多线程（4）-线程池

Java5之后提供了线程池ThreadPool,

concurrent包下的Executors提供了很多创建线程池的方法，根据不同需要，分开使用。包括但不限于
newFixedThreadPool固定大小的线程池，newCachedThreadPool缓存线程池（根据线程多少创建线程数），newSingleThreadExecutor单线程（保证有一个线程），newScheduledThreadPool（定时器线程池）.....

上代码

public class ThreadPoolTest {

	public static void main(String[] args) {
		//ExecutorService threadPool = Executors.newFixedThreadPool(3);
		//ExecutorService threadPool = Executors.newCachedThreadPool();
		ExecutorService threadPool = Executors.newSingleThreadExecutor();
		for(int i=1;i<=10;i++){
			final int task = i;
			threadPool.execute(new Runnable(){
				@Override
				public void run() {
					for(int j=1;j<=10;j++){
						try {
							Thread.sleep(20);
						} catch (InterruptedException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						}
						System.out.println(Thread.currentThread().getName() + " is looping of " + j + " for  task of " + task);
					}
				}
			});
		}
		System.out.println("all of 10 tasks have committed! ");
		//threadPool.shutdownNow();
		
		Executors.newScheduledThreadPool(3).scheduleAtFixedRate(
				new Runnable(){
					@Override
				public void run() {
					System.out.println("bombing!");
					
				}},
				6,
				2,
				TimeUnit.SECONDS);
	}
}

实际代码要注意使用try catch finally，根据需要在finally里关闭线程池。上述代码在jdk1.8之后使用lambda表达式后为

public class ThreadPoolTest {

	public static void main(String[] args) {
		//ExecutorService threadPool = Executors.newFixedThreadPool(3);
		//ExecutorService threadPool = Executors.newCachedThreadPool();
		ExecutorService threadPool = Executors.newSingleThreadExecutor();
		for(int i=1;i<=10;i++){
			final int task = i;
			threadPool.execute(() -> {
                for(int j=1;j<=10;j++){
                    try {
                        Thread.sleep(20);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + " is looping of " + j + " for  task of " + task);
                }
            });
		}
		System.out.println("all of 10 tasks have committed! ");
		//threadPool.shutdownNow();
		
		Executors.newScheduledThreadPool(3).scheduleAtFixedRate(
				() -> System.out.println("bombing!"),
				6,
				2,
				TimeUnit.SECONDS);
	}
}

当有返回结果时，使用callable和future代替runnable

上代码学姿势

ExecutorService threadPool =  Executors.newSingleThreadExecutor();
		Future future =
			threadPool.submit(
					() -> {
						Thread.sleep(2000);
						return "hello";
					}
			);
		System.out.println("等待结果");
		try {
			System.out.println("拿到结果：" + future.get());
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

当有一组Callable任务需要提交时，使用CompletionService，期take()方法返回对应的Future对象

上代码

ExecutorService threadPool2 =  Executors.newFixedThreadPool(10);
		CompletionService completionService = new ExecutorCompletionService(threadPool2);
		for(int i=1;i<=10;i++){
			final int seq = i;
			completionService.submit(() -> {
                Thread.sleep(new Random().nextInt(5000));
                return seq;
            });
		}
		for(int i=0;i<10;i++){
			try {
				System.out.println(
						completionService.take().get());
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (ExecutionException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}