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：

 /*
  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  *
  */

 /*
  *
  *
  *
  *
  *
  * Written by Doug Lea with assistance from members of JCP JSR-166
  * Expert Group and released to the public domain, as explained at
  * http://creativecommons.org/publicdomain/zero/1.0/
  */

 package java.util.concurrent.locks;
 import java.util.concurrent.*;
 import sun.misc.Unsafe;

 /**
  * Basic thread blocking primitives for creating locks and other
  * synchronization classes.
  *
  * <p>This class associates, with each thread that uses it, a permit
  * (in the sense of the {@link java.util.concurrent.Semaphore
  * Semaphore} class). A call to {@code park} will return immediately
  * if the permit is available, consuming it in the process; otherwise
  * it <em>may</em> block.  A call to {@code unpark} makes the permit
  * available, if it was not already available. (Unlike with Semaphores
  * though, permits do not accumulate. There is at most one.)
  *
  * <p>Methods {@code park} and {@code unpark} provide efficient
  * means of blocking and unblocking threads that do not encounter the
  * problems that cause the deprecated methods {@code Thread.suspend}
  * and {@code Thread.resume} to be unusable for such purposes: Races
  * between one thread invoking {@code park} and another thread trying
  * to {@code unpark} it will preserve liveness, due to the
  * permit. Additionally, {@code park} will return if the caller's
  * thread was interrupted, and timeout versions are supported. The
  * {@code park} method may also return at any other time, for "no
  * reason", so in general must be invoked within a loop that rechecks
  * conditions upon return. In this sense {@code park} serves as an
  * optimization of a "busy wait" that does not waste as much time
  * spinning, but must be paired with an {@code unpark} to be
  * effective.
  *
  * <p>The three forms of {@code park} each also support a
  * {@code blocker} object parameter. This object is recorded while
  * the thread is blocked to permit monitoring and diagnostic tools to
  * identify the reasons that threads are blocked. (Such tools may
  * access blockers using method {@link #getBlocker}.) The use of these
  * forms rather than the original forms without this parameter is
  * strongly encouraged. The normal argument to supply as a
  * {@code blocker} within a lock implementation is {@code this}.
  *
  * <p>These methods are designed to be used as tools for creating
  * higher-level synchronization utilities, and are not in themselves
  * useful for most concurrency control applications.  The {@code park}
  * method is designed for use only in constructions of the form:
  * <pre>while (!canProceed()) { ... LockSupport.park(this); }</pre>
  * where neither {@code canProceed} nor any other actions prior to the
  * call to {@code park} entail locking or blocking.  Because only one
  * permit is associated with each thread, any intermediary uses of
  * {@code park} could interfere with its intended effects.
  *
  * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out
  * non-reentrant lock class:
  * <pre>{@code
  * class FIFOMutex {
  *   private final AtomicBoolean locked = new AtomicBoolean(false);
  *   private final Queue<Thread> waiters
  *     = new ConcurrentLinkedQueue<Thread>();
  *
  *   public void lock() {
  *     boolean wasInterrupted = false;
  *     Thread current = Thread.currentThread();
  *     waiters.add(current);
  *
  *     // Block while not first in queue or cannot acquire lock
  *     while (waiters.peek() != current ||
  *            !locked.compareAndSet(false, true)) {
  *        LockSupport.park(this);
  *        if (Thread.interrupted()) // ignore interrupts while waiting
  *          wasInterrupted = true;
  *     }
  *
  *     waiters.remove();
  *     if (wasInterrupted)          // reassert interrupt status on exit
  *        current.interrupt();
  *   }
  *
  *   public void unlock() {
  *     locked.set(false);
  *     LockSupport.unpark(waiters.peek());
  *   }
  * }}</pre>
  */

 public class LockSupport {
     private LockSupport() {} // Cannot be instantiated.

     // Hotspot implementation via intrinsics API
     private static final Unsafe unsafe = Unsafe.getUnsafe();
     private static final long parkBlockerOffset;

     static {
         try {
             parkBlockerOffset = unsafe.objectFieldOffset
                 (java.lang.Thread.class.getDeclaredField("parkBlocker"));
         } catch (Exception ex) { throw new Error(ex); }
     }

     private static void setBlocker(Thread t, Object arg) {
         // Even though volatile, hotspot doesn't need a write barrier here.
         unsafe.putObject(t, parkBlockerOffset, arg);
     }

     /**
      * Makes available the permit for the given thread, if it
      * was not already available.  If the thread was blocked on
      * {@code park} then it will unblock.  Otherwise, its next call
      * to {@code park} is guaranteed not to block. This operation
      * is not guaranteed to have any effect at all if the given
      * thread has not been started.
      *
      * @param thread the thread to unpark, or {@code null}, in which case
      *        this operation has no effect
      */
     public static void unpark(Thread thread) {
         if (thread != null)
             unsafe.unpark(thread);
     }

     /**
      * Disables the current thread for thread scheduling purposes unless the
      * permit is available.
      *
      * <p>If the permit is available then it is consumed and the call returns
      * immediately; otherwise
      * the current thread becomes disabled for thread scheduling
      * purposes and lies dormant until one of three things happens:
      *
      * <ul>
      * <li>Some other thread invokes {@link #unpark unpark} with the
      * current thread as the target; or
      *
      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
      * the current thread; or
      *
      * <li>The call spuriously (that is, for no reason) returns.
      * </ul>
      *
      * <p>This method does <em>not</em> report which of these caused the
      * method to return. Callers should re-check the conditions which caused
      * the thread to park in the first place. Callers may also determine,
      * for example, the interrupt status of the thread upon return.
      *
      * @param blocker the synchronization object responsible for this
      *        thread parking
      * @since 1.6
      */
     public static void park(Object blocker) {
         Thread t = Thread.currentThread();
         setBlocker(t, blocker);
         unsafe.park(false, 0L);
         setBlocker(t, null);
     }

     /**
      * Disables the current thread for thread scheduling purposes, for up to
      * the specified waiting time, unless the permit is available.
      *
      * <p>If the permit is available then it is consumed and the call
      * returns immediately; otherwise the current thread becomes disabled
      * for thread scheduling purposes and lies dormant until one of four
      * things happens:
      *
      * <ul>
      * <li>Some other thread invokes {@link #unpark unpark} with the
      * current thread as the target; or
      *
      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
      * the current thread; or
      *
      * <li>The specified waiting time elapses; or
      *
      * <li>The call spuriously (that is, for no reason) returns.
      * </ul>
      *
      * <p>This method does <em>not</em> report which of these caused the
      * method to return. Callers should re-check the conditions which caused
      * the thread to park in the first place. Callers may also determine,
      * for example, the interrupt status of the thread, or the elapsed time
      * upon return.
      *
      * @param blocker the synchronization object responsible for this
      *        thread parking
      * @param nanos the maximum number of nanoseconds to wait
      * @since 1.6
      */
     public static void parkNanos(Object blocker, long nanos) {
         if (nanos > 0) {
             Thread t = Thread.currentThread();
             setBlocker(t, blocker);
             unsafe.park(false, nanos);
             setBlocker(t, null);
         }
     }

     /**
      * Disables the current thread for thread scheduling purposes, until
      * the specified deadline, unless the permit is available.
      *
      * <p>If the permit is available then it is consumed and the call
      * returns immediately; otherwise the current thread becomes disabled
      * for thread scheduling purposes and lies dormant until one of four
      * things happens:
      *
      * <ul>
      * <li>Some other thread invokes {@link #unpark unpark} with the
      * current thread as the target; or
      *
      * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
      * current thread; or
      *
      * <li>The specified deadline passes; or
      *
      * <li>The call spuriously (that is, for no reason) returns.
      * </ul>
      *
      * <p>This method does <em>not</em> report which of these caused the
      * method to return. Callers should re-check the conditions which caused
      * the thread to park in the first place. Callers may also determine,
      * for example, the interrupt status of the thread, or the current time
      * upon return.
      *
      * @param blocker the synchronization object responsible for this
      *        thread parking
      * @param deadline the absolute time, in milliseconds from the Epoch,
      *        to wait until
      * @since 1.6
      */
     public static void parkUntil(Object blocker, long deadline) {
         Thread t = Thread.currentThread();
         setBlocker(t, blocker);
         unsafe.park(true, deadline);
         setBlocker(t, null);
     }

     /**
      * Returns the blocker object supplied to the most recent
      * invocation of a park method that has not yet unblocked, or null
      * if not blocked.  The value returned is just a momentary
      * snapshot -- the thread may have since unblocked or blocked on a
      * different blocker object.
      *
      * @param t the thread
      * @return the blocker
      * @throws NullPointerException if argument is null
      * @since 1.6
      */
     public static Object getBlocker(Thread t) {
         if (t == null)
             throw new NullPointerException();
         return unsafe.getObjectVolatile(t, parkBlockerOffset);
     }

     /**
      * Disables the current thread for thread scheduling purposes unless the
      * permit is available.
      *
      * <p>If the permit is available then it is consumed and the call
      * returns immediately; otherwise the current thread becomes disabled
      * for thread scheduling purposes and lies dormant until one of three
      * things happens:
      *
      * <ul>
      *
      * <li>Some other thread invokes {@link #unpark unpark} with the
      * current thread as the target; or
      *
      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
      * the current thread; or
      *
      * <li>The call spuriously (that is, for no reason) returns.
      * </ul>
      *
      * <p>This method does <em>not</em> report which of these caused the
      * method to return. Callers should re-check the conditions which caused
      * the thread to park in the first place. Callers may also determine,
      * for example, the interrupt status of the thread upon return.
      */
     public static void park() {
         unsafe.park(false, 0L);
     }

     /**
      * Disables the current thread for thread scheduling purposes, for up to
      * the specified waiting time, unless the permit is available.
      *
      * <p>If the permit is available then it is consumed and the call
      * returns immediately; otherwise the current thread becomes disabled
      * for thread scheduling purposes and lies dormant until one of four
      * things happens:
      *
      * <ul>
      * <li>Some other thread invokes {@link #unpark unpark} with the
      * current thread as the target; or
      *
      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
      * the current thread; or
      *
      * <li>The specified waiting time elapses; or
      *
      * <li>The call spuriously (that is, for no reason) returns.
      * </ul>
      *
      * <p>This method does <em>not</em> report which of these caused the
      * method to return. Callers should re-check the conditions which caused
      * the thread to park in the first place. Callers may also determine,
      * for example, the interrupt status of the thread, or the elapsed time
      * upon return.
      *
      * @param nanos the maximum number of nanoseconds to wait
      */
     public static void parkNanos(long nanos) {
         if (nanos > 0)
             unsafe.park(false, nanos);
     }

     /**
      * Disables the current thread for thread scheduling purposes, until
      * the specified deadline, unless the permit is available.
      *
      * <p>If the permit is available then it is consumed and the call
      * returns immediately; otherwise the current thread becomes disabled
      * for thread scheduling purposes and lies dormant until one of four
      * things happens:
      *
      * <ul>
      * <li>Some other thread invokes {@link #unpark unpark} with the
      * current thread as the target; or
      *
      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
      * the current thread; or
      *
      * <li>The specified deadline passes; or
      *
      * <li>The call spuriously (that is, for no reason) returns.
      * </ul>
      *
      * <p>This method does <em>not</em> report which of these caused the
      * method to return. Callers should re-check the conditions which caused
      * the thread to park in the first place. Callers may also determine,
      * for example, the interrupt status of the thread, or the current time
      * upon return.
      *
      * @param deadline the absolute time, in milliseconds from the Epoch,
      *        to wait until
      */
     public static void parkUntil(long deadline) {
         unsafe.park(true, deadline);
     }
 }

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

 public class WaitTest1 {

     public static void main(String[] args) {

         ThreadA ta = new ThreadA("ta");

         synchronized(ta) { // 通过synchronized(ta)获取“对象ta的同步锁”
             try {
                 System.out.println(Thread.currentThread().getName()+" start ta");
                 ta.start();

                 System.out.println(Thread.currentThread().getName()+" block");
                 // 主线程等待
                 ta.wait();

                 System.out.println(Thread.currentThread().getName()+" continue");
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
         }
     }

     static class ThreadA extends Thread{

         public ThreadA(String name) {
             super(name);
         }

         public void run() {
             synchronized (this) { // 通过synchronized(this)获取“当前对象的同步锁”
                 System.out.println(Thread.currentThread().getName()+" wakup others");
                 notify();    // 唤醒“当前对象上的等待线程”
             }
         }
     }
 }

示例2

 import java.util.concurrent.locks.LockSupport;

 public class LockSupportTest1 {

     private static Thread mainThread;

     public static void main(String[] args) {

         ThreadA ta = new ThreadA("ta");
         // 获取主线程
         mainThread = Thread.currentThread();

         System.out.println(Thread.currentThread().getName()+" start ta");
         ta.start();

         System.out.println(Thread.currentThread().getName()+" block");
         // 主线程阻塞
         LockSupport.park(mainThread);

         System.out.println(Thread.currentThread().getName()+" continue");
     }

     static class ThreadA extends Thread{

         public ThreadA(String name) {
             super(name);
         }

         public void run() {
             System.out.println(Thread.currentThread().getName()+" wakup others");
             // 唤醒“主线程”
             LockSupport.unpark(mainThread);
         }
     }
 }

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

1. Java多线程系列--“JUC锁”01之 框架

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)

5. Java多线程系列--“JUC锁”05之 非公平锁

6. Java多线程系列--“JUC锁”06之 Condition条件

7. Java多线程系列目录(共xx篇)