简介
CountDownLatch这个类使一个线程等待其他线程各自执行完毕后再执行。其原理是通过一个计数器来实现的,计数器的初始值是线程的数量。每当一个线程执行完毕后,计数器的值就-1,当计数器的值为0时,表示所有线程都执行完毕,然后在闭锁上等待的线程就可以恢复工作了。
重要的方法
//调用await()方法的线程会被挂起,它会等待直到count值为0才继续执行
public void await() throws InterruptedException { };
//和await()类似,只不过等待一定的时间后count值还没变为0的话就会继续执行
public boolean await(long timeout, TimeUnit unit) throws InterruptedException { };
//将count值减1
public void countDown() { };
CountDownLatch的不足
CountDownLatch是一次性的,计算器的值只能在构造方法中初始化一次,之后没有任何机制再次对其设置值,当CountDownLatch使用完毕后,它不能再次被使用。
简单示例
假如要计算5个线程执行完毕所需要花的时间,这个时候就可以考虑使用闭锁。
import java.util.concurrent.CountDownLatch;
/**
* @Auther: 罗罗
* @Description:
*/
public class Test03 {
//CountDownLatch: 闭锁,在完成某些运算时,只有在其他线程的运算全部完成时,当前运算才可以继续执行
public static void main(String[] args) {
final CountDownLatch countDownLatch = new CountDownLatch(5);
CountDownlatchDemo demo = new CountDownlatchDemo(countDownLatch);
long start = System.currentTimeMillis();
for (int i = 0; i < 5; i++) {
new Thread(demo).start();
}
//没有执行完毕前,等待
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
long end = System.currentTimeMillis();
System.out.println("所花时间为:"+(end-start));
}
}
class CountDownlatchDemo implements Runnable{
private CountDownLatch countDownLatch;
public CountDownlatchDemo(CountDownLatch countDownLatch) {
this.countDownLatch = countDownLatch;
}
@Override
public void run() {
//加锁,确保安全
synchronized (this){
try {
for (int i = 0; i < 500; i++) {
if (i % 2 == 0){
System.out.println(i);
}
}
}finally {
//保证一定执行
countDownLatch.countDown();
}
}
}
}
FutureTask也可以实现闭锁操作,只有FutureTask所在的线程执行完毕之后,等待的线程才可以继续执行
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
/**
* @Auther: 罗罗
* @Description:
*/
public class FutureTaskDemo {
public static void main(String[] args) {
Task task = new Task();
FutureTask<Integer> result = new FutureTask<>(task);
new Thread(result).start();
try {
//接受运算后的结果
Integer sum = result.get();
System.out.println(sum);
System.out.println("==============");
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}
class Task implements Callable<Integer>{
@Override
public Integer call() throws Exception {
int sum = 0;
for (int i = 0; i <= 100; i++) {
sum+=i;
}
return sum;
}
}
"C:\Program Files\Java\jdk1.8.0_144\bin\java.exe" "-javaagent:D:\IntelliJ IDEA\IntelliJ IDEA 2018.2.4\lib\idea_rt.jar=65323:D:\IntelliJ IDEA\IntelliJ IDEA 2018.2.4\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-Files\Java\jdk1.8.0_144\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\rt.jar;D:\IDEA2018\volatile\out\production\volatile" demo.FutureTaskDemo
5050
==============
Process finished with exit code 0