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。

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

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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