线程结束等待(join)与线程优先级(Priority)
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。
线程优先级
线程优先级被线程调度用来判定何时每个线程允许运行。理论上,优先级高的线程比优先级低的线程获得更多的CPU时间。实际上,线程获得的CPU时间通常由包括优先级在内的多个因素决定。
一个优先级高的线程自然比优先级低的线程优先。设置线程优化级只是让线程先执行的概率增大并不保证该线程一定先执行.
设置线程的优先级,用setPriority()方法,该方法也是Tread 的成员。它的通常形式为:
final void setPriority(int level)
这 里 , level 指定了对所调用的线程的新的优先权的设置。Level的值必须在MIN_PRIORITY到MAX_PRIORITY范围内。通常,它们的值分别是1和10。要返回一个线程为默认的优先级,指定NORM_PRIORITY,通常值为5。这些优先级在Thread中都被定义为final型变量。
你可以通过调用Thread的getPriority()方法来获得当前的优先级设置。该方法如下:
final int getPriority( )
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