Java LockSupport常用方法的源码分析

目录

• LockSupport类常用方法源码
• wait/notify方法和park/unpark方法区别

LockSupport类常用方法源码

LockSupport只是一个简单的基础类，位于java.util.concurrent.locks包下，多用于线程的阻塞和唤醒，因此LockSupport也被称为其他线程的工具类。

LockSupport类的源码有标注，LockSupport类无法实例化。LockSupport类的底层是有Unsafe类实现的，LockSupport加载时的初始化也用到了Unsafe获取成员的偏移量，其源码如下：

    // Hotspot implementation via intrinsics API
    private static final sun.misc.Unsafe UNSAFE;
    private static final long parkBlockerOffset;
    private static final long SEED;
    private static final long PROBE;
    private static final long SECONDARY;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> tk = Thread.class;
            parkBlockerOffset = UNSAFE.objectFieldOffset
                (tk.getDeclaredField(\"parkBlocker\"));
            SEED = UNSAFE.objectFieldOffset
                (tk.getDeclaredField(\"threadLocalRandomSeed\"));
            PROBE = UNSAFE.objectFieldOffset
                (tk.getDeclaredField(\"threadLocalRandomProbe\"));
            SECONDARY = UNSAFE.objectFieldOffset
                (tk.getDeclaredField(\"threadLocalRandomSecondarySeed\"));
        } catch (Exception ex) { throw new Error(ex); }
    }

LockSupport类中有一些核心的线程操作方法，多用于线程的阻塞与唤醒。

调用park()方法使线程阻塞：

    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        UNSAFE.park(false, 0L);
        setBlocker(t, null);
    }
    private static void setBlocker(Thread t, Object arg) {
        // Even though volatile, hotspot doesn\'t need a write barrier here.
        UNSAFE.putObject(t, parkBlockerOffset, arg);
    }

调用park(Object blocker)对传入的线程进行阻塞

    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        UNSAFE.park(false, 0L);
        setBlocker(t, null);
    }

在截止时间之前阻塞传入的某个线程：

    public static void parkUntil(Object blocker, long deadline) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        UNSAFE.park(true, deadline);
        setBlocker(t, null);
    }

在nanos的时间范围内阻塞传入的线程：

    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);
        }
    }

唤醒传入的线程：

    public static void unpark(Thread thread) {
        if (thread != null)
            UNSAFE.unpark(thread);
    }

wait/notify方法和park/unpark方法区别

LockSupport类中的方法还有很多，在此先列举到这里。当我们看到阻塞和唤醒方法时，我们会联想到另一组唤醒方法wait()和notify()，这两组方法还是有所区别的。

这里直接记录下结论：wait和notify方法只能在同步代码块中使用（即必须与synchronized连用）；必须先执行wait方法，然后再执行notify方法唤醒线程，调换顺序的话线程仍处于阻塞状态。

而park()和unpark()方法与之不同，这里可以通过代码运行结果来看：

package XIAOWEI;
import java.util.concurrent.locks.LockSupport;

​​​​​​​public class Xiaowei{
    public static void main(String[] args) {
        Thread A = new Thread(()-> {
            System.out.println(\"线程A已经被阻塞QWQ\");
            LockSupport.park();
            System.out.println(\"线程A被线程B唤醒啦~~~\");
        });
        A.start();
        new Thread(()->{
            System.out.println(\"线程B在唤醒线程A ing~~~\");
            LockSupport.unpark(A);
        },\"B\").start();
    }
}

那如果我们先通过线程B唤醒线程A，然后再让线程A阻塞呢（让线程A的阻塞休眠两秒）？

package XIAOWEI;
import java.util.concurrent.locks.LockSupport;
public class Xiaowei {
    public static void main(String[] args) {
        Thread A = new Thread(()-> {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(\"线程A已经被阻塞QWQ(第二版)\");
            LockSupport.park();
            System.out.println(\"线程A被线程B唤醒啦~~~（第二版）\");
        });
        A.start();
        new Thread(()->{
            System.out.println(\"线程B在唤醒线程A ing~~~驾驾驾\");
            LockSupport.unpark(A);
        },\"B\").start();
    }
}

由上面输出结果来看，虽然线程B先唤醒了线程A，然后线程A再开始阻塞，但是线程A还是处于唤醒状态，这是为什么呢？

接下来我找了段LockSupport类中的注释，其实有时看看注释也挺有意思的哈哈：

 * <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.)

这段话大意是说，LockSupport类使用permits这个东西来实现线程的阻塞和唤醒。每一个线程都会使用到（拥有）permit，且permit的值默认为0。接着它又说，这个概念和Semaphore信号量差不多，但是permit的值只有0和1两个值。哦~原来是这样。

对于上面例子，线程B调用unpark方法唤醒A后，会使得线程A的permit值为1，当线程调用park方法使自己阻塞时，发现自己已经有许可（permit）了，就会继续向下执行业务，而不会阻塞不动。