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文章标签 ‘java’

细节背后：为什么线程协作之前必须先获得锁?

2009年10月3日 Killman 没有评论

为什么Object.wait()/notify()/notifyAll() 之前必须获得锁？ 这是JLS的规定。Wait-notify机制是围绕监控器锁进行的，获得锁是很自然的前提，自身没有拿到锁之前，怎么能够尝试去操作靠锁来调控的线程呢？不过今天偶尔有时间，就看下Sun Hotspot是怎么实现这一机制的。

当我们执行下面的代码时，线程会抛出异常java.lang.IllegalMonitorStateException: current thread not owner。

public class WaitNotifyCompilerCode {
	private String aString = "Hello World!";
 
	public static void main(String[] args) {
		System.out.println("Execute start ....");
		final WaitNotifyCompilerCode w = new WaitNotifyCompilerCode();
		w.wait1SecAndPrintString();
		System.out.println("Execute end ....");
	}
 
	public void wait1SecAndPrintString() {
		try {
			this.wait(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println(aString);
	}
 
}

异常栈的信息如下：

Exception in thread "main" java.lang.IllegalMonitorStateException: current thread not owner
	at java.lang.Object.wait(Native Method)
	at com.feihoo.test.waitnotify.WaitNotifyCompilerCode.wait1SecAndPrintString(WaitNotifyCompilerCode.java:35)
	at com.feihoo.test.waitnotify.WaitNotifyCompilerCode.main(WaitNotifyCompilerCode.java:27)

深入查看 OpenJDK的源码，找到 Object.wait() 函数本地代码：

JVM_ENTRY(void, JVM_MonitorWait(JNIEnv* env, jobject handle, jlong ms))
  JVMWrapper("JVM_MonitorWait");
  Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
  assert(obj->is_instance() || obj->is_array(), "JVM_MonitorWait must apply to an object");
  JavaThreadInObjectWaitState jtiows(thread, ms != 0);
  if (JvmtiExport::should_post_monitor_wait()) {
    JvmtiExport::post_monitor_wait((JavaThread *)THREAD, (oop)obj(), ms);
  }
  ObjectSynchronizer::wait(obj, ms, CHECK);
JVM_END

重点看倒数第二行，调用的函数如下面的代码：

// NOTE: must use heavy weight monitor to handle wait()
void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
  if (UseBiasedLocking) {
    BiasedLocking::revoke_and_rebias(obj, false, THREAD);
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
  }
  if (millis < 0) {
    TEVENT (wait - throw IAX) ;
    THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
  }
  ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
  DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
  monitor->wait(millis, true, THREAD);

而上面的代码中最后一行的函数里，在做实际操作之前调用了下面的宏：

// A macro is used below because there may already be a pending
// exception which should not abort the execution of the routines
// which use this (which is why we don't put this into check_slow and
// call it with a CHECK argument).
 
#define CHECK_OWNER()                                                             \
  do {                                                                            \
    if (THREAD != _owner) {                                                       \
      if (THREAD->is_lock_owned((address) _owner)) {                              \
        _owner = THREAD ;  /* Convert from basiclock addr to Thread addr */       \
        _recursions = 0;                                                          \
        OwnerIsThread = 1 ;                                                       \
      } else {                                                                    \
        TEVENT (Throw IMSX) ;                                                     \
        THROW(vmSymbols::java_lang_IllegalMonitorStateException());               \
      }                                                                           \
    }                                                                             \
  } while (false)
分类: $Performance, java 标签: , ,