakup others"); 32 notify(); // 唤醒“当前对象上的等待线程” 33 } 34 } 35 } 36 }
示例2
1 import java.util.concurrent.locks.LockSupport; 2 3 public class LockSupportTest1 { 4 5 private static Thread mainThread; 6 7 public static void main(String[] args) { 8 9 ThreadA ta = new ThreadA("ta"); 10 // 获取主线程 11 mainThread = Thread.currentThread(); 12 13 System.out.println(Thread.currentThread().getName()+" start ta"); 14 ta.start(); 15 16 System.out.println(Thread.currentThread().getName()+" block"); 17 // 主线程阻塞 18 LockSupport.park(mainThread); 19 20 System.out.println(Thread.currentThread().getName()+" continue"); 21 } 22 23 static class ThreadA extends Thread{ 24 25 public ThreadA(String name) { 26 super(name); 27 } 28 29 public void run() { 30 System.out.println(Thread.currentThread().getName()+" wakup others"); 31 // 唤醒“主线程” 32 LockSupport.unpark(mainThread); 33 } 34 } 35 }
运行结果：
main start ta main block ta wakup others main continue
说明：park和wait的区别。wait让线程阻塞前，必须通过synchronized获取同步锁。
更多内容
2. Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock
3. Java多线程系列--“JUC锁”03之 公平锁(一)
4. Java多线程系列--“JUC锁”04之 公平锁(二)
6. Java多线程系列--“JUC锁”06之 Condition条件