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java wait()和sleep()的区别

sleep可以在任何地方调用。wait必须在synchronized中调用，否则抛出 IllegalMonitorStateException

Object a = new Object();
try {
    a.wait();
} catch (InterruptedException e) {
    e.printStackTrace();
}

输出

java.lang.IllegalMonitorStateException
	at java.lang.Object.wait(Native Method)
	at java.lang.Object.wait(Object.java:502)
	at com.example.threads.TestSleepAndWait.testWait(TestSleepAndWait.java:11)

正确使用wait：

@Test
public void testWait() {
    Object lock = new Object();
    synchronized (lock) {
        try {
            lock.wait(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

@Test
public synchronized void testWait2() {
    try {
        wait(1000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

是否释放监视器锁

sleep不会释放监视器锁。wait释放监视器锁（因此wait必须在synchronized中使用）

sleep的javadoc

/**
 * Causes the currently executing thread to sleep (temporarily cease
 * execution) for the specified number of milliseconds, subject to
 * the precision and accuracy of system timers and schedulers. The thread
 * does not lose ownership of any monitors.
 */
public static native void sleep(long millis) throws InterruptedException;

wait的javadoc，原文太长做了截断

/*
     * The current thread must own this object's monitor. The thread
     * releases ownership of this monitor and waits until another thread
     * notifies threads waiting on this object's monitor to wake up
     * either through a call to the {@code notify} method or the
     * {@code notifyAll} method. The thread then waits until it can
     * re-obtain ownership of the monitor and resumes execution.
*/
public final void wait() throws InterruptedException {
    wait(0);
}

测试sleep

@Test
public void testSleepForMonitor(){
    try {
        Thread.sleep(60000L);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

jps找到pid，再jstack打印thread dump。

C:/Users/ycwu>jps
3312
7744 RemoteMavenServer
13352 Jps
1884 Launcher
8476 JUnitStarter

C:/Users/ycwu>jstack 8476

输出

"main" #1 prio=5 os_prio=0 tid=0x00000000029f3000 nid=0x2fdc waiting on condition [0x00000000028ae000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at com.example.threads.TestSleepAndWait.testSleepForMonitor(TestSleepAndWait.java:42)

测试wait

@Test
public synchronized void testWaitForMonitor(){
    try {
        wait(60000L);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

jps找到pid，再jstack打印thread dump。

C:/Users/ycwu>jps
13408 JUnitStarter
3312
7744 RemoteMavenServer
13220 Launcher
13224 Jps

C:/Users/ycwu>jstack 13408

输出

"main" #1 prio=5 os_prio=0 tid=0x0000000002ae3000 nid=0x91c in Object.wait() [0x000000000294e000]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000076c56cc18> (a com.example.threads.TestSleepAndWait)
        at com.example.threads.TestSleepAndWait.testWaitForMonitor(TestSleepAndWait.java:50)
        - locked <0x000000076c56cc18> (a com.example.threads.TestSleepAndWait)

从下往上看，先获取了锁（ locked <0x000000076c56cc18> ），然后释放并且等待（ waiting on <0x000000076c56cc18> ）。

原文 https://ycwu314.github.io/p/java-wait-vs-sleep/

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