Object#wait()与Object#wait(long)的区别
2024-08-24 09:07:20
例子
例1 最基础的等待-通知
下面一个例子，一个线程"waiting"在同步代码块调用了Object#wait()方法，另一个线程"timedWaiting"调用了Object#wait(3000)等待3000ms，主线程sleep 5000ms后唤醒所有线程。
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.TimeUnit;
import lombok.extern.slf4j.Slf4j; /**
* @see Object#wait(long) 等待对应的毫秒数(不为0)或者被唤醒，执行后续代码
* @see Object#wait() 只有被唤醒(底层源码调用wait 0 ms)才能执行后续代码
* @author rhyme
* @date 2020/5/30 11:38
*/
@Slf4j
public class ObjectWaitMain {
private static final Object lock = new Object(); private static final Runnable waiting =
() -> {
synchronized (lock) {
try {
log.info("Thread: {}, will Object#wait()", Thread.currentThread().getName());
lock.wait();
log.info("Thread: {} is notified.", Thread.currentThread().getName());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}; private static final Runnable timedWaiting =
() -> {
synchronized (lock) {
try {
log.info(
"Thread: {}, will be notified or wait 3000 milliseconds.",
Thread.currentThread().getName());
lock.wait(3000);
log.info(
"Thread: {}, after being notified or wait 3000 milliseconds.",
Thread.currentThread().getName()); } catch (InterruptedException e) {
e.printStackTrace();
}
}
}; public static void main(String[] args) throws InterruptedException { CompletableFuture.allOf(
CompletableFuture.runAsync(waiting), CompletableFuture.runAsync(timedWaiting)); // 主线程sleep5000ms，当3000ms后"timedWaiting"线程执行"wait(3000)"后的代码块
// 如果"timedWaiting"线程在3000ms被notify，那么会立即执行后续代码，不会wait 3000ms
TimeUnit.MILLISECONDS.sleep(5000); synchronized (lock) {
log.info("main will notifyAll waiting thread.");
lock.notifyAll();
}
log.info("main end.");
}
}
执行结果:
例2 Object#wait(long)的参数大于0与等于0
测试类代码
/**
* @author rhyme
* @date 2020/5/31 0:43
*/
@Slf4j
public class ThreadPoolExecutorTest {
private ThreadPoolExecutor threadPoolExecutor; @Before
public void initializeThreadPoolExecutor() {
threadPoolExecutor =
new ThreadPoolExecutor(4, 8, 10, TimeUnit.SECONDS, new LinkedBlockingDeque<>(4));
} @After
public void teardown() throws InterruptedException {
threadPoolExecutor.shutdown(); TimeUnit.SECONDS.sleep(2);
threadPoolExecutor = null;
}
大于0
/**
* 当 {@link Object#wait(long)}的参数是大于0, 线程wait对象的毫秒数后, 不需被唤醒, 就可再次获取锁执行wait后续代码
* @see Object#wait(long)
*/
@Test
public void synchronizedTimedWaitingTest() {
Runnable timedWaiting =
() -> {
synchronized (this) {
while (true) {
try {
log.info("before Object#wait(1);");
this.wait(1);
log.info("after Object#wait(1);");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}; threadPoolExecutor.execute(timedWaiting);
}
执行结果:
等于0或Object#wait()
/**
* 当 {@link Object#wait(long)}的参数是0, 线程只能是被唤醒后, 才能再次获取锁执行wait后续代码
* @see Object#wait()
*/
@Test
public void synchronizedWaitingTest() {
Runnable timedWaiting =
() -> {
synchronized (this) {
while (true) {
try {
log.info("before Object#wait(0);");
// 与"this.wait()"等价
this.wait(0);
log.info("after Object#wait(0);");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}; threadPoolExecutor.execute(timedWaiting);
}
执行结果, 该线程一直wait, 线程池关闭结束, UT结束:
总结
Object#wait(long) 等待对应的毫秒数后(不为0)或者在等待过程中被唤醒后，就能再次获取锁执行wait后面代码；
Object#wait() 只有被唤醒后(底层源码调用wait 0 ms)才能再次获取锁执行wait后面代码。
源码如下：
public final void wait() throws InterruptedException {
wait(0);
}
public final native void wait(long timeout) throws InterruptedException;
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