Java isAlive()和join()的使用

Bertha 。 2022-05-21 10:25 157阅读 0赞

如前所述，通常你希望主线程最后结束。在前面的例子中，这点是通过在main()中调用sleep()来实现的，经过足够长时间的延迟以确保所有子线程都先于主线程结束。然而，这不是一个令人满意的解决方法，它也带来一个大问题：一个线程如何知道另一线程已经结束？幸运的是，Thread类提供了回答此问题的方法。  
  
有两种方法可以判定一个线程是否结束。第一，可以在线程中调用isAlive()。这种方法由Thread定义，它的通常形式如下：  
 final boolean isAlive( )  
  
如果所调用线程仍在运行，isAlive()方法返回true，如果不是则返回false。但isAlive（）很少用到，等待线程结束的更常用的方法是调用join()，描述如下：  
 final void join( ) throws InterruptedException  
  
该方法等待所调用线程结束。该名字来自于要求线程等待直到指定线程参与的概念。join()的附加形式允许给等待指定线程结束定义一个最大时间。下面是前面例子的改进版本。运用join()以确保主线程最后结束。同样，它也演示了isAlive()方法。  
// Using join() to wait for threads to finish.  
class NewThread implements Runnable \{   
 String name; // name of thread  
 Thread t;  
 NewThread(String threadname) \{   
 name = threadname;  
 t = new Thread(this, name);  
 System.out.println("New thread: " + t);  
 t.start(); // Start the thread  
 \}  
 // This is the entry point for thread.  
 public void run() \{   
 try \{   
 for(int i = 5; i > 0; i--) \{   
 System.out.println(name + ": " + i);  
 Thread.sleep(1000);  
 \}  
 \} catch (InterruptedException e) \{   
 System.out.println(name + " interrupted.");  
 \}  
 System.out.println(name + " exiting.");  
 \}  
\}  
  
class DemoJoin \{   
 public static void main(String args\[\]) \{   
 NewThread ob1 = new NewThread("One");  
 NewThread ob2 = new NewThread("Two");  
 NewThread ob3 = new NewThread("Three");  
 System.out.println("Thread One is alive: "+ ob1.t.isAlive());  
 System.out.println("Thread Two is alive: "+ ob2.t.isAlive());  
 System.out.println("Thread Three is alive: "+ ob3.t.isAlive());  
 // wait for threads to finish  
 try \{   
 System.out.println("Waiting for threads to finish.");  
 ob1.t.join();  
 ob2.t.join();  
 ob3.t.join();  
 \} catch (InterruptedException e) \{   
 System.out.println("Main thread Interrupted");  
 \}  
 System.out.println("Thread One is alive: "+ ob1.t.isAlive());  
 System.out.println("Thread Two is alive: "+ ob2.t.isAlive());  
 System.out.println("Thread Three is alive: "+ ob3.t.isAlive());  
 System.out.println("Main thread exiting.");  
 \}  
\}  
  
程序输出如下所示：  
New thread: Thread\[One,5,main\]  
New thread: Thread\[Two,5,main\]  
New thread: Thread\[Three,5,main\]  
Thread One is alive: true  
Thread Two is alive: true  
Thread Three is alive: true  
Waiting for threads to finish.  
One: 5  
Two: 5  
Three: 5  
One: 4  
Two: 4  
Three: 4  
One: 3  
Two: 3  
Three: 3  
One: 2  
Two: 2  
Three: 2  
One: 1  
Two: 1  
Three: 1  
Two exiting.  
Three exiting.  
One exiting.  
Thread One is alive: false  
Thread Two is alive: false  
Thread Three is alive: false  
Main thread exiting.

如你所见，调用join()后返回，线程终止执行。

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