Android多线程(Executor线程池篇)

【齐天的博客】转载请注明出处（万分感谢！）：
https://blog.youkuaiyun.com/qijinglai/article/details/80750088

关联文章:
Android多线程(Handler篇)
Android多线程(AsyncTask篇)
Android多线程(HandlerThread篇)
Android多线程(IntentService篇)

前言

在Android多线程(AsyncTask篇)中提到可以通过自定义Executor来实现并行，这里涉及到线程池的概念。
Executor是Java中的概念，是一个接口，真正的线程池实现是ThreadPoolExecutor。它提供了一系列的参数来配置不同的线程池。按功能来分Android线程池主要分4类，均通过Executors提供的工厂方法得到。
由于Android的线程池都是直接或间接通过配置ThreadPoolExecutor得到的，所以先来看一下ThreadPoolExecutor。

ThreadPoolExecutor

ThreadPoolExecutor是线程池的真正实现，他的构造方法提供了一系列参数来配置线程池，下面先介绍一下其构造方法中的参数的含义。
这是一个很常用的构造方法：

public ThreadPoolExecutor(
            int corePoolSize,//核心线程数
            int maximumPoolSize,//最大线程数
            long keepAliveTime, //非核心线程的超时时间
            TimeUnit unit,//单位
            BlockingQueue<Runnable> workQueue,//任务队列 
            ThreadFactory threadFactory//线程工厂
            )

• corePoolSize
  核心线程数，核心线程池默认会在线程池中一直存活，无论它是不是闲置。如果将ThreadPoolExecute的allowCoreThreadTimeOut设置为true，那么闲置的核心线程会执行超时策略，这个超时策略的时间间隔是由keepAliveTime所指定的。
• maximumPoolSize
  线程池所能容纳的最大线程数，当活动的线程数达到这个数值后，后续的任务会被阻塞。
• keepAliveTime
  非核心线程池的超时时长，非核心线程池闲置的时间超过这个时间就会被回收。当ThreadPoolExecute的allowCoreThreadTimeOut设置为true时，它同样也作用于核心线程。
• unit
  用于指定keepAliveTime的单位。
• workQueue
  线程池中的任务队列，通过线程池的execute方法提交的Runnable对象会储存在这个参数中。

• threadFactory
  线程工厂，为线程池提供创建新线程的功能，ThreadFactory是一个接口，他只有一个方法：

  public interface ThreadFactory {
      Thread newThread(Runnable var1);
  }

  除了这些参数外还有个很不常用的参数RejectedExecutionHandler handler。当线程池无法执行新任务时(任务队列满了或者无法成功执行)会调用handler的rejectExecutionException。

执行规则

AsyncTask的线程池配置

private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE_SECONDS = 30;
private static final BlockingQueue<Runnable> sPoolWorkQueue =
            new LinkedBlockingQueue<Runnable>(128);
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger mCount = new AtomicInteger(1);

        public Thread newThread(Runnable r) {
            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
        }
    };


public static final Executor THREAD_POOL_EXECUTOR;

    static {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
                sPoolWorkQueue, sThreadFactory);
        threadPoolExecutor.allowCoreThreadTimeOut(true);
        THREAD_POOL_EXECUTOR = threadPoolExecutor;
    }

在我的Android多线程(AsyncTask篇)这一篇中可以知道AsyncTask配置了THREAD_POOL_EXECUTOR这个线程池，由上面的代码可以知道此线程池的规格：

• 核心线程数=((CPU核心数-1)和4比取小)后和2比取大
• 最大线程数=2*CPU核心数+1
• 核心线程无超时，非核心线程超时时间为30秒
• 任务队列容量=128

线程池分类

这里介绍4中常见的线程池

1. FixedThreadPool
   通过Executors.newFixedThreadPool创建
   ①只有核心线程且数量固定
   ②没有超时机制
   ③队列没大小限制

   /**
    * 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>());
   }
   

2. CachedThreadPool
   通过Executors.newFixedThreadPool创建
   ①只有非核心线程，且数量不定。
   ②最大线程数Integer.MAX_VALUE，最多65535个线程
   ③超时时间60s
   他会立即执行任何任务，适用于大量耗时较少的操作

   /**
    * 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>());
   }
   

3. ScheduledThreadPool
   通过Executors.newScheduledThreadPool来创建
   ①核心线程数固定
   ②非核心线程没限制
   ③非核心线程闲置时立即回收
   用于执行定时任务和有固定周期的任务

   /**
    * Creates a thread pool that can schedule commands to run after a
    * given delay, or to execute periodically.
    * @param corePoolSize the number of threads to keep in the pool,
    * even if they are idle
    * @return a newly created scheduled thread pool
    * @throws IllegalArgumentException if {@code corePoolSize < 0}
    */
   public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
       return new ScheduledThreadPoolExecutor(corePoolSize);
   }

4. SingleThreadExecutor
   通过Executors.newSingleThreadExecutor来创建
   ①一个核心线程
   同意外界任务进来按顺序执行

   /**
    * Creates an Executor that uses a single worker thread operating
    * off an unbounded queue. (Note however that if this single
    * thread terminates due to a failure during execution prior to
    * shutdown, a new one will take its place if needed to execute
    * subsequent tasks.)  Tasks are guaranteed to execute
    * sequentially, and no more than one task will be active at any
    * given time. Unlike the otherwise equivalent
    * {@code newFixedThreadPool(1)} the returned executor is
    * guaranteed not to be reconfigurable to use additional threads.
    *
    * @return the newly created single-threaded Executor
    */
   public static ExecutorService newSingleThreadExecutor() {
       return new FinalizableDelegatedExecutorService
           (new ThreadPoolExecutor(1, 1,
                                   0L, TimeUnit.MILLISECONDS,
                                   new LinkedBlockingQueue<Runnable>()));
   }

线程池的好处

除了前面介绍的配置好的线程池之外，我们也可以根据需求自己配置线程池，就写到这吧，最后总结下线程池的好处。

1. 重用线程池中的线程，避免因线程的创建和销毁所带来的性能上的开销；
2. 能有效地控制线程池的最大并发数，避免大量的线程之间互相抢占资源而导致阻塞；
3. 能够对线程进行简单的管理，并提供定时执行以及间隔循环执行等功能。

本文参考《Android开发艺术探索》第十一章内容