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？

 package com.mianshi.easy;
 class JoinDemo implements Runnable{
     public void run() {
         for(int i=0;i<100;i++){
             System.out.println(Thread.currentThread().getName()+"……"+ i);
         }
     }
 }

 public class JoinMethod {

     public static void main(String[] args) throws InterruptedException {
         JoinDemo jd = new JoinDemo();
         Thread T1 = new Thread(jd);
         Thread T2 = new Thread(jd);
         Thread T3 = new Thread(jd);

         T1.start();
         //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
         T1.join();        

         T2.start();
         T2.join();

         T3.start();
         T3.join();

     }
 }

结果：

Thread-0……0
Thread-0……1
Thread-0……2
Thread-0……3
Thread-0……4
Thread-0……5
Thread-0……6
Thread-0……7
Thread-0……8
Thread-0……9
Thread-0……10
Thread-0……11
Thread-0……12
Thread-0……13
Thread-0……14
Thread-0……15
Thread-0……16
Thread-0……17
Thread-0……18
Thread-0……19
Thread-0……20
Thread-0……21
Thread-0……22
Thread-0……23
Thread-0……24
Thread-0……25
Thread-0……26
Thread-0……27
Thread-0……28
Thread-0……29
Thread-0……30
Thread-0……31
Thread-0……32
Thread-0……33
Thread-0……34
Thread-0……35
Thread-0……36
Thread-0……37
Thread-0……38
Thread-0……39
Thread-0……40
Thread-0……41
Thread-0……42
Thread-0……43
Thread-0……44
Thread-0……45
Thread-0……46
Thread-0……47
Thread-0……48
Thread-0……49
Thread-0……50
Thread-0……51
Thread-0……52
Thread-0……53
Thread-0……54
Thread-0……55
Thread-0……56
Thread-0……57
Thread-0……58
Thread-0……59
Thread-0……60
Thread-0……61
Thread-0……62
Thread-0……63
Thread-0……64
Thread-0……65
Thread-0……66
Thread-0……67
Thread-0……68
Thread-0……69
Thread-0……70
Thread-0……71
Thread-0……72
Thread-0……73
Thread-0……74
Thread-0……75
Thread-0……76
Thread-0……77
Thread-0……78
Thread-0……79
Thread-0……80
Thread-0……81
Thread-0……82
Thread-0……83
Thread-0……84
Thread-0……85
Thread-0……86
Thread-0……87
Thread-0……88
Thread-0……89
Thread-0……90
Thread-0……91
Thread-0……92
Thread-0……93
Thread-0……94
Thread-0……95
Thread-0……96
Thread-0……97
Thread-0……98
Thread-0……99
Thread-1……0
Thread-1……1
Thread-1……2
Thread-1……3
Thread-1……4
Thread-1……5
Thread-1……6
Thread-1……7
Thread-1……8
Thread-1……9
Thread-1……10
Thread-1……11
Thread-1……12
Thread-1……13
Thread-1……14
Thread-1……15
Thread-1……16
Thread-1……17
Thread-1……18
Thread-1……19
Thread-1……20
Thread-1……21
Thread-1……22
Thread-1……23
Thread-1……24
Thread-1……25
Thread-1……26
Thread-1……27
Thread-1……28
Thread-1……29
Thread-1……30
Thread-1……31
Thread-1……32
Thread-1……33
Thread-1……34
Thread-1……35
Thread-1……36
Thread-1……37
Thread-1……38
Thread-1……39
Thread-1……40
Thread-1……41
Thread-1……42
Thread-1……43
Thread-1……44
Thread-1……45
Thread-1……46
Thread-1……47
Thread-1……48
Thread-1……49
Thread-1……50
Thread-1……51
Thread-1……52
Thread-1……53
Thread-1……54
Thread-1……55
Thread-1……56
Thread-1……57
Thread-1……58
Thread-1……59
Thread-1……60
Thread-1……61
Thread-1……62
Thread-1……63
Thread-1……64
Thread-1……65
Thread-1……66
Thread-1……67
Thread-1……68
Thread-1……69
Thread-1……70
Thread-1……71
Thread-1……72
Thread-1……73
Thread-1……74
Thread-1……75
Thread-1……76
Thread-1……77
Thread-1……78
Thread-1……79
Thread-1……80
Thread-1……81
Thread-1……82
Thread-1……83
Thread-1……84
Thread-1……85
Thread-1……86
Thread-1……87
Thread-1……88
Thread-1……89
Thread-1……90
Thread-1……91
Thread-1……92
Thread-1……93
Thread-1……94
Thread-1……95
Thread-1……96
Thread-1……97
Thread-1……98
Thread-1……99
Thread-2……0
Thread-2……1
Thread-2……2
Thread-2……3
Thread-2……4
Thread-2……5
Thread-2……6
Thread-2……7
Thread-2……8
Thread-2……9
Thread-2……10
Thread-2……11
Thread-2……12
Thread-2……13
Thread-2……14
Thread-2……15
Thread-2……16
Thread-2……17
Thread-2……18
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Thread-2……24
Thread-2……25
Thread-2……26
Thread-2……27
Thread-2……28
Thread-2……29
Thread-2……30
Thread-2……31
Thread-2……32
Thread-2……33
Thread-2……34
Thread-2……35
Thread-2……36
Thread-2……37
Thread-2……38
Thread-2……39
Thread-2……40
Thread-2……41
Thread-2……42
Thread-2……43
Thread-2……44
Thread-2……45
Thread-2……46
Thread-2……47
Thread-2……48
Thread-2……49
Thread-2……50
Thread-2……51
Thread-2……52
Thread-2……53
Thread-2……54
Thread-2……55
Thread-2……56
Thread-2……57
Thread-2……58
Thread-2……59
Thread-2……60
Thread-2……61
Thread-2……62
Thread-2……63
Thread-2……64
Thread-2……65
Thread-2……66
Thread-2……67
Thread-2……68
Thread-2……69
Thread-2……70
Thread-2……71
Thread-2……72
Thread-2……73
Thread-2……74
Thread-2……75
Thread-2……76
Thread-2……77
Thread-2……78
Thread-2……79
Thread-2……80
Thread-2……81
Thread-2……82
Thread-2……83
Thread-2……84
Thread-2……85
Thread-2……86
Thread-2……87
Thread-2……88
Thread-2……89
Thread-2……90
Thread-2……91
Thread-2……92
Thread-2……93
Thread-2……94
Thread-2……95
Thread-2……96
Thread-2……97
Thread-2……98
Thread-2……99

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。

 package com.mianshi.easy;

 class JoinDemo implements Runnable{

     public void run() {
         for(int i=0;i<100;i++){
             System.out.println(Thread.currentThread().getName()+"……"+ i);
         }
     }
 }

 /**
  * 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
  * 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。
  * */
 public class JoinMethod {

     public static void main(String[] args) throws InterruptedException {
         JoinDemo jd = new JoinDemo();
         Thread t1 = new Thread(jd);
         Thread t2 = new Thread(jd);

         t1.start();

         t1.join();        

         t2.start();

         for(int j =0; j<100; j++){
             System.out.println(Thread.currentThread().getName()+"……"+j);
         }
     }
 }

结果：

Thread-0……0
Thread-0……1
Thread-0……2
Thread-0……3
Thread-0……4
Thread-0……5
Thread-0……6
Thread-0……7
Thread-0……8
Thread-0……9
Thread-0……10
Thread-0……11
Thread-0……12
Thread-0……13
Thread-0……14
Thread-0……15
Thread-0……16
Thread-0……17
Thread-0……18
Thread-0……19
Thread-0……20
Thread-0……21
Thread-0……22
Thread-0……23
Thread-0……24
Thread-0……25
Thread-0……26
Thread-0……27
Thread-0……28
Thread-0……29
Thread-0……30
Thread-0……31
Thread-0……32
Thread-0……33
Thread-0……34
Thread-0……35
Thread-0……36
Thread-0……37
Thread-0……38
Thread-0……39
Thread-0……40
Thread-0……41
Thread-0……42
Thread-0……43
Thread-0……44
Thread-0……45
Thread-0……46
Thread-0……47
Thread-0……48
Thread-0……49
Thread-0……50
Thread-0……51
Thread-0……52
Thread-0……53
Thread-0……54
Thread-0……55
Thread-0……56
Thread-0……57
Thread-0……58
Thread-0……59
Thread-0……60
Thread-0……61
Thread-0……62
Thread-0……63
Thread-0……64
Thread-0……65
Thread-0……66
Thread-0……67
Thread-0……68
Thread-0……69
Thread-0……70
Thread-0……71
Thread-0……72
Thread-0……73
Thread-0……74
Thread-0……75
Thread-0……76
Thread-0……77
Thread-0……78
Thread-0……79
Thread-0……80
Thread-0……81
Thread-0……82
Thread-0……83
Thread-0……84
Thread-0……85
Thread-0……86
Thread-0……87
Thread-0……88
Thread-0……89
Thread-0……90
Thread-0……91
Thread-0……92
Thread-0……93
Thread-0……94
Thread-0……95
Thread-0……96
Thread-0……97
Thread-0……98
Thread-0……99
main……0
main……1
main……2
main……3
main……4
main……5
main……6
main……7
main……8
main……9
main……10
main……11
main……12
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main……24
main……25
main……26
main……27
main……28
main……29
main……30
main……31
main……32
main……33
main……34
main……35
Thread-1……0
main……36
Thread-1……1
main……37
Thread-1……2
main……38
Thread-1……3
main……39
Thread-1……4
main……40
Thread-1……5
main……41
Thread-1……6
Thread-1……7
main……42
Thread-1……8
main……43
Thread-1……9
main……44
Thread-1……10
main……45
Thread-1……11
Thread-1……12
main……46
Thread-1……13
main……47
Thread-1……14
main……48
Thread-1……15
main……49
Thread-1……16
main……50
Thread-1……17
main……51
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main……52
Thread-1……19
main……53
Thread-1……20
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Thread-1……22
Thread-1……23
Thread-1……24
Thread-1……25
main……54
main……55
main……56
main……57
Thread-1……26
main……58
Thread-1……27
main……59
Thread-1……28
main……60
Thread-1……29
main……61
Thread-1……30
main……62
Thread-1……31
main……63
Thread-1……32
main……64
Thread-1……33
main……65
Thread-1……34
main……66
Thread-1……35
main……67
Thread-1……36
main……68
Thread-1……37
main……69
Thread-1……38
main……70
Thread-1……39
main……71
Thread-1……40
main……72
Thread-1……41
main……73
Thread-1……42
main……74
Thread-1……43
main……75
Thread-1……44
main……76
Thread-1……45
main……77
Thread-1……46
main……78
Thread-1……47
main……79
Thread-1……48
main……80
Thread-1……49
main……81
Thread-1……50
main……82
Thread-1……51
main……83
Thread-1……52
main……84
Thread-1……53
main……85
Thread-1……54
main……86
Thread-1……55
main……87
Thread-1……56
main……88
Thread-1……57
main……89
Thread-1……58
main……90
Thread-1……59
main……91
Thread-1……60
main……92
Thread-1……61
main……93
Thread-1……62
main……94
Thread-1……63
main……95
Thread-1……64
main……96
Thread-1……65
main……97
Thread-1……66
main……98
Thread-1……67
main……99
Thread-1……68
Thread-1……69
Thread-1……70
Thread-1……71
Thread-1……72
Thread-1……73
Thread-1……74
Thread-1……75
Thread-1……76
Thread-1……77
Thread-1……78
Thread-1……79
Thread-1……80
Thread-1……81
Thread-1……82
Thread-1……83
Thread-1……84
Thread-1……85
Thread-1……86
Thread-1……87
Thread-1……88
Thread-1……89
Thread-1……90
Thread-1……91
Thread-1……92
Thread-1……93
Thread-1……94
Thread-1……95
Thread-1……96
Thread-1……97
Thread-1……98
Thread-1……99

（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。

 package com.mianshi.easy;

 class JoinDemo implements Runnable{

     public void run() {
         for(int i=0;i<200;i++){
             System.out.println(Thread.currentThread().getName()+"……"+ i);
         }
     }
 }

 /**
  * 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。
  * 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
  * 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
  * 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
  * 让它把执行权放出来，它就等谁死。至于谁去抢，它不管。
  *
  * */
 public class JoinMethod {

     public static void main(String[] args) throws InterruptedException {
         JoinDemo jd = new JoinDemo();
         Thread t1 = new Thread(jd);
         Thread t2 = new Thread(jd);

         t1.start();

         t2.start();

         t1.join();        

         for(int j =0; j<100; j++){
             System.out.println(Thread.currentThread().getName()+"……"+j);
         }
     }
 }

结果：

Thread-1……0
Thread-0……0
Thread-0……1
Thread-1……1
Thread-0……2
Thread-1……2
Thread-0……3
Thread-1……3
Thread-0……4
Thread-0……5
Thread-0……6
Thread-0……7
Thread-0……8
Thread-0……9
Thread-1……4
Thread-1……5
Thread-1……6
Thread-1……7
Thread-1……8
Thread-1……9
Thread-1……10
Thread-1……11
Thread-1……12
Thread-0……10
Thread-1……13
Thread-1……14
Thread-1……15
Thread-1……16
Thread-1……17
Thread-1……18
Thread-1……19
Thread-1……20
Thread-1……21
Thread-1……22
Thread-1……23
Thread-1……24
Thread-1……25
Thread-1……26
Thread-0……11
Thread-0……12
Thread-0……13
Thread-0……14
Thread-0……15
Thread-0……16
Thread-0……17
Thread-1……27
Thread-0……18
Thread-1……28
Thread-0……19
Thread-1……29
Thread-0……20
Thread-1……30
Thread-0……21
Thread-1……31
Thread-1……32
Thread-1……33
Thread-1……34
Thread-1……35
Thread-1……36
Thread-1……37
Thread-1……38
Thread-1……39
Thread-1……40
Thread-1……41
Thread-1……42
Thread-1……43
Thread-1……44
Thread-1……45
Thread-1……46
Thread-1……47
Thread-0……22
Thread-1……48
Thread-0……23
Thread-1……49
Thread-0……24
Thread-1……50
Thread-0……25
Thread-1……51
Thread-0……26
Thread-1……52
Thread-0……27
Thread-1……53
Thread-0……28
Thread-1……54
Thread-0……29
Thread-1……55
Thread-0……30
Thread-1……56
Thread-0……31
Thread-1……57
Thread-1……58
Thread-1……59
Thread-1……60
Thread-1……61
Thread-1……62
Thread-1……63
Thread-1……64
Thread-1……65
Thread-1……66
Thread-1……67
Thread-1……68
Thread-1……69
Thread-1……70
Thread-1……71
Thread-0……32
Thread-1……72
Thread-0……33
Thread-1……73
Thread-0……34
Thread-1……74
Thread-0……35
Thread-1……75
Thread-0……36
Thread-1……76
Thread-0……37
Thread-1……77
Thread-0……38
Thread-0……39
Thread-0……40
Thread-0……41
Thread-0……42
Thread-0……43
Thread-1……78
Thread-0……44
Thread-1……79
Thread-0……45
Thread-1……80
Thread-0……46
Thread-1……81
Thread-0……47
Thread-1……82
Thread-0……48
Thread-1……83
Thread-0……49
Thread-1……84
Thread-0……50
Thread-1……85
Thread-0……51
Thread-1……86
Thread-0……52
Thread-1……87
Thread-0……53
Thread-1……88
Thread-0……54
Thread-1……89
Thread-0……55
Thread-1……90
Thread-0……56
Thread-1……91
Thread-0……57
Thread-1……92
Thread-0……58
Thread-1……93
Thread-0……59
Thread-1……94
Thread-0……60
Thread-1……95
Thread-0……61
Thread-1……96
Thread-0……62
Thread-1……97
Thread-0……63
Thread-1……98
Thread-0……64
Thread-1……99
Thread-0……65
Thread-1……100
Thread-0……66
Thread-1……101
Thread-0……67
Thread-1……102
Thread-0……68
Thread-1……103
Thread-0……69
Thread-1……104
Thread-0……70
Thread-1……105
Thread-0……71
Thread-1……106
Thread-0……72
Thread-1……107
Thread-0……73
Thread-1……108
Thread-1……109
Thread-0……74
Thread-1……110
Thread-0……75
Thread-1……111
Thread-0……76
Thread-1……112
Thread-0……77
Thread-1……113
Thread-1……114
Thread-0……78
Thread-1……115
Thread-0……79
Thread-1……116
Thread-0……80
Thread-1……117
Thread-0……81
Thread-1……118
Thread-0……82
Thread-1……119
Thread-0……83
Thread-1……120
Thread-0……84
Thread-1……121
Thread-0……85
Thread-1……122
Thread-0……86
Thread-1……123
Thread-0……87
Thread-1……124
Thread-0……88
Thread-1……125
Thread-0……89
Thread-1……126
Thread-0……90
Thread-1……127
Thread-0……91
Thread-1……128
Thread-0……92
Thread-1……129
Thread-0……93
Thread-1……130
Thread-0……94
Thread-1……131
Thread-0……95
Thread-1……132
Thread-0……96
Thread-1……133
Thread-0……97
Thread-1……134
Thread-0……98
Thread-1……135
Thread-0……99
Thread-1……136
Thread-0……100
Thread-1……137
Thread-0……101
Thread-1……138
Thread-0……102
Thread-1……139
Thread-0……103
Thread-1……140
Thread-0……104
Thread-1……141
Thread-0……105
Thread-1……142
Thread-0……106
Thread-1……143
Thread-0……107
Thread-1……144
Thread-0……108
Thread-1……145
Thread-1……146
Thread-1……147
Thread-0……109
Thread-1……148
Thread-0……110
Thread-1……149
Thread-0……111
Thread-1……150
Thread-0……112
Thread-1……151
Thread-0……113
Thread-1……152
Thread-0……114
Thread-1……153
Thread-0……115
Thread-1……154
Thread-0……116
Thread-1……155
Thread-0……117
Thread-1……156
Thread-0……118
Thread-1……157
Thread-0……119
Thread-1……158
Thread-0……120
Thread-1……159
Thread-0……121
Thread-0……122
Thread-1……160
Thread-0……123
Thread-1……161
Thread-0……124
Thread-1……162
Thread-0……125
Thread-0……126
Thread-0……127
Thread-0……128
Thread-0……129
Thread-0……130
Thread-0……131
Thread-0……132
Thread-0……133
Thread-0……134
Thread-0……135
Thread-0……136
Thread-0……137
Thread-0……138
Thread-0……139
Thread-0……140
Thread-0……141
Thread-0……142
Thread-0……143
Thread-0……144
Thread-0……145
Thread-0……146
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Thread-0……150
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Thread-0……156
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Thread-0……193
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Thread-0……198
Thread-0……199
Thread-1……163
Thread-1……164
main……0
main……1
main……2
main……3
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main……5
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顺便贴下java.lang.Thread中关于join()方法的实现代码：

     /**
      * Waits at most {@code millis} milliseconds for this thread to
      * die. A timeout of {@code 0} means to wait forever.
      *
      * <p> This implementation uses a loop of {@code this.wait} calls
      * conditioned on {@code this.isAlive}. As a thread terminates the
      * {@code this.notifyAll} method is invoked. It is recommended that
      * applications not use {@code wait}, {@code notify}, or
      * {@code notifyAll} on {@code Thread} instances.
      *
      * @param  millis
      *         the time to wait in milliseconds
      *
      * @throws  IllegalArgumentException
      *          if the value of {@code millis} is negative
      *
      * @throws  InterruptedException
      *          if any thread has interrupted the current thread. The
      *          <i>interrupted status</i> of the current thread is
      *          cleared when this exception is thrown.
      */
     public final synchronized void join(long millis)
     throws InterruptedException {
         long base = System.currentTimeMillis();
         long now = 0;

         if (millis < 0) {
             throw new IllegalArgumentException("timeout value is negative");
         }

         if (millis == 0) {
             while (isAlive()) {
                 wait(0);
             }
         } else {
             while (isAlive()) {
                 long delay = millis - now;
                 if (delay <= 0) {
                     break;
                 }
                 wait(delay);
                 now = System.currentTimeMillis() - base;
             }
         }
     }

     /**
      * Waits at most {@code millis} milliseconds plus
      * {@code nanos} nanoseconds for this thread to die.
      *
      * <p> This implementation uses a loop of {@code this.wait} calls
      * conditioned on {@code this.isAlive}. As a thread terminates the
      * {@code this.notifyAll} method is invoked. It is recommended that
      * applications not use {@code wait}, {@code notify}, or
      * {@code notifyAll} on {@code Thread} instances.
      *
      * @param  millis
      *         the time to wait in milliseconds
      *
      * @param  nanos
      *         {@code 0-999999} additional nanoseconds to wait
      *
      * @throws  IllegalArgumentException
      *          if the value of {@code millis} is negative, or the value
      *          of {@code nanos} is not in the range {@code 0-999999}
      *
      * @throws  InterruptedException
      *          if any thread has interrupted the current thread. The
      *          <i>interrupted status</i> of the current thread is
      *          cleared when this exception is thrown.
      */
     public final synchronized void join(long millis, int nanos)
     throws InterruptedException {

         if (millis < 0) {
             throw new IllegalArgumentException("timeout value is negative");
         }

         if (nanos < 0 || nanos > 999999) {
             throw new IllegalArgumentException(
                                 "nanosecond timeout value out of range");
         }

         if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
             millis++;
         }

         join(millis);
     }

     /**
      * Waits for this thread to die.
      *
      * <p> An invocation of this method behaves in exactly the same
      * way as the invocation
      *
      * <blockquote>
      * {@linkplain #join(long) join}{@code (0)}
      * </blockquote>
      *
      * @throws  InterruptedException
      *          if any thread has interrupted the current thread. The
      *          <i>interrupted status</i> of the current thread is
      *          cleared when this exception is thrown.
      */
     public final void join() throws InterruptedException {
         join(0);
     }