如何确保三个线程顺序执行
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、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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 |
public class ThreadTest1 {
// T1、T2、T3三个线程顺序执行
public static void main(String[] args) {
Thread t1 = new Thread( new Work( null ));
Thread t2 = new Thread( new Work(t1));
Thread t3 = new Thread( new Work(t2));
t1.start();
t2.start();
t3.start();
}
static class Work implements Runnable {
private Thread beforeThread;
public Work(Thread beforeThread) {
this .beforeThread = beforeThread;
}
public void run() {
if (beforeThread != null ) {
try {
beforeThread.join();
System.out.println( "thread start:" + Thread.currentThread().getName());
} catch (InterruptedException e) {
e.printStackTrace();
}
} else {
System.out.println( "thread start:" + Thread.currentThread().getName());
}
}
}
}
|
2、使用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。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 |
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) {
}
}
}
}
|