JDK8 LockSupport源码

  虚拟机使用的Unsafe为每个线程维护一个许可变量_counter。它有3个特点：

• _counter默认为0，unpark会将其变为1
• park会尝试将_counter从1变为0，如果不为1则阻塞。
• 可以先unpark再park，这时的park不会再阻塞

class UnsafeDemo {
    
    private int _counter;

    
    UnsafeDemo(){
        _counter= 0;
    }

    
    public void park() {
        while (_counter!= 1){
            if (Thread.currentThread().isInterrupted()){
                break;
            }
        }
        _counter= 0;
    }

    
    public void unpark() {
        _count = 1;
    }
}

都是基于Unsafe的park和unpark的操作。

其中，

• blocker的作用是记录阻塞时的信息，用于允许监视和诊断工具确定线程被阻塞的原因
• LockSupport不需要获取同步锁，直接就能阻塞某线程和释放某线程

package java.util.concurrent.locks;

public class LockSupport {
    
    private LockSupport() {}

    
    private static void setBlocker(Thread t, Object arg) {
        UNSAFE.putObject(t, parkBlockerOffset, arg);
    }

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

    
    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        // 设置blocker，用于记录阻塞情况
        setBlocker(t, blocker);
        // 阻塞线程，获取许可
        UNSAFE.park(false, 0L);
        setBlocker(t, null);
    }

    
    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);
        }
    }

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

    
    public static Object getBlocker(Thread t) {
        if (t == null)
            throw new NullPointerException();
        return UNSAFE.getObjectVolatile(t, parkBlockerOffset);
    }

    
    public static void park() {
        UNSAFE.park(false, 0L);
    }

    
    public static void parkNanos(long nanos) {
        if (nanos > 0)
            UNSAFE.park(false, nanos);
    }

    
    public static void parkUntil(long deadline) {
        UNSAFE.park(true, deadline);
    }

    
    static final int nextSecondarySeed() {
        int r;
        Thread t = Thread.currentThread();
        if ((r = UNSAFE.getInt(t, SECONDARY)) != 0) {
            r ^= r << 13;   // xorshift
            r ^= r >>> 17;
            r ^= r << 5;
        }
        else if ((r = java.util.concurrent.ThreadLocalRandom.current().nextInt()) == 0)
            r = 1; // avoid zero
        UNSAFE.putInt(t, SECONDARY, r);
        return r;
    }

    
    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); }
    }

}