Jdk1.6 JUC源码解析(20)-Executors

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Jdk1.6 JUC源码解析(20)-Executors

作者：大飞

 

功能简介：

• Executors是JUC包提供的一个工具性质的帮助类，它针对ExecutorService、ScheduledExecutorService、ThreadFactory和Callable提供了一系列工厂方法和工具方法。

源码分析：

• 首先看下针对ExecutorService提供的一些工厂方法：

 

    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue());
    }

       通过之前文章中对ThreadPoolExecutor的分析可知：

 

              1.这个方法创建了一个核心线程数量和最大线程数量一致的，并且任务队列是无界队列的线程池。

              2.由于默认核心线程不会超时，所以超时相关的参数也没有意义。

              3.如果在线程关闭之前，一个工作线程由于某种原因挂了，那么线程池会自动补上一个新的工作线程。

 

    public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue(),
                                      threadFactory);

       除了能定制ThreadFactory之外，和上个方法一样。

 

    public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue()));
    }

       这个工厂方法看上去有点类似newFixedThreadPool(1) ，但有一点儿区别，这个不能重新调整配置(比如动态增大核心线程数量)了，由于方法内返回的不是ThreadPoolExecutor实例，而是一个包装类：

    static class FinalizableDelegatedExecutorService
	extends DelegatedExecutorService {
	    FinalizableDelegatedExecutorService(ExecutorService executor) {
	        super(executor);
	    }
	    protected void finalize()  {
	        super.shutdown();//被垃圾回收时，关闭线程池。
	    }
    }
    /**
     * 包装类，方法代理到内部的ExecutorService，只暴漏ExecutorService定义的方法。
     */
    static class DelegatedExecutorService extends AbstractExecutorService {
        private final ExecutorService e;
        DelegatedExecutorService(ExecutorService executor) { e = executor; }
        public void execute(Runnable command) { e.execute(command); }
        public void shutdown() { e.shutdown(); }
        public List shutdownNow() { return e.shutdownNow(); }
        public boolean isShutdown() { return e.isShutdown(); }
        public boolean isTerminated() { return e.isTerminated(); }
        public boolean awaitTermination(long timeout, TimeUnit unit)
            throws InterruptedException {
            return e.awaitTermination(timeout, unit);
        }
        public Future submit(Runnable task) {
            return e.submit(task);
        }
        public  Future submit(Callable task) {
            return e.submit(task);
        }
        public  Future submit(Runnable task, T result) {
            return e.submit(task, result);
        }
        public  List> invokeAll(Collection> tasks)
            throws InterruptedException {
            return e.invokeAll(tasks);
        }
        public  List> invokeAll(Collection> tasks,
                                             long timeout, TimeUnit unit)
            throws InterruptedException {
            return e.invokeAll(tasks, timeout, unit);
        }
        public  T invokeAny(Collection> tasks)
            throws InterruptedException, ExecutionException {
            return e.invokeAny(tasks);
        }
        public  T invokeAny(Collection> tasks,
                               long timeout, TimeUnit unit)
            throws InterruptedException, ExecutionException, TimeoutException {
            return e.invokeAny(tasks, timeout, unit);
        }
    }

 

    public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue(),
                                    threadFactory));
    }

       除了能定制ThreadFactory之外，和上个方法一样。

 

    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue());
    }

       通过之前文章中对ThreadPoolExecutor的分析可知：

              1.这个方法创建了一个核心线程数量为0，最大线程(可以认为)无上限，并且任务队列是同步队列(无实际容量)的线程池。

              2.针对每一个新任务，如果当前没有空闲线程，都会创建一个新的工作线程来处理任务。工作线程默认空闲超过60秒超时被回收。

 

    public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue(),
                                      threadFactory);

       除了能定制ThreadFactory之外，和上个方法一样。

 

    public static ExecutorService unconfigurableExecutorService(ExecutorService executor) {
        if (executor == null)
            throw new NullPointerException();
        return new DelegatedExecutorService(executor);
    }

       DelegatedExecutorService这面已经看到过，这个方法就相当于将一个ExecutorService包装成一个不可配置的ExecutorService。

 

• 继续看下针对ScheduledExecutorService提供的一些工厂方法：

    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }

    public static ScheduledExecutorService newScheduledThreadPool(
            int corePoolSize, ThreadFactory threadFactory) {
        return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
    }

       通过之前文章中对ScheduledThreadPoolExecutor的分析可知：

              1.这个方法创建了一个给定(核心)线程数量的ScheduledThreadPoolExecutor(由于其内部的任务队列是无界的，所以尽管继承自ThreadPoolExecutor，但最大线程数量无意义)。

 

    public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
        return new DelegatedScheduledExecutorService
            (new ScheduledThreadPoolExecutor(1));
    }

    public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {
        return new DelegatedScheduledExecutorService
            (new ScheduledThreadPoolExecutor(1, threadFactory));
    }

        类似newScheduledThreadPool(1)  (这个其实看起来更像是一个加强版的Timer)，但不能调整配置：

    static class DelegatedScheduledExecutorService
            extends DelegatedExecutorService
            implements ScheduledExecutorService {
        private final ScheduledExecutorService e;
        DelegatedScheduledExecutorService(ScheduledExecutorService executor) {
            super(executor);
            e = executor;
        }
        public ScheduledFuture schedule(Runnable command, long delay,  TimeUnit unit) {
            return e.schedule(command, delay, unit);
        }
        public  ScheduledFuture schedule(Callable callable, long delay, TimeUnit unit) {
            return e.schedule(callable, delay, unit);
        }
        public ScheduledFuture scheduleAtFixedRate(Runnable command, long initialDelay,  long period, TimeUnit unit) {
            return e.scheduleAtFixedRate(command, initialDelay, period, unit);
        }
        public ScheduledFuture scheduleWithFixedDelay(Runnable command, long initialDelay,  long delay, TimeUnit unit) {
            return e.scheduleWithFixedDelay(command, initialDelay, delay, unit);
        }
    }

 

    public static ScheduledExecutorService unconfigurableScheduledExecutorService(ScheduledExecutorService executor) {
        if (executor == null)
            throw new NullPointerException();
        return new DelegatedScheduledExecutorService(executor);
    }

       相当于将一个ScheduledExecutorService包装成一个不可配置的ScheduledExecutorService。

 

• 再看下针对ThreadFactory提供的一些工厂方法：

    public static ThreadFactory defaultThreadFactory() {
        return new DefaultThreadFactory();
    }

       返回一个DefaultThreadFactory实例。在创建ThreadPoolExecutor和ScheduledThreadPoolExecutor时如果没有显式指定ThreadFactory，会默认使用这个，看下实现：

    static class DefaultThreadFactory implements ThreadFactory {
        static final AtomicInteger poolNumber = new AtomicInteger(1);
        final ThreadGroup group;
        final AtomicInteger threadNumber = new AtomicInteger(1);
        final String namePrefix;
        DefaultThreadFactory() {
            SecurityManager s = System.getSecurityManager();
            group = (s != null)? s.getThreadGroup() :
                                 Thread.currentThread().getThreadGroup();
            namePrefix = "pool-" +
                          poolNumber.getAndIncrement() +
                         "-thread-";
        }
        public Thread newThread(Runnable r) {
            Thread t = new Thread(group, r,
                                  namePrefix + threadNumber.getAndIncrement(),
                                  0);
            if (t.isDaemon())
                t.setDaemon(false);
            if (t.getPriority() != Thread.NORM_PRIORITY)
                t.setPriority(Thread.NORM_PRIORITY);
            return t;
        }
    }

 

    public static ThreadFactory privilegedThreadFactory() {
        return new PrivilegedThreadFactory();
    }
    static class PrivilegedThreadFactory extends DefaultThreadFactory {
        private final ClassLoader ccl;
        private final AccessControlContext acc;
        PrivilegedThreadFactory() {
            super();
            this.ccl = Thread.currentThread().getContextClassLoader();
            this.acc = AccessController.getContext();
            acc.checkPermission(new RuntimePermission("setContextClassLoader"));
        }
        public Thread newThread(final Runnable r) {
            return super.newThread(new Runnable() {
                public void run() {
                    AccessController.doPrivileged(new PrivilegedAction