Java 线程池详解学习：FixedThreadPool，CachedThreadPool，ScheduledThreadPool...

Java常用的线程池有FixedThreadPool和CachedThreadPool，我们可以通过查看他们的源码来进行学习。

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Java的源码下载参考这篇文章：Java源码下载和阅读（JDK1.8) - zhangpeterx的博客

在源码的目录java/util/concurrent下找到Executors.java文件查看源码。
里面提到的封装类有：
newScheduledThreadPool，newSingleThreadScheduledExecutor，newCachedThreadPool，newSingleThreadExecutor，newFixedThreadPool，newWorkStealingPool，newFixedThreadPool

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我这里只把newFixedThreadPool，newCachedThreadPool的源码放出来：

public class Executors {

    /**
     * Creates a thread pool that reuses a fixed number of threads
     * operating off a shared unbounded queue.  At any point, at most
     * {@code nThreads} threads will be active processing tasks.
     * If additional tasks are submitted when all threads are active,
     * they will wait in the queue until a thread is available.
     * If any thread terminates due to a failure during execution
     * prior to shutdown, a new one will take its place if needed to
     * execute subsequent tasks.  The threads in the pool will exist
     * until it is explicitly {@link ExecutorService#shutdown shutdown}.
     *
     * @param nThreads the number of threads in the pool
     * @return the newly created thread pool
     * @throws IllegalArgumentException if {@code nThreads <= 0}
     */
    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }
    
    /**
     * Creates a thread pool that reuses a fixed number of threads
     * operating off a shared unbounded queue, using the provided
     * ThreadFactory to create new threads when needed.  At any point,
     * at most {@code nThreads} threads will be active processing
     * tasks.  If additional tasks are submitted when all threads are
     * active, they will wait in the queue until a thread is
     * available.  If any thread terminates due to a failure during
     * execution prior to shutdown, a new one will take its place if
     * needed to execute subsequent tasks.  The threads in the pool will
     * exist until it is explicitly {@link ExecutorService#shutdown
     * shutdown}.
     *
     * @param nThreads the number of threads in the pool
     * @param threadFactory the factory to use when creating new threads
     * @return the newly created thread pool
     * @throws NullPointerException if threadFactory is null
     * @throws IllegalArgumentException if {@code nThreads <= 0}
     */
    public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>(),
                                      threadFactory);
    }

    /**
     * Creates a thread pool that creates new threads as needed, but
     * will reuse previously constructed threads when they are
     * available.  These pools will typically improve the performance
     * of programs that execute many short-lived asynchronous tasks.
     * Calls to {@code execute} will reuse previously constructed
     * threads if available. If no existing thread is available, a new
     * thread will be created and added to the pool. Threads that have
     * not been used for sixty seconds are terminated and removed from
     * the cache. Thus, a pool that remains idle for long enough will
     * not consume any resources. Note that pools with similar
     * properties but different details (for example, timeout parameters)
     * may be created using {@link ThreadPoolExecutor} constructors.
     *
     * @return the newly created thread pool
     */
    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

    /**
     * Creates a thread pool that creates new threads as needed, but
     * will reuse previously constructed threads when they are
     * available, and uses the provided
     * ThreadFactory to create new threads when needed.
     * @param threadFactory the factory to use when creating new threads
     * @return the newly created thread pool
     * @throws NullPointerException if threadFactory is null
     */
    public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>(),
                                      threadFactory);
    }

上面的代码说明了newFixedThreadPool，newCachedThreadPool只是把ThreadPoolExecutor进行了封装而已，所以建议直接使用ThreadPoolExecutor而不是这些封装类。

阿里巴巴代码规范中也提到这件事：

【强制】线程池不允许使用 Executors 去创建，而是通过 ThreadPoolExecutor 的方式，这样
的处理方式让写的同学更加明确线程池的运行规则，规避资源耗尽的风险。
说明：Executors 返回的线程池对象的弊端如下：
1）FixedThreadPool 和 SingleThreadPool:
允许的请求队列长度为 Integer.MAX_VALUE，可能会堆积大量的请求，从而导致 OOM。
2）CachedThreadPool 和 ScheduledThreadPool:
允许的创建线程数量为 Integer.MAX_VALUE，可能会创建大量的线程，从而导致 OOM。

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