场景:有三个线程t1、t2、t3。确保三个线程t1执行完后t2执行,t2执行完成后t3执行。
1.使用join
thread.Join把指定的线程加入到当前线程,可以将两个交替执行的线程合并为顺序执行的线程。比如在线程B中调用了线程A的Join()方法,直到线程A执行完毕后,才会继续执行线程B。
t.join(); //调用join方法,等待线程t执行完毕
t.join(1000); //等待 t 线程,等待时间是1000毫秒。
一、join
package Thread;
/**
* @author Muluo
* @create 2021-03-13 8:13
*/
public class testThreadt1t2t3 {
public static void main(String[] args) {
Thread thread1 = new Thread(new work(null));
Thread thread2 = new Thread(new work(thread1));
Thread thread3 = new Thread(new work(thread2));
thread1.setName("线程一");
thread2.setName("线程二");
thread3.setName("线程三");
thread1.start();
thread2.start();
thread3.start();
}
}
class work implements Runnable {
private Thread thread;
public work(Thread thread) {
this.thread = thread;
}
@Override
public void run() {
if (thread != null) {
try {
thread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName());
} else {
System.out.println(Thread.currentThread().getName());
}
}
}
二、CountDownLatch
CountDownLatch(闭锁)是一个很有用的工具类,利用它我们可以拦截一个或多个线程使其在某个条件成熟后再执行。它的内部提供了一个计数器,在构造闭锁时必须指定计数器的初始值,且计数器的初始值必须大于0。另外它还提供了一个countDown方法来操作计数器的值,每调用一次countDown方法计数器都会减1,直到计数器的值减为0时就代表条件已成熟,所有因调用await方法而阻塞的线程都会被唤醒。这就是CountDownLatch的内部机制,看起来很简单,无非就是阻塞一部分线程让其在达到某个条件之后再执行。
public class ThreadTest2 {
// T1、T2、T3三个线程顺序执行
public static void main(String[] args) {
CountDownLatch c0 = new CountDownLatch(0); //计数器为0
CountDownLatch c1 = new CountDownLatch(1); //计数器为1
CountDownLatch c2 = new CountDownLatch(1); //计数器为1
Thread t1 = new Thread(new Work(c0, c1));
//c0为0,t1可以执行。t1的计数器减1
Thread t2 = new Thread(new Work(c1, c2));
//t1的计数器为0时,t2才能执行。t2的计数器c2减1
Thread t3 = new Thread(new Work(c2, c2));
//t2的计数器c2为0时,t3才能执行
t1.start();
t2.start();
t3.start();
}
//定义Work线程类,需要传入开始和结束的CountDownLatch参数
static class Work implements Runnable {
CountDownLatch c1;
CountDownLatch c2;
Work(CountDownLatch c1, CountDownLatch c2) {
super();
this.c1 = c1;
this.c2 = c2;
}
public void run() {
try {
c1.await();//前一线程为0才可以执行
System.out.println("thread start:" + Thread.currentThread().getName());
c2.countDown();//本线程计数器减少
} catch (InterruptedException e) {
}
}
}
}