Java自身的线程池就是一个例子,ExecutorService,当调用submit(Runnable task)方法后,如果不显示的调用shutdown()方法,线程池线程是不会终止的。
The ExecutorService does not kill the running threads, and since threads are created as non-daemon, the JVM doesn't exit.
那么它是如何保持线程池线程的生命的呢?
ExecutorService通过阻塞队列LinkedBlockingQueue来实现
import java.util.concurrent.LinkedBlockingQueue; public class LinkedBlockingQueueDemo { public static void main(String[] args) throws InterruptedException { LinkedBlockingQueue queue = new LinkedBlockingQueue(10); System.out.println("debug1"); System.out.println(queue.take()); System.out.println("debug2"); } }
LinkedBlockingQueue的take()方法如下:
public E take() throws InterruptedException { E x; int c = -1; final AtomicInteger count = this.count; final ReentrantLock takeLock = this.takeLock; takeLock.lockInterruptibly(); try { while (count.get() == 0) { notEmpty.await(); } x = dequeue(); c = count.getAndDecrement(); if (c > 1) notEmpty.signal(); } finally { takeLock.unlock(); } if (c == capacity) signalNotFull(); return x; }
如果对类为空,它将被阻塞
while (count.get() == 0) {
notEmpty.await();
}
直到调用put(E e)方法被唤醒。
tomcat web服务器又是如何保持运行状态的呢,通过代码发现tomcat的启动类为
org.apache.catalina.startup.Bootstrap
Bootstrap依赖org.apache.catalina.startup.Catalina
Catalina依赖org.apache.catalina.core.StandardServer
通过StandardServer的public void await()实现,代码片段如下:
while(!stopAwait) {
try {
Thread.sleep( 10000 );
} catch( InterruptedException ex ) {
// continue and check the flag
}
}
还有一中方式保证一个线程的active,如下:
public class Main { public static void main(String[] args) throws Exception { ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext( new String[] { "applicationProvider.xml" }); context.start(); System.out.println("按任意键退出"); System.in.read(); } }
因此可以总结出,保持一个线程的方式如下:
1. 通过线程间通讯wait() 和notify()或者是Condition的awati()和signal()加上一个循环判断,例如ExecutorService
2. 通过一个死循环。例如tomcat,为了减轻服务器负担,可以加上Thread.sleep( 10000 ); 让循环体执行得慢点。
3.通过阻塞API的调用,例如IO流的read和write