• 转: java并发编程Executor框架


    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:

    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

    java.util.concurrent.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

    int[] numbers = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 10, 11 };
    ConcurrentCalculator calc = new ConcurrentCalculator();
    Long sum = calc.sum(numbers);
    System.out.println(sum);
    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();
        }
    }

     

     

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  • 原文地址:https://www.cnblogs.com/vigarbuaa/p/2791084.html
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