07
4月
2021

Java isAlive()和join()的使用

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。

Share

自由看

自由看世界一人折腾 一人维护

You may also like...