簡談Java的join()方法

時間 2019-11-18

標籤 java join 方法欄目 Java 简体版

原文原文鏈接

join()是Thread類的一個方法。根據jdk文檔的定義：html

public final void join()throws InterruptedException: Waits for this thread to die.java

join()方法的做用，是等待這個線程結束；但顯然，這樣的定義並不清晰。我的認爲"Java 7 Concurrency Cookbook"的定義較爲清晰：api

join() method suspends the execution of the calling thread until the object called finishes its execution.oracle

也就是說，t.join()方法阻塞調用此方法的線程(calling thread)，直到線程t完成，此線程再繼續；一般用於在main()主線程內，等待其它線程完成再結束main()主線程。例如：this

 1 public class JoinTester01 implements Runnable {
 2 
 3     private String name;
 4 
 5     public JoinTester01(String name) {
 6     this.name = name;
 7     }
 8 
 9     public void run() {
10     System.out.printf("%s begins: %s\n", name, new Date());
11     try {
12         TimeUnit.SECONDS.sleep(4);
13     } catch (InterruptedException e) {
14         e.printStackTrace();
15     }
16     System.out.printf("%s has finished: %s\n", name, new Date());
17     }
18 
19     public static void main(String[] args) {
20     Thread thread1 = new Thread(new JoinTester01("One"));
21     Thread thread2 = new Thread(new JoinTester01("Two"));
22     thread1.start();
23     thread2.start();
24     
25     try {
26         thread1.join();
27         thread2.join();
28     } catch (InterruptedException e) {
29         // TODO Auto-generated catch block
30         e.printStackTrace();
31     }
32     
33     System.out.println("Main thread is finished");
34     }
35 
36 }

上述代碼若是沒有join()方法，輸出以下：spa

Main thread is finished
One begins: Wed Aug 28 10:21:36 CST 2013
Two begins: Wed Aug 28 10:21:36 CST 2013
Two has finished: Wed Aug 28 10:21:40 CST 2013
One has finished: Wed Aug 28 10:21:40 CST 2013線程

能夠看出主線程main比其它兩個線程先結束。code

最後來深刻了解一下join()，請看其源碼：htm

 1 /**
 2      *  Waits at most <code>millis</code> milliseconds for this thread to  
 3      * die. A timeout of <code>0</code> means to wait forever.    
 4      */
 5     //此處A timeout of 0 means to wait forever 字面意思是永遠等待，實際上是等到t結束後。
 6     public final synchronized void join(long millis)    throws InterruptedException {
 7         long base = System.currentTimeMillis();
 8         long now = 0;
 9 
10         if (millis < 0) {
11             throw new IllegalArgumentException("timeout value is negative");
12         }
13         
14         if (millis == 0) {
15             while (isAlive()) {
16                 wait(0);
17             }
18         } else {
19             while (isAlive()) {
20                 long delay = millis - now;
21                 if (delay <= 0) {
22                     break;
23                 }
24                 wait(delay);
25                 now = System.currentTimeMillis() - base;
26             }
27         }
28     }

能夠看出，Join方法實現是經過wait（小提示：Object 提供的方法）。當main線程調用t.join時候，main線程會得到線程對象t的鎖（wait 意味着拿到該對象的鎖),調用該對象的wait(等待時間)，直到該對象喚醒main線程，好比退出後。這就意味着main 線程調用t.join時，必須可以拿到線程t對象的鎖。對象

 1 public class JoinTester02 implements Runnable {
 2 
 3     Thread thread;
 4 
 5     public JoinTester02(Thread thread) {
 6     this.thread = thread;
 7     }
 8 
 9     public void run() {
10     synchronized (thread) {
11         System.out.println("getObjectLock");
12         try {
13         Thread.sleep(9000);
14         } catch (InterruptedException ex) {
15         ex.printStackTrace();
16         }
17         System.out.println("ReleaseObjectLock");
18     }
19     }
20 
21     public static void main(String[] args) {
22     Thread thread = new Thread(new JoinTester01("Three"));
23     Thread getLockThread = new Thread(new JoinTester02(thread));
24     
25     getLockThread.start();
26     thread.start();
27     
28     try {
29         thread.join();
30     } catch (InterruptedException e) {
31         // TODO Auto-generated catch block
32         e.printStackTrace();
33     }
34     System.out.println("Main finished!");
35     }
36 
37 }public class JoinTester02 implements Runnable {
38 
39     Thread thread;
40 
41     public JoinTester02(Thread thread) {
42     this.thread = thread;
43     }
44 
45     public void run() {
46     synchronized (thread) {
47         System.out.println("getObjectLock");
48         try {
49         Thread.sleep(9000);
50         } catch (InterruptedException ex) {
51         ex.printStackTrace();
52         }
53         System.out.println("ReleaseObjectLock");
54     }
55     }
56 
57     public static void main(String[] args) {
58     Thread thread = new Thread(new JoinTester01("Three"));
59     Thread getLockThread = new Thread(new JoinTester02(thread));
60     
61     getLockThread.start();
62     thread.start();
63     
64     try {
65         thread.join();
66     } catch (InterruptedException e) {
67         // TODO Auto-generated catch block
68         e.printStackTrace();
69     }
70     System.out.println("Main finished!");
71     }
72 
73 }