线程池(Executors)

在java类库中,任务执行的主要抽象不是Thread,而是Executor,将任务的提交过程和执行过程解耦

	public interface Executor {

	    void execute(Runnable command);
	}

 

public class RunMain implements Executor{

	@Override
	public void execute(Runnable command) {
		new Thread(command).start();
	}

	
}

 为了解决执行服务的生命周期问题,Executor扩展了ExecutorService接口

	public interface ExecutorService extends Executor {
		void shutdown();
		List<Runnable> shutdownNow();
		boolean isShutdown();
		boolean isTerminated();
		boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException;
		// ....其他任务提交的便利方法

	}

 

  • ExecutorService的生命周期有3种状态,运行、关闭、已终止。
  • shutdown执行平缓的关闭过程:不再接受新的任务,同时等待已经提交的任务执行完成
  • shutdownNow方法将执行粗暴的关闭方式:它将尝试取消所有运行中的任务,并且不再启动队列中尚未开始执行的任务
  • 可以调用awaitTermination来等待ExecutorService到达终止状态,或者通过调用isTerminated来轮询ExecutorService是否已经终止

线程池:

newFixedThreadPool :创建一个固定长度的线程池,每当提交一任务就创建一个线程,知道达到线程池的最大数量(如果某个线程由于发生了未预期的Exception而结束,那么线程池会补充一个新的线程)

newCachedThreadPool:创建一个可缓存的线程池,如果线程池规模超过了处理需求时,那么将回收空闲的线程,而当需求增加时,则可以添加新的线程,线程池的规模不存在任何限制

newSingleThreadExecutor:单线程的Executor,如果这个线程结束会创建一个新的线程来替代,能确保依照在队列中的顺序来穿行执行

newScheduledThreadPool:创一个固定长度的线程池,而且以延迟或定时的方式来执行任务,用于替换Timer

import static java.util.concurrent.TimeUnit.SECONDS;
import java.util.Date;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
public class TestScheduledThread {
public static void main(String[] args) {
   final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(2);
   final Runnable beeper = new Runnable() {
    int count = 0;
    public void run() {
     System.out.println(new Date() + " beep " + (++count));
    }
   };
   // 1秒钟后运行,并每隔2秒运行一次
   final ScheduledFuture beeperHandle = scheduler.scheduleAtFixedRate(beeper, 1, 2, SECONDS);
   // 2秒钟后运行,并每次在上次任务运行完后等待5秒后重新运行
   final ScheduledFuture beeperHandle2 = scheduler.scheduleWithFixedDelay(beeper, 2, 5, SECONDS);
   // 30秒后结束关闭任务,并且关闭Scheduler
   scheduler.schedule(new Runnable() {
    public void run() {
     beeperHandle.cancel(true);
     beeperHandle2.cancel(true);
     scheduler.shutdown();
    }
   }, 30, SECONDS);
}
}

 

 

Runnable 和 Callable 描述的都是抽象的计算任务,后者有返回值,并可能抛出一个异常

Executor执行的任务有4个生命周期阶段:创建、提交、开始和完成

ExecutorService中所有的submit方法豆浆返回一个Future,

Callable 与 Future接口

	public interface Callable<V> {
		V call() throws Exception;
	}

	public interface Future<V> {

		/**
		 * 试图取消对此任务的执行,
		 * 如果任务已经启动,则mayInterruptIfRunning参数决定是否执行此任务的线程中断
		 * 
		 * 此方法返回后,后续调用isDone将始终返回true。
		 * 后续调用isCancelled将始终返回true
		 * 
		 */
		boolean cancel(boolean mayInterruptIfRunning);

		/**
		 *如果这个任务被取消之前,正常完成。返回true
		 */
		boolean isCancelled();

		boolean isDone();

		/**
		 * 等待计算完成,然后获取其结果
		 * @throws CancellationException
		 *             if the computation was cancelled
		 * @throws ExecutionException
		 *             if the computation threw an exception
		 * @throws InterruptedException
		 *             if the current thread was interrupted while waiting
		 */
		V get() throws InterruptedException, ExecutionException;

		V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;
	}

 

CompletionService用于封装
public class RunMain implements Callable<String> {
	private int id;

	public RunMain(int i) {
		this.id = i;
	}

	public static void main(String[] args) throws Exception {
		ExecutorService service = Executors.newCachedThreadPool();
		CompletionService<String> completion = new ExecutorCompletionService<String>(service);
		for (int i = 0; i < 10; i++) {
			completion.submit(new RunMain(i));
		}
		for (int i = 0; i < 10; i++) {
			System.out.println(completion.take().get());
		}
		service.shutdown();
	}

	public String call() throws Exception {
		Integer time = (int) (Math.random() * 1000);
		try {
			System.out.println(this.id + " start");
			Thread.sleep(time);
			System.out.println(this.id + " end");
		} catch (Exception e) {
			e.printStackTrace();
		}
		return this.id + ":" + time;
	}
}
 

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