一.两种线程池

线程池成员有ThreadPoolExecutor与ScheduledThreadPoolExecutor

1.ThreadPoolExecutor

ThreadPoolExecutor有3种调用的最终实例化方法都是一个,但参数差异较大

1.1 FixedThreadPool

适用于为了满足资源管理的需求而需要限制当前线程数量的应用场景适用于负载较重的服务器

      //不传ThreadFactory() 默认是Executors.defaultThreadFactory()
      ExecutorService fixedService = Executors.newFixedThreadPool(10);
      ThreadPoolExecutor fixedExecutor = new ThreadPoolExecutor(10, 10,
              0L, TimeUnit.MILLISECONDS,
              new LinkedBlockingQueue<Runnable>());
      //传完整参数 线程工厂 和reject异常处理
      ThreadPoolExecutor fullParams = new ThreadPoolExecutor(10, 10,
              0L, TimeUnit.MILLISECONDS,
              new LinkedBlockingQueue<Runnable>(), Executors.privilegedThreadFactory(),
              new RejectedExecutionHandler() {
                  public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
                      System.out.println("rejectedExecution发生");
                  }
              });

1.2 SingleThreadExecutor

创建使用单个线程,使用于需要保证顺序执行各个任务并且在任意时间点不会有多线程活动适用于单线程场景，如某个服务异步启动
SingleThreadExecutor的初始化通过FinalizableDelegatedExecutorService包装
FinalizableDelegatedExecutorService对象就是对代理对象DelegatedExecutorService又包了一层确保垃圾回收销毁线程。

 ExecutorService singleExecutor = Executors.newSingleThreadExecutor();
//方法实现
//这里是LinkedBlockingQueue
   public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }

1.3 CachedThreadPool

大小无界的线程池适用于执行很多短期异步任务的小程序或者是负载较轻的服务器
CachedThreadPool 使用没有容量的SynchronousQueue 当主线程提交任务的速度高于线程池处理任务的速度时候由于CachedThreadPool会不断创建新的线程有可能会耗尽内存资源

ExecutorService cachedExecutor = Executors.newCachedThreadPool();
//方法实现
//这里用 SynchronousQueue实现
public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }