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java中wait和sleep的区别

在本篇文章中,我们将会讨论一下java中wait()和sleep()方法的区别。并讨论一下怎么使用这两个方法。

Wait和sleep的区别

wait() 是Object中定义的native方法:

public final native void wait(long timeout) throws InterruptedException;

所以每一个类的实例都可以调用这个方法。wait()只能在synchronized block中调用。它会释放synchronized时加在object上的锁。

sleep()是定义Thread中的native静态类方法:

public static native void sleep(long millis) throws InterruptedException;

所以Thread.sleep()可以在任何情况下调用。Thread.sleep()将会暂停当前线程,并且不会释放任何锁资源。

我们先看一下一个简单的wait使用:

@Slf4j
public class WaitUsage {

private static Object LOCK = new Object();

public static void WaitExample() throws InterruptedException {
synchronized (LOCK) {
LOCK.wait(1000);
log.info("Object '" + LOCK + "' is woken after" +
" waiting for 1 second");
}
}
}

再看一下sleep的使用:

@Slf4j
public class SleepUsage {

public static void sleepExample() throws InterruptedException {
Thread.sleep(1000);
log.info(
"Thread '" + Thread.currentThread().getName() +
"' is woken after sleeping for 1 second");
}
}

唤醒wait和sleep

sleep()方法自带sleep时间,时间过后,Thread会自动被唤醒。
或者可以通过调用interrupt()方法来中断。

相比而言wait的唤醒会比较复杂,我们需要调用notify() 和 notifyAll()方法来唤醒等待在特定wait object上的线程。

notify()会根据线程调度的机制选择一个线程来唤醒,而notifyAll()会唤醒所有等待的线程,由这些线程重新争夺资源锁。

wait,notity通常用在生产者和消费者情形,我们看下怎么使用:

@Slf4j
public class WaitNotifyUsage {

private int count =0;

public void produceMessage() throws InterruptedException {

while(true) {
synchronized (this) {
while (count == 5) {
log.info("count == 5 , wait ....");
wait();
}
count++;
log.info("produce count {}", count);
notify();
}
}
}

public void consumeMessage() throws InterruptedException {

while (true) {
synchronized (this) {
while (count == 0) {
log.info("count == 0, wait ...");
wait();
}
log.info("consume count {}", count);
count--;
notify();
}
}
}
}

看下怎么调用:

@Test
public void testWaitNotifyUsage() throws InterruptedException{
WaitNotifyUsage waitNotifyUsage=new WaitNotifyUsage();

ExecutorService executorService=Executors.newFixedThreadPool(4);
executorService.submit(()-> {
try {
waitNotifyUsage.produceMessage();
} catch (InterruptedException e) {
e.printStackTrace();
}
});

executorService.submit(()-> {
try {
waitNotifyUsage.consumeMessage();
} catch (InterruptedException e) {
e.printStackTrace();
}
});

Thread.sleep(50000);
}

本文的例子可以参考​​https://github.com/ddean2009/learn-java-concurrency/tree/master/wait-sleep​