Java并发编程-21-在执行器中分离任务的启动与结果的处理

一、在执行器中分离任务的启动与结果的处理

1、通常情况下，使用线程执行器来执行并发任务，将runnable或callable任务发送给线程执行器，并获得Future对象来控制任务

2、特殊情形：在一个对象里发送任务给执行器，然后在另一个对象里处理结果

3、应运而生的CompletionService类

二、CompletionService

1、CompletionService类里有一个方法迎来发送任务给线程执行器，还有一个方法为下一个已经执行结束的任务获取Future对象

2、内部实现机制：CompletionService类使用Executor对象来执行任务

3、优势：可以共享CompletionService对象，并发送任务到执行器，然后其他的对象可以处理任务的结果

4、缺点：只能为已经执行结束的任务获取Future对象，因此这些Future对象只能被用来获取任务的结果

5、CompletionService可以执行Runnable类型的任务或者Callable的任务,但是Runnable并不返回结果，所以基本没用

三、CompletionService提供的方法

Future<V> poll();

用于检查队列是否有Future对象，如果队列为空，返回null，否则，返回第一个元素，并移除这个元素

Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;

访问执行任务的队列，查看是否有任务已经完成，如果有，返回队列的第一个元素，然后删除这个元素

Future<V> take() throws InterruptedException;

检查队列中是否有Future对象，如果队列为空，它将阻塞线程直到队列中有可用的元素，否则，返回第一个元素，并移除这个元素

四、模拟

ReportRequest执行CompletionService中的两个ReportGenerator任务

ReportProcessor将处理被发送到执行器里的ReportRequest所产生的结果

package com.concurrency.executor;

import java.util.concurrent.Callable;
import java.util.concurrent.TimeUnit;

/**
 * 模拟生成一份报告
 * @author Nicholas
 *
 */
public class ReportGenerator implements Callable<String> {

	private String sender;
	private String title;

	public ReportGenerator(String sender, String title) {
		this.sender = sender;
		this.title = title;
	}

	@Override
	public String call() throws Exception {
		try {
			long duration = (long) (Math.random() * 10);
			System.out
					.printf("%s_%s : ReportGenerator : Generating a report during %d seconds",
							this.sender, this.title, duration);
			TimeUnit.SECONDS.sleep(duration);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		String retString = sender + " : " + title;
		return retString;
	}
}

package com.concurrency.executor;

import java.util.concurrent.CompletionService;

/**
 * 这个类模拟请求获取报告
 * 但是不在这个线程里面处理报告
 * @author Nicholas
 *
 */
public class ReportRequest implements Runnable {

	private String name;
	
	//创建CompletionService属性
	private CompletionService<String> completionService;

	public ReportRequest(String name,
			CompletionService<String> completionService) {
		this.name = name;
		this.completionService = completionService;
	}

	@Override
	public void run() {
		//生成一份报告
		ReportGenerator reportGenerator = new ReportGenerator(name, "Report");
		
		//将报告交给completionService对象
		completionService.submit(reportGenerator);
	}
}

package com.concurrency.executor;

import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;

/**
 * 处理报告，也就是说报告的请求和处理不在一个线程中
 * 
 * @author Nicholas
 *
 */
public class ReportProcessor implements Runnable {

	private CompletionService<String> completionService;
	private boolean end;

	public ReportProcessor(CompletionService<String> completionService) {
		this.completionService = completionService;
		end = false;
	}

	@Override
	public void run() {

		// end 为false，调用completionService接口的poll()方法，来获取下一个已经完成任务的Future对象
		// 这个任务是采用completionService完成的
		while (!end) {
			try {
				Future<String> result = completionService.poll(20,
						TimeUnit.SECONDS);
				if (result != null) {
					// 调用Future对象的get()方法获取任务的结果
					String report = result.get();
					System.out.println("ReportReceiver : Report Received : "
							+ report);
				}
			} catch (InterruptedException | ExecutionException e) {
				e.printStackTrace();
			}
		}
		System.out.println("ReportSender : End");
	}

	public void setEnd(boolean end) {
		this.end = end;
	}
}

package com.concurrency.executor;

import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class Main {

	public void testReportProcessor() {

		// 创建线程执行器
		ExecutorService executorService = Executors.newCachedThreadPool();

		// 创建completionService对象，使用executorService初始化
		CompletionService<String> completionService = new ExecutorCompletionService<String>(
				executorService);

		// 创建两个ReportRequest对象，然后创建两个线程分别执行他们
		ReportRequest faceRequest = new ReportRequest("Face", completionService);
		ReportRequest onlineRequest = new ReportRequest("Online",
				completionService);
		Thread feceThread = new Thread(faceRequest);
		Thread onlineThread = new Thread(onlineRequest);

		// 创建一个ReportProcessor对象，然后创建一个线程对象执行
		ReportProcessor reportProcessor = new ReportProcessor(completionService);
		Thread threadReportProcessor = new Thread(reportProcessor);

		// 启动这三个线程
		System.out.println("Main : Starting for the report generators");
		feceThread.start();
		onlineThread.start();
		threadReportProcessor.start();

		// 等待ReportRequest线程处理结束
		try {
			System.out.println("Main : waiting for the report generators");
			feceThread.join();
			onlineThread.join();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		// 结束线程执行器
		System.out.println("Main : shutting down the executor");
		executorService.shutdown();

		// 调用awaitTermination等待所有的任务执行结束
		try {
			executorService.awaitTermination(1, TimeUnit.DAYS);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		// 结束ReportProcessor的执行
		reportProcessor.setEnd(true);
		System.out.println("Main : Ends");
	}

	public static void main(String[] args) {

		new Main().testReportProcessor();
	}
}