java - 同步部分不阻塞!

Question

昨天我注意到一些非常奇怪的事情。看起来两个线程正在同时进入锁定在同一个对象上的两个同步块(synchronized block)。

包含相关代码的类 (MyClass) 看起来与此类似:

private static int[]    myLock  = new int[0];

protected static int methodA(final long handle, final byte[] sort) {
    synchronized (myLock) {
        return xsMethodA(handle, sort);
    }
}

protected static int methodB(final long handle) {
    synchronized (myLock) {
        return xsMethodB(handle);
    }
}

我为运行上述类的应用程序创建了一个线程转储，当我看到这个时非常惊讶:

"http-8080-136" daemon prio=10 tid=0x00000000447df000 nid=0x70ed waiting for monitor entry [0x00007fd862aea000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.MyClass.methodA(MyClass.java:750)
    - locked <0x00007fd8a6b8c790> (a [I)
    at com.SomeOtherClass.otherMethod(SomeOtherClass.java:226)
    ...

"http-8080-111" daemon prio=10 tid=0x00007fd87d1a0000 nid=0x70c8 waiting for monitor entry [0x00007fd86e15f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.MyClass.methodB(MyClass.java:991)
    - locked <0x00007fd8a6b8c790> (a [I)
    at com.SomeOtherClass.yetAnotherMethod(SomeOtherClass.java:3231)
    ...

(为了简单起见，我更改了类和方法的名称，所以不要被愚蠢的名称弄糊涂了。)

似乎线程 http-8080-136 和 http-8080-111 都获得了 myLock 上的锁。它是同一个对象，因为对象地址相同:0x00007fd8a6b8c790。 Java 运行时规范对 synchronized 关键字是这样说的:

A synchronized statement acquires a mutual-exclusion lock (§17.1) on behalf of the executing thread, executes a block, then releases the lock. While the executing thread owns the lock, no other thread may acquire the lock. [The Java Language Specification, 14.19]

这怎么可能呢？

线程转储中还有另外 44 个线程在“等待”锁。这是线程等待时的样子:

"http-8080-146" daemon prio=10 tid=0x00007fd786dab000 nid=0x184b waiting for monitor entry [0x00007fd8393b6000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.MyClass.methodC(MyClass.java:750)
    - waiting to lock <0x00007fd8a6b8c790> (a [I)
    at com.SomeOtherClass.yetAnoterMethod2(SomeOtherClass.java:226)

Answer 1

我在 hotspot-dev 邮件列表上问过同样的问题，并从 Christopher Phillips 那里得到了非常好的回答:

---

嗨爱德华

我认为是线程转储具有误导性。

如果您真的认为这 2 个同时处于锁定状态，您可能应该获得一个 gcore(外部一致)。

你看到的“waiting for monitor entry”的状态实际上是MONITOR_WAIT，在实际获取热锁之前可以表示如下代码: (另见 osThread.hpp 中的 OSThreadContendState)调用自: src/share/vm/runtime/synchronizer.cpp

3413      OSThreadContendState osts(Self->osthread());
3414      ThreadBlockInVM tbivm(jt);
3415
3416      Self->set_current_pending_monitor(this);
3417
3418      // TODO-FIXME: change the following for(;;) loop to straight-line code.
3419      for (;;) {
3420        jt->set_suspend_equivalent();
3421        // cleared by handle_special_suspend_equivalent_condition()
3422        // or java_suspend_self()
3423
3424        EnterI (THREAD) ;
3425
3426        if (!ExitSuspendEquivalent(jt)) break ;
3427
3428        //
3429        // We have acquired the contended monitor, but while we were
3430        // waiting another thread suspended us. We don't want to enter
3431        // the monitor while suspended because that would surprise the
3432        // thread that suspended us.

克里斯