1.Thread pool创建

ThreadPoolExecutor

  public ThreadPoolExecutor(int corePoolSize,
                            int maximumPoolSize,
                            long keepAliveTime,
                            TimeUnit unit,
                            BlockingQueue<Runnable> workQueue,
                            ThreadFactory threadFactory,
                            RejectedExecutionHandler handler) 

参数解释：
corePoolSize 核心线程池大小
When a new task is submitted in method [execute(Runnable)], and fewer than corePoolSize threads are running, a new thread is created to handle the request, even if other worker threads are idle

maximumPoolSize 线程池最大容量大小
If there are more than corePoolSize but less than maximumPoolSize threads running, a new thread will be created only if the queue is full

BlockingQueue 阻塞队列
Any [BlockingQueue]may be used to transfer and hold submitted tasks
If fewer than corePoolSize threads are running, the Executor always prefers adding a new thread rather than queuing.
If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a new thread.
If a request cannot be queued, a new thread is created unless this would exceed maximumPoolSize, in which case, the task will be rejected.

三个一般的策略

1. 直接交接
   default 实现 -- SynchronousQueue
   特点：不会持有任何task，直接抛给Thread来执行，如果当前没有可用线程，且不可申请新线程，任务会被拒绝、通常为了避免任务拒绝，会指定maximumPoolSize为最大值，避免任务被拒绝。

2.无限队列
default 实现 -- LinkedBlockingQueue
特点：如果所有的corePoolSize都在busy的状态，将会把task入队列，不会创建新的线程，直到有线程空闲

3.有限队列
default 实现 -- ArrayBlockingQueue
特点：队列大小有限，当队列已满，将去创建新的线程，如果已经达到maximumPoolSize，任务会被拒绝

3.类型判断

CPU密集
所谓CPU密集型就是指系统大部分时间是在做程序正常的计算任务，例如数字运算、赋值、分配内存、内存拷贝、循环、查找、排序等，这些处理都需要CPU来完成。

IO密集
IO密集型的话，是指系统大部分时间在跟I/O交互，而这个时间线程不会占用CPU来处理，即在这个时间范围内，可以由其他线程来使用CPU，因而可以多配置一些线程。

混合型
混合型的话，是指两者都占有一定的时间。