JAVA 1.5 并发之 Executor框架 (内容为转载)

时间:2021-10-01 14:36:26

本文内容转自 http://www.iteye.com/topic/366591

Executor框架是指java 5中引入的一系列并发库中与executor相关的一些功能类,其中包括线程池,Executor,Executors,ExecutorService,CompletionService,Future,Callable等。他们的关系为:


JAVA 1.5 并发之 Executor框架 (内容为转载)

 

并发编程的一种编程方式是把任务拆分为一些列的小任务,即Runnable,然后在提交给一个Executor执行,Executor.execute(Runnalbe) 。Executor在执行时使用内部的线程池完成操作。

一、创建线程池

Executors类,提供了一系列工厂方法用于创先线程池,返回的线程池都实现了ExecutorService接口。

public static ExecutorService newFixedThreadPool(int nThreads)

创建固定数目线程的线程池。

public static ExecutorService newCachedThreadPool()

创建一个可缓存的线程池,调用execute 将重用以前构造的线程(如果线程可用)。如果现有线程没有可用的,则创建一个新线程并添加到池中。终止并从缓存中移除那些已有 60 秒钟未被使用的线程。

public static ExecutorService newSingleThreadExecutor()

创建一个单线程化的Executor。

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize)

创建一个支持定时及周期性的任务执行的线程池,多数情况下可用来替代Timer类。

Executor executor = Executors.newFixedThreadPool(10);
Runnable task = new Runnable() {
	@Override
	public void run() {
		System.out.println("task over");
	}
};
executor.execute(task);

executor = Executors.newScheduledThreadPool(10);
ScheduledExecutorService scheduler = (ScheduledExecutorService) executor;
scheduler.scheduleAtFixedRate(task, 10, 10, TimeUnit.SECONDS);

  

 二、ExecutorService与生命周期

ExecutorService扩展了Executor并添加了一些生命周期管理的方法。一个Executor的生命周期有三种状态,运行 ,关闭 ,终止。Executor创建时处于运行状态。当调用ExecutorService.shutdown()后,处于关闭状态,isShutdown()方法返回true。这时,不应该再想Executor中添加任务,所有已添加的任务执行完毕后,Executor处于终止状态,isTerminated()返回true。

如果Executor处于关闭状态,往Executor提交任务会抛出unchecked exception RejectedExecutionException。

ExecutorService executorService = (ExecutorService) executor;
while (!executorService.isShutdown()) {
	try {
		executorService.execute(task);
	} catch (RejectedExecutionException ignored) {
		
	}
}
executorService.shutdown();

  

 三、使用Callable,Future返回结果

Future<V>代表一个异步执行的操作,通过get()方法可以获得操作的结果,如果异步操作还没有完成,则,get()会使当前线程阻塞。FutureTask<V>实现了Future<V>和Runable<V>。Callable代表一个有返回值得操作。

		Callable<Integer> func = new Callable<Integer>(){
			public Integer call() throws Exception {
				System.out.println("inside callable");
				Thread.sleep(1000);
				return new Integer(8);
			}		
		};		
		FutureTask<Integer> futureTask  = new FutureTask<Integer>(func);
		Thread newThread = new Thread(futureTask);
		newThread.start();
		
		try {
			System.out.println("blocking here");
			Integer result = futureTask.get();
			System.out.println(result);
		} catch (InterruptedException ignored) {
		} catch (ExecutionException ignored) {
		}

   ExecutoreService提供了submit()方法,传递一个Callable,或Runnable,返回Future。如果Executor后台线程池还没有完成Callable的计算,这调用返回Future对象的get()方法,会阻塞直到计算完成。

例子:并行计算数组的和。

package executorservice;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;

public class ConcurrentCalculator {

	private ExecutorService exec;
	private int cpuCoreNumber;
	private List<Future<Long>> tasks = new ArrayList<Future<Long>>();

	// 内部类
	class SumCalculator implements Callable<Long> {
		private int[] numbers;
		private int start;
		private int end;

		public SumCalculator(final int[] numbers, int start, int end) {
			this.numbers = numbers;
			this.start = start;
			this.end = end;
		}

		public Long call() throws Exception {
			Long sum = 0l;
			for (int i = start; i < end; i++) {
				sum += numbers[i];
			}
			return sum;
		}
	}

	public ConcurrentCalculator() {
		cpuCoreNumber = Runtime.getRuntime().availableProcessors();
		exec = Executors.newFixedThreadPool(cpuCoreNumber);
	}

	public Long sum(final int[] numbers) {
		// 根据CPU核心个数拆分任务,创建FutureTask并提交到Executor
		for (int i = 0; i < cpuCoreNumber; i++) {
			int increment = numbers.length / cpuCoreNumber + 1;
			int start = increment * i;
			int end = increment * i + increment;
			if (end > numbers.length)
				end = numbers.length;
			SumCalculator subCalc = new SumCalculator(numbers, start, end);
			FutureTask<Long> task = new FutureTask<Long>(subCalc);
			tasks.add(task);
			if (!exec.isShutdown()) {
				exec.submit(task);
			}
		}
		return getResult();
	}

	/**
	 * 迭代每个只任务,获得部分和,相加返回
	 * 
	 * @return
	 */
	public Long getResult() {
		Long result = 0l;
		for (Future<Long> task : tasks) {
			try {
				// 如果计算未完成则阻塞
				Long subSum = task.get();
				result += subSum;
			} catch (InterruptedException e) {
				e.printStackTrace();
			} catch (ExecutionException e) {
				e.printStackTrace();
			}
		}
		return result;
	}

	public void close() {
		exec.shutdown();
	}
}

  

 Main

Java代码   JAVA 1.5 并发之 Executor框架 (内容为转载)
  1. int[] numbers = new int[] { 123456781011 };  
  2. ConcurrentCalculator calc = new ConcurrentCalculator();  
  3. Long sum = calc.sum(numbers);  
  4. System.out.println(sum);  
  5. calc.close();  

 四、CompletionService

在刚在的例子中,getResult()方法的实现过程中,迭代了FutureTask的数组,如果任务还没有完成则当前线程会阻塞,如果我们希望任意字任务完成后就把其结果加到result中,而不用依次等待每个任务完成,可以使CompletionService。生产者submit()执行的任务。使用者take()已完成的任务,并按照完成这些任务的顺序处理它们的结果 。也就是调用CompletionService的take方法是,会返回按完成顺序放回任务的结果,CompletionService内部维护了一个阻塞队列BlockingQueue,如果没有任务完成,take()方法也会阻塞。修改刚才的例子使用CompletionService:

public class ConcurrentCalculator2 {

	private ExecutorService exec;
	private CompletionService<Long> completionService;


	private int cpuCoreNumber;

	// 内部类
	class SumCalculator implements Callable<Long> {
		......
	}

	public ConcurrentCalculator2() {
		cpuCoreNumber = Runtime.getRuntime().availableProcessors();
		exec = Executors.newFixedThreadPool(cpuCoreNumber);
		completionService = new ExecutorCompletionService<Long>(exec);


	}

	public Long sum(final int[] numbers) {
		// 根据CPU核心个数拆分任务,创建FutureTask并提交到Executor
		for (int i = 0; i < cpuCoreNumber; i++) {
			int increment = numbers.length / cpuCoreNumber + 1;
			int start = increment * i;
			int end = increment * i + increment;
			if (end > numbers.length)
				end = numbers.length;
			SumCalculator subCalc = new SumCalculator(numbers, start, end);	
			if (!exec.isShutdown()) {
				completionService.submit(subCalc);


			}
			
		}
		return getResult();
	}

	/**
	 * 迭代每个只任务,获得部分和,相加返回
	 * 
	 * @return
	 */
	public Long getResult() {
		Long result = 0l;
		for (int i = 0; i < cpuCoreNumber; i++) {			
			try {
				Long subSum = completionService.take().get();
				result += subSum;			
			} catch (InterruptedException e) {
				e.printStackTrace();
			} catch (ExecutionException e) {
				e.printStackTrace();
			}
		}
		return result;
	}

	public void close() {
		exec.shutdown();
	}
}

  

因为是转载的我就不加内容了,在联系中有一些小细节写在了下一篇

execute VS submit

http://www.cnblogs.com/rockman12352/p/3789275.html