Executor框架是指java5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:
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。
Executor:一个接口,其定义了一个接收Runnable对象的方法executor,其方法签名为executor(Runnable command),
ExecutorService:是一个比Executor使用更广泛的子类接口,其提供了生命周期管理的方法,以及可跟踪一个或多个异步任务执行状况返回Future的方法
AbstractExecutorService:ExecutorService执行方法的默认实现
ScheduledExecutorService:一个可定时调度任务的接口
ScheduledThreadPoolExecutor:ScheduledExecutorService的实现,一个可定时调度任务的线程池
ThreadPoolExecutor:线程池,可以通过调用Executors以下静态工厂方法来创建线程池并返回一个ExecutorService对象
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。
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Executor executor = Executors.newFixedThreadPool(10);
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Runnable task = new Runnable() {
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@Override
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public void run() {
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System.out.println("task over");
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}
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};
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executor.execute(task);
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executor = Executors.newScheduledThreadPool(10);
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ScheduledExecutorService scheduler = (ScheduledExecutorService) executor;
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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。
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ExecutorService executorService = (ExecutorService) executor;
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while (!executorService.isShutdown()) {
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try {
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executorService.execute(task);
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} catch (RejectedExecutionException ignored) {
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}
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}
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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。
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Callable<Integer> func = new Callable<Integer>(){
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public Integer call() throws Exception {
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System.out.println("inside callable");
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Thread.sleep(1000);
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return new Integer(8);
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}
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};
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FutureTask<Integer> futureTask = new FutureTask<Integer>(func);
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Thread newThread = new Thread(futureTask);
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newThread.start();
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try {
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System.out.println("blocking here");
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Integer result = futureTask.get();
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System.out.println(result);
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} catch (InterruptedException ignored) {
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} catch (ExecutionException ignored) {
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}
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。
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package executorservice;
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import java.util.ArrayList;
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import java.util.List;
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import java.util.concurrent.Callable;
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import java.util.concurrent.ExecutionException;
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import java.util.concurrent.ExecutorService;
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import java.util.concurrent.Executors;
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import java.util.concurrent.Future;
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import java.util.concurrent.FutureTask;
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public class ConcurrentCalculator {
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private ExecutorService exec;
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private int cpuCoreNumber;
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private List<Future<Long>> tasks = new ArrayList<Future<Long>>();
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// 内部类
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class SumCalculator implements Callable<Long> {
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private int[] numbers;
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private int start;
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private int end;
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public SumCalculator(final int[] numbers, int start, int end) {
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this.numbers = numbers;
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this.start = start;
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this.end = end;
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}
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public Long call() throws Exception {
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Long sum = 0l;
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for (int i = start; i < end; i++) {
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sum += numbers[i];
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}
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return sum;
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}
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}
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public ConcurrentCalculator() {
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cpuCoreNumber = Runtime.getRuntime().availableProcessors();
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exec = Executors.newFixedThreadPool(cpuCoreNumber);
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}
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public Long sum(final int[] numbers) {
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// 根据CPU核心个数拆分任务,创建FutureTask并提交到Executor
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for (int i = 0; i < cpuCoreNumber; i++) {
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int increment = numbers.length / cpuCoreNumber + 1;
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int start = increment * i;
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int end = increment * i + increment;
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if (end > numbers.length)
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end = numbers.length;
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SumCalculator subCalc = new SumCalculator(numbers, start, end);
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FutureTask<Long> task = new FutureTask<Long>(subCalc);
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tasks.add(task);
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if (!exec.isShutdown()) {
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exec.submit(task);
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}
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}
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return getResult();
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}
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/**
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* 迭代每个只任务,获得部分和,相加返回
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*
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* @return
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*/
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public Long getResult() {
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Long result = 0l;
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for (Future<Long> task : tasks) {
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try {
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// 如果计算未完成则阻塞
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Long subSum = task.get();
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result += subSum;
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} catch (InterruptedException e) {
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e.printStackTrace();
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} catch (ExecutionException e) {
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e.printStackTrace();
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}
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}
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return result;
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}
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public void close() {
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exec.shutdown();
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}
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}
Main
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int[] numbers = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 10, 11 };
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ConcurrentCalculator calc = new ConcurrentCalculator();
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Long sum = calc.sum(numbers);
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System.out.println(sum);
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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:
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public class ConcurrentCalculator2 {
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private ExecutorService exec;
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private CompletionService<Long> completionService;
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private int cpuCoreNumber;
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// 内部类
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class SumCalculator implements Callable<Long> {
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......
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}
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public ConcurrentCalculator2() {
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cpuCoreNumber = Runtime.getRuntime().availableProcessors();
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exec = Executors.newFixedThreadPool(cpuCoreNumber);
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completionService = new ExecutorCompletionService<Long>(exec);
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}
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public Long sum(final int[] numbers) {
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// 根据CPU核心个数拆分任务,创建FutureTask并提交到Executor
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for (int i = 0; i < cpuCoreNumber; i++) {
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int increment = numbers.length / cpuCoreNumber + 1;
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int start = increment * i;
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int end = increment * i + increment;
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if (end > numbers.length)
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end = numbers.length;
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SumCalculator subCalc = new SumCalculator(numbers, start, end);
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if (!exec.isShutdown()) {
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completionService.submit(subCalc);
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}
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}
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return getResult();
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}
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/**
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* 迭代每个只任务,获得部分和,相加返回
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*
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* @return
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*/
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public Long getResult() {
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Long result = 0l;
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for (int i = 0; i < cpuCoreNumber; i++) {
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try {
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Long subSum = completionService.take().get();
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result += subSum;
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} catch (InterruptedException e) {
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e.printStackTrace();
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} catch (ExecutionException e) {
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e.printStackTrace();
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}
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}
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return result;
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}
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public void close() {
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exec.shutdown();
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}
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}