用来进行线程间同步写作，等待所有线程完成倒计时。
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| package com.chat;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.CountDownLatch;
@Slf4j
public class Test {
public static void main(String[] args) throws InterruptedException {
CountDownLatch countDownLatch = new CountDownLatch(3);
new Thread(() ->{
log.debug("begin...");
try {
Thread.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
countDownLatch.countDown();
log.debug("end...");
}).start();
new Thread(() ->{
log.debug("begin...");
try {
Thread.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
countDownLatch.countDown();
log.debug("end...");
}).start();
new Thread(() ->{
log.debug("begin...");
try {
Thread.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
countDownLatch.countDown();
log.debug("end...");
}).start();
log.debug("wait...");
countDownLatch.await();
log.debug("wait end...");
}
}
|
输出
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| 23:02:46.900 [main] DEBUG com.chat.Test - wait...
23:02:46.900 [Thread-0] DEBUG com.chat.Test - begin...
23:02:46.900 [Thread-2] DEBUG com.chat.Test - begin...
23:02:46.900 [Thread-1] DEBUG com.chat.Test - begin...
23:02:46.904 [Thread-0] DEBUG com.chat.Test - end...
23:02:46.905 [Thread-1] DEBUG com.chat.Test - end...
23:02:46.906 [Thread-2] DEBUG com.chat.Test - end...
23:02:46.907 [main] DEBUG com.chat.Test - wait end...
|
为什么要用 CountdownLatch 不用 join？
这是因为 CountdownLatch 是更高级的用法，用起来简单，好懂。并且，如果用的是线程池而不是多进程，就必须要用 CountdownLatch 达到 join 效果。
线程池
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| package com.chat;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
@Slf4j
public class Test {
public static void main(String[] args) throws InterruptedException {
CountDownLatch countDownLatch = new CountDownLatch(3);
ExecutorService executorService = Executors.newFixedThreadPool(4);
executorService.submit(() -> {
log.debug("begin...");
try {
Thread.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
countDownLatch.countDown();
log.debug("end...");
});
executorService.submit(() -> {
log.debug("begin...");
try {
Thread.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
countDownLatch.countDown();
log.debug("end...");
});
executorService.submit(() -> {
log.debug("begin...");
try {
Thread.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
countDownLatch.countDown();
log.debug("end...");
});
executorService.submit(() -> {
log.debug("wait...");
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
log.debug("wait end...");
});
}
}
|
3 个线程池中的线程计算，一个等待。
