Java多线程系列--“JUC锁”07之 LockSupport

Java多线程系列-目录
[笔记][Java7并发编程实战手册]系列目录

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概述

本章介绍JUC(java.util.concurrent)包中的LockSupport。

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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。自己的理解：简单来说：从表面看到的现象就是：调用object的wait需要有一个监视器对象，否则抛出异常，而LockSupport不需要，可以直接针对某个线程使用该效果

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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)

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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。

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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”            }        }    }}

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示例2

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 tamain blockta wakup othersmain 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。

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本文From：http://www.cnblogs.com/skywang12345/p/3505784.html