wait()，notify()和notifyAll()介绍

1.wait()
使当前线程等待，直到另一个线程调用notify(),notifyAll()或者中断，当前线程调用wait()之前必须持有锁，调用wait()之后会释放锁。等到当前线程重新获得锁，恢复执行。
只能在同步方法调用wait()。
2.notify()
唤醒一个正在等待锁的线程，如果有多个线程在等待锁，那么任意唤醒其中一个wait的线程。被唤醒的线程(wait的线程)只有在当前线程(notify的线程)释放锁后才可以执行。
只能在同步方法调用notify()。
3.notifyAll()
唤醒所有正在等待锁的线程。
只能在同步方法调用notifyAll()。

native wait()

openjdk找到src/share/native/java/lang/Object.c

static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode},
    {"wait",        "(J)V",                   (void *)&JVM_MonitorWait},
    {"notify",      "()V",                    (void *)&JVM_MonitorNotify},
    {"notifyAll",   "()V",                    (void *)&JVM_MonitorNotifyAll},
    {"clone",       "()Ljava/lang/Object;",   (void *)&JVM_Clone},
}

我们先看下wait方法
src/share/vm/prims/jvm.cpp
搜索JVM_MonitorWait

JVM_ENTRY(void, JVM_MonitorWait(JNIEnv* env, jobject handle, jlong ms))
  JVMWrapper("JVM_MonitorWait");
  Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
  JavaThreadInObjectWaitState jtiows(thread, ms != 0);
  if (JvmtiExport::should_post_monitor_wait()) {
    //当前线程拥有锁，尚且没有加入到等待队列，所以要推迟wait()
    JvmtiExport::post_monitor_wait((JavaThread *)THREAD, (oop)obj(), ms);
  }
    //进入等待状态
  ObjectSynchronizer::wait(obj, ms, CHECK);
JVM_END

我们看下ObjectSynchronizer::wait方法
src/share/vm/runtime/synchronizer.cpp

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

  dtrace_waited_probe(monitor, obj, THREAD);
}

我们看下monitor->wait方法
src/share/vm/runtime/objectMonitor.cpp

void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
   Thread * const Self = THREAD ;
   assert(Self->is_Java_thread(), "Must be Java thread!");
   JavaThread *jt = (JavaThread *)THREAD;

   DeferredInitialize () ;

   // Throw IMSX or IEX.
   CHECK_OWNER();

   // check for a pending interrupt 是否有中断信号
   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
     // post monitor waited event.  Note that this is past-tense, we are done waiting.
     if (JvmtiExport::should_post_monitor_waited()) {
        // Note: 'false' parameter is passed here because the
        // wait was not timed out due to thread interrupt.
        JvmtiExport::post_monitor_waited(jt, this, false);
     }
     TEVENT (Wait - Throw IEX) ;
     THROW(vmSymbols::java_lang_InterruptedException());
     return ;
   }
   TEVENT (Wait) ;

   assert (Self->_Stalled == 0, "invariant") ;
   Self->_Stalled = intptr_t(this) ;
   //设置线程的监视锁
   jt->set_current_waiting_monitor(this);

   // create a node to be put into the queue
   // Critically, after we reset() the event but prior to park(), we must check
   // for a pending interrupt.
   //添加一个节点放入到等待队列中
   ObjectWaiter node(Self);
   node.TState = ObjectWaiter::TS_WAIT ;
   Self->_ParkEvent->reset() ;
   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
   //获取锁
   Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
   //添加节点
   AddWaiter (&node) ;
   //释放锁
   Thread::SpinRelease (&_WaitSetLock) ;

   if ((SyncFlags & 4) == 0) {
      _Responsible = NULL ;
   }
   intptr_t save = _recursions; // record the old recursion count
   //增加等待线程数
   _waiters++;                  // increment the number of waiters
   _recursions = 0;             // set the recursion level to be 1
   exit (Self) ;                    // exit the monitor
   guarantee (_owner != Self, "invariant") ;

   ......
}

native notify()

我们看下notify()
src/share/vm/prims/jvm.cpp

JVM_ENTRY(void, JVM_MonitorNotify(JNIEnv* env, jobject handle))
  JVMWrapper("JVM_MonitorNotify");
  Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
  ObjectSynchronizer::notify(obj, CHECK);
JVM_END

src/share/vm/runtime/synchronizer.cpp

void ObjectSynchronizer::notify(Handle obj, TRAPS) {
    //偏向锁校验
 if (UseBiasedLocking) {
    BiasedLocking::revoke_and_rebias(obj, false, THREAD);
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
  }

  markOop mark = obj->mark();
    //检验线程是否有锁
  if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
    return;
  }
  ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
}

src/share/vm/runtime/objectMonitor.cpp

void ObjectMonitor::notify(TRAPS) {
  CHECK_OWNER();
  if (_WaitSet == NULL) {
     TEVENT (Empty-Notify) ;
     return ;
  }
  DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);

  int Policy = Knob_MoveNotifyee ;

  Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
  ObjectWaiter * iterator = DequeueWaiter() ;
  if (iterator != NULL) {
     TEVENT (Notify1 - Transfer) ;
     guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
     guarantee (iterator->_notified == 0, "invariant") ;
     if (Policy != 4) {
        iterator->TState = ObjectWaiter::TS_ENTER ;
     }
     iterator->_notified = 1 ;
     Thread * Self = THREAD;
     iterator->_notifier_tid = Self->osthread()->thread_id();

     ObjectWaiter * List = _EntryList ;
     if (List != NULL) {
        assert (List->_prev == NULL, "invariant") ;
        assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
        assert (List != iterator, "invariant") ;
     }
......


  }
    //释放锁
  Thread::SpinRelease (&_WaitSetLock) ;

  if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
     ObjectMonitor::_sync_Notifications->inc() ;
  }
}

native notifyAll()

我们看下notifyAll
和notify()相似
src/share/vm/prims/jvm.cpp

JVM_ENTRY(void, JVM_MonitorNotifyAll(JNIEnv* env, jobject handle))
  JVMWrapper("JVM_MonitorNotifyAll");
  Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
  ObjectSynchronizer::notifyall(obj, CHECK);
JVM_END

我们看下ObjectSynchronizer::notifyall方法
和notify()相似

void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
    //偏向锁校验
  if (UseBiasedLocking) {
    BiasedLocking::revoke_and_rebias(obj, false, THREAD);
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
  }

  markOop mark = obj->mark();
    //检测线程是否有锁
  if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
    return;
  }
  ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
}

src/share/vm/runtime/objectMonitor.cpp
比notify()多了等待队列

void ObjectMonitor::notifyAll(TRAPS) {
  CHECK_OWNER();
  ObjectWaiter* iterator;
  if (_WaitSet == NULL) {
      TEVENT (Empty-NotifyAll) ;
      return ;
  }
  DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);

  int Policy = Knob_MoveNotifyee ;
  int Tally = 0 ;
  Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;

  for (;;) {
     iterator = DequeueWaiter () ;
     if (iterator == NULL) break ;
     TEVENT (NotifyAll - Transfer1) ;
     ++Tally ;

     // Disposition - what might we do with iterator ?
     // a.  add it directly to the EntryList - either tail or head.
     // b.  push it onto the front of the _cxq.
     // For now we use (a).

     guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
     guarantee (iterator->_notified == 0, "invariant") ;
     iterator->_notified = 1 ;
     Thread * Self = THREAD;
     iterator->_notifier_tid = Self->osthread()->thread_id();
     if (Policy != 4) {
        iterator->TState = ObjectWaiter::TS_ENTER ;
     }

     ObjectWaiter * List = _EntryList ;
     if (List != NULL) {
        assert (List->_prev == NULL, "invariant") ;
        assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
        assert (List != iterator, "invariant") ;
     }

   ......

     if (Policy < 4) {
       iterator->wait_reenter_begin(this);
     }

  }
   //释放锁
  Thread::SpinRelease (&_WaitSetLock) ;

  if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
     ObjectMonitor::_sync_Notifications->inc(Tally) ;
  }
}

wait()和notfiy()实例

MyThread30_0类，使用wait()

public class MyThread30_0 extends Thread {
    private Object lock;

    public MyThread30_0(Object lock)
    {
        this.lock = lock;
    }

    public void run()
    {
        try
        {
            synchronized (lock)
            {
                System.out.println("开始------wait time = " + System.currentTimeMillis());
                lock.wait();
                System.out.println("结束------wait time = " + System.currentTimeMillis());
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyThread30_1类，使用notify()

public class MyThread30_1 extends Thread {
    private Object lock;

    public MyThread30_1(Object lock)
    {
        this.lock = lock;
    }

    public void run()
    {
        synchronized (lock)
        {
            System.out.println("开始------notify time = " + System.currentTimeMillis());
            lock.notify();
            System.out.println("结束------notify time = " + System.currentTimeMillis());
        }
    }
}

main方法,sleep保证mt0线程先执行。

public class MyThread30_main {
    public static void main(String[] args) throws Exception
    {
        Object lock = new Object();
        MyThread30_0 mt0 = new MyThread30_0(lock);
        mt0.start();
        Thread.sleep(3000);
        MyThread30_1 mt1 = new MyThread30_1(lock);
        mt1.start();
    }
}

运行结果如下

开始------wait time = 1563183002681
开始------notify time = 1563183005683
结束------notify time = 1563183005683
结束------wait time = 1563183005683

先执行“开始——wait time = 1563183002681”，lock.wait()会释放锁，mt0线程进入等待状态。
mt1线程获得锁，执行“开始——notify time = 1563183005683”，lock.notify()唤醒wait的线程(mt0)，使mt0退出等待状态，进入可运行状态，mt0线程想要执行必须等待notify的线程(mt1)释放锁，所以执行“结束——notify time = 1563183005683”
最后再恢复执行mt0线程，输出“结束——wait time = 1563183005683”

wait()释放锁

ThreadDomain31类,调用wait方法

public class ThreadDomain31 {
    public void testMethod(Object lock)
    {
        try
        {
            synchronized (lock)
            {
                System.out.println(Thread.currentThread().getName() + " Begin wait()");
                lock.wait();
                System.out.println(Thread.currentThread().getName() + " End wait");
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyThread31类

public class MyThread31 extends Thread{
    private Object lock;

    public MyThread31(Object lock)
    {
        this.lock = lock;
    }

    public void run()
    {
        ThreadDomain31 td = new ThreadDomain31();
        td.testMethod(lock);
    }

    public static void main(String[] args)
    {
        Object lock = new Object();
        MyThread31 mt0 = new MyThread31(lock);
        MyThread31 mt1 = new MyThread31(lock);
        mt0.start();
        mt1.start();
    }
}

运行结果如下

Thread-0 Begin wait()
Thread-1 Begin wait()

假如wait()不释放锁，我们应该执行完同步方法，我们应该打印

Thread-1 Begin wait()
Thread-1 End wait
Thread-0 Begin wait()
Thread-0 End wait

反之，证明了wait释放锁，放弃了同步块的执行权。

notify()不释放锁

ThreadDomain32类，用sleep为了说明此时没有其他线程进入同步代码块，即不释放锁。

public class ThreadDomain32 {
    public void testMethod(Object lock)
    {
        try
        {
            synchronized (lock)
            {
                System.out.println("Begin wait(), ThreadName = " + Thread.currentThread().getName());
                lock.wait();
                System.out.println("End wait(), ThreadName = " + Thread.currentThread().getName());
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }

    public void synNotifyMethod(Object lock)
    {
        try
        {
            synchronized (lock)
            {
                System.out.println("Begin notify(), ThreadName = " + Thread.currentThread().getName());
                lock.notify();
                Thread.sleep(5000);
                System.out.println("End notify(), ThreadName = " + Thread.currentThread().getName());
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyThread32_0类

public class MyThread32_0 extends Thread{
    private Object lock;

    public MyThread32_0(Object lock)
    {
        this.lock = lock;
    }

    public void run()
    {
        ThreadDomain32 td = new ThreadDomain32();
        td.testMethod(lock);
    }
}

MyThread32_1类

public class MyThread32_1 extends Thread{
    private Object lock;

    public MyThread32_1(Object lock)
    {
        this.lock = lock;
    }

    public void run()
    {
        ThreadDomain32 td = new ThreadDomain32();
        td.synNotifyMethod(lock);
    }
}

输出结果如下

Begin wait(), ThreadName = Thread-0
Begin notify(), ThreadName = Thread-1
End notify(), ThreadName = Thread-1
End wait(), ThreadName = Thread-0
Begin notify(), ThreadName = Thread-2
End notify(), ThreadName = Thread-2

Begin wait()和End notify()紧密相连，我们已经使用了sleep(5000),5s之间足够让其他线程进入同步代码块了，说明notify的线程(mt1和mt2)没有释放锁。

notifyAll()唤醒所有线程

ThreadDomain34类

public class ThreadDomain34 {

    public void testMethod(Object lock)
    {
        try
        {
            synchronized (lock)
            {
                System.out.println("Begin wait(), ThreadName = " + Thread.currentThread().getName());
                lock.wait();
                System.out.println("End wait(), ThreadName = " + Thread.currentThread().getName());
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }

}

MyThread34_0类

public class MyThread34_0 extends Thread {

    private Object lock;

    public MyThread34_0(Object lock)
    {
        this.lock = lock;
    }

    public void run()
    {
        ThreadDomain34 td = new ThreadDomain34();
        td.testMethod(lock);
    }
}

MyThread34_1类

public class MyThread34_1 extends Thread{

    private Object lock;

    public MyThread34_1(Object lock)
    {
        this.lock = lock;
    }

    public void run()
    {
        synchronized (lock)
        {
            lock.notifyAll();
        }
    }
}

main方法，启动三个线程并使其wait，启动mt3线程notifyAll

public class MyThread34_main {

    public static void main(String[] args) throws InterruptedException {
        Object lock = new Object();
        MyThread34_0 mt0 = new MyThread34_0(lock);
        MyThread34_0 mt1 = new MyThread34_0(lock);
        MyThread34_0 mt2 = new MyThread34_0(lock);
        mt0.start();
        mt1.start();
        mt2.start();
        Thread.sleep(1000);
        MyThread34_1 mt3 = new MyThread34_1(lock);
        mt3.start();
    }

}

输出结果如下

Begin wait(), ThreadName = Thread-0
Begin wait(), ThreadName = Thread-2
Begin wait(), ThreadName = Thread-1
End wait(), ThreadName = Thread-1
End wait(), ThreadName = Thread-2
End wait(), ThreadName = Thread-0

notifyAll方法唤醒处于同一监视器下所有wait状态的线程，启动线程顺序是随机的，唤醒线程的顺序是随机的。