join()方法

1、现在有T1、T2、T3三个线程，你怎样保证T2在T1执行完后执行，T3在T2执行完后执行？

 1 package com.mianshi.easy;
 2 class JoinDemo implements Runnable{
 3     public void run() {
 4         for(int i=0;i<100;i++){
 5             System.out.println(Thread.currentThread().getName()+"……"+ i);
 6         }
 7     }
 8 }
 9 
10 public class JoinMethod {
11 
12     public static void main(String[] args) throws InterruptedException {
13         JoinDemo jd = new JoinDemo();
14         Thread T1 = new Thread(jd);
15         Thread T2 = new Thread(jd);
16         Thread T3 = new Thread(jd);
17 
18         T1.start();        
19         //当t1拿着执行权把这些数据都打印完了，主线程才恢复到运行中来
20         T1.join();        
21 
22         T2.start();
23         T2.join();
24 
25         T3.start();
26         T3.join();
27 
28     }
29 }
View Code

结果：

Thread-0……0
Thread-0……1
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View Code

2、练习demo

（1）主线程向下转时，碰到了t1.join(),t1要申请加入到运行中来，就是要CPU执行权。这时CPU执行权在主线程手里，主线程就把CPU执行权给放开，陷入冻结状态。活着的只有t1了，只有当t1拿着执行权把这些数据都打印完了，主线程才恢复到运行中来。

 1 package com.mianshi.easy;
 2 
 3 class JoinDemo implements Runnable{
 4 
 5     public void run() {
 6         for(int i=0;i<100;i++){
 7             System.out.println(Thread.currentThread().getName()+"……"+ i);
 8         }
 9     }
10 }
11 
12 /**
13  * 主线程向下转时，碰到了t1.join(),t1要申请加入到运行中来，就是要CPU执行权。
14  * 这时候CPU执行权在主线程手里，主线程就把CPU执行权给放开，陷入冻结状态。活着的
15  * 只有t1了，只有当t1拿着执行权把这些数据都打印完了，主线程才恢复到运行中来。
16  * */
17 public class JoinMethod {
18 
19     public static void main(String[] args) throws InterruptedException {
20         JoinDemo jd = new JoinDemo();
21         Thread t1 = new Thread(jd);
22         Thread t2 = new Thread(jd);
23 
24         t1.start();
25 
26         t1.join();        
27 
28         t2.start();
29         
30         for(int j =0; j<100; j++){
31             System.out.println(Thread.currentThread().getName()+"……"+j);
32         }
33     }
34 }
View Code

结果：

Thread-0……0
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main……0
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View Code

（2）改变顺序，主线程启动了两个子线程A、B时，其中一个子线程A调用了join()方法后，主线程将释放执行权，进入冻结状态。子线程A并不会影响子线程B，两个子线程将会交替执行，直到A线程run()方法执行完，主线程将会重新获得执行权。

 1 package com.mianshi.easy;
 2 
 3 class JoinDemo implements Runnable{
 4 
 5     public void run() {
 6         for(int i=0;i<200;i++){
 7             System.out.println(Thread.currentThread().getName()+"……"+ i);
 8         }
 9     }
10 }
11 
12 /**
13  * 将t1.join()和t2.start()位置互换。主线程开启了t1、t2，这时候CPU执行权还在主线程手里。
14  * 当主线程碰到了t1.join(),释放执行权，处于冻结状态。活着的t1、t2都具备执行资格，这时候CPU
15  * 便对t1和t2交替执行。主线程要等到t1结束才能活，至于t2结不结束，与主线程没有丝毫关系。如果t1
16  * 结束了，t2还没结束，主线程就会和t2抢执行权执行。主线程碰到谁的join，它就等谁。也就是说，谁
17  * 让它把执行权放出来，它就等谁死。至于谁去抢，它不管。
18  * 
19  * */
20 public class JoinMethod {
21 
22     public static void main(String[] args) throws InterruptedException {
23         JoinDemo jd = new JoinDemo();
24         Thread t1 = new Thread(jd);
25         Thread t2 = new Thread(jd);
26 
27         t1.start();
28 
29         t2.start();
30         
31         t1.join();        
32 
33         for(int j =0; j<100; j++){
34             System.out.println(Thread.currentThread().getName()+"……"+j);
35         }
36     }
37 }
View Code

结果：

Thread-1……0
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顺便贴下java.lang.Thread中关于join()方法的实现代码：

  1     /**
  2      * Waits at most {@code millis} milliseconds for this thread to
  3      * die. A timeout of {@code 0} means to wait forever.
  4      *
  5      * <p> This implementation uses a loop of {@code this.wait} calls
  6      * conditioned on {@code this.isAlive}. As a thread terminates the
  7      * {@code this.notifyAll} method is invoked. It is recommended that
  8      * applications not use {@code wait}, {@code notify}, or
  9      * {@code notifyAll} on {@code Thread} instances.
 10      *
 11      * @param  millis
 12      *         the time to wait in milliseconds
 13      *
 14      * @throws  IllegalArgumentException
 15      *          if the value of {@code millis} is negative
 16      *
 17      * @throws  InterruptedException
 18      *          if any thread has interrupted the current thread. The
 19      *          <i>interrupted status</i> of the current thread is
 20      *          cleared when this exception is thrown.
 21      */
 22     public final synchronized void join(long millis)
 23     throws InterruptedException {
 24         long base = System.currentTimeMillis();
 25         long now = 0;
 26 
 27         if (millis < 0) {
 28             throw new IllegalArgumentException("timeout value is negative");
 29         }
 30 
 31         if (millis == 0) {
 32             while (isAlive()) {
 33                 wait(0);
 34             }
 35         } else {
 36             while (isAlive()) {
 37                 long delay = millis - now;
 38                 if (delay <= 0) {
 39                     break;
 40                 }
 41                 wait(delay);
 42                 now = System.currentTimeMillis() - base;
 43             }
 44         }
 45     }
 46 
 47     /**
 48      * Waits at most {@code millis} milliseconds plus
 49      * {@code nanos} nanoseconds for this thread to die.
 50      *
 51      * <p> This implementation uses a loop of {@code this.wait} calls
 52      * conditioned on {@code this.isAlive}. As a thread terminates the
 53      * {@code this.notifyAll} method is invoked. It is recommended that
 54      * applications not use {@code wait}, {@code notify}, or
 55      * {@code notifyAll} on {@code Thread} instances.
 56      *
 57      * @param  millis
 58      *         the time to wait in milliseconds
 59      *
 60      * @param  nanos
 61      *         {@code 0-999999} additional nanoseconds to wait
 62      *
 63      * @throws  IllegalArgumentException
 64      *          if the value of {@code millis} is negative, or the value
 65      *          of {@code nanos} is not in the range {@code 0-999999}
 66      *
 67      * @throws  InterruptedException
 68      *          if any thread has interrupted the current thread. The
 69      *          <i>interrupted status</i> of the current thread is
 70      *          cleared when this exception is thrown.
 71      */
 72     public final synchronized void join(long millis, int nanos)
 73     throws InterruptedException {
 74 
 75         if (millis < 0) {
 76             throw new IllegalArgumentException("timeout value is negative");
 77         }
 78 
 79         if (nanos < 0 || nanos > 999999) {
 80             throw new IllegalArgumentException(
 81                                 "nanosecond timeout value out of range");
 82         }
 83 
 84         if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
 85             millis++;
 86         }
 87 
 88         join(millis);
 89     }
 90 
 91     /**
 92      * Waits for this thread to die.
 93      *
 94      * <p> An invocation of this method behaves in exactly the same
 95      * way as the invocation
 96      *
 97      * <blockquote>
 98      * {@linkplain #join(long) join}{@code (0)}
 99      * </blockquote>
100      *
101      * @throws  InterruptedException
102      *          if any thread has interrupted the current thread. The
103      *          <i>interrupted status</i> of the current thread is
104      *          cleared when this exception is thrown.
105      */
106     public final void join() throws InterruptedException {
107         join(0);
108     }
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原文地址：https://www.cnblogs.com/gongxing/p/4651624.html