应用Executors来建立Thread pool

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Java Gossip: Executors

有时候您需要建立一堆Thread来执行一些小任务，然而频繁的建立Thread有时会是个开销，因为Thread的建立必须与作业系统互动，如果能建立一个Thread pool来管理这些小的Thread并加以重复使用，对于系统效能会是个改善的方式。

您可以使用Executors来建立Thread pool，Executors有几个static方法，列出如下：
方法说明 newCachedThreadPool 建立可以快取的Thread，每个Thread预设可idle 60秒

newFixedThreadPool

包括固定数量的Thread

newSingleThreadExecutor

只有一个Thread，循序的执行指定给它的每个任务 newScheduledThreadPool 可排程的Thread newSingleThreadScheduledExecutor 单一可排程的Thread
举个简单的实例，下面的程式使用newFixedThreadPool方法建立Thread pool，当中包括五个可以重复使用的Thread，您可以指定Runnable物件给它，程式中会产生十个Runnable物件，由于Thread pool中只有五个可用的Thread，所以后来建立的五个Runnable必须等待有空闲的Thread才会被执行：

ExecutorDemo.java

package onlyfun.caterpillar;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ExecutorDemo {
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(5);

for(int i = 0; i < 10; i++) {
final int count = i;
service.submit(new Runnable() {
public void run() {
System.out.println(count);
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}

}
});
}

service.shutdown(); // 最后记得关闭Thread pool
}
}

submit()方法也接受实作Callable介面的物件，最后传回Future物件，可以取得Callable执行过后的传回结果。

如果想利用Executors进行排程，例如排定某个工作30秒后执行：
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor( );
        scheduler.schedule(new Runnable( ) {
                               public void run() {
                                   // 排程工作
                               }
                           },
                           30, TimeUnit.SECONDS);

或排定某个工作5秒后执行，之后每30秒执行一次：
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor( );
        final ScheduledFuture future = scheduler.scheduleAtFixedRate(new Runnable( ) {
                               public void run() {
                                   // 排程工作
                                   System.out.println("t");
                               }
                           },
                           0, 5, TimeUnit.SECONDS);

        // 排定 60 秒后取消future
        scheduler.schedule(new Runnable( ) {
            public void run( ) {
              future.cancel(false);
            }
          }, 60, TimeUnit.SECONDS);

如上所示，想要取消排程任务，可以呼叫ScheduledFuture的cancel()方法。
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