多线程线程池

Posted 2021-05-29 飞翔在天

参考：JDK文档

ThreadPoolExecutor

1. ThreadPoolExecutor实现

• fixed

• single

• cache类型阻塞队列区别，有界还是无界队列

2. ThreadPoolExecutor

　　When a new task is submitted in method execute(Runnable)：

　　if fewer than corePoolSize threads are running, a new thread is created to handle the request, even if other worker threads are idle.

　　// 场景：当前线程数少于corePoolSize，创建新线程（尽管有其他工作线程是空闲状态）

　　Else if fewer than maximumPoolSize threads are running, a new thread will be created to handle the request only if the queue is full.

　　// 场景：线程数大于corePoolSize但小于maxinum，仅当队列满时才创建新的线程 

2.1 几个参数

1. ThreadFactory: 定义线程组名称，线程优先级和daemon参数

2. Keep-alive：If the pool currently has more than corePoolSize threads, excess threads will be terminated if they have been idle for more than the keepAliveTime // 线程池数大于corePoolSize，如果线程空闲时间大于keepAliveTime将被终止。

3. BlockingQueue： transfer and hold submitted tasks

• • If fewer than corePoolSize threads are running, the Executor always prefers adding a new thread rather than queuing.

　　　　　   // 线程数量小于corePoolSize，直接新增线程执行任务

• • If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a new thread.

    // 大于等于corePoolSize时，新来的任务将被排队

  • If a request cannot be queued, a new thread is created unless this would exceed maximumPoolSize, in which case, the task will be rejected.

　　　　　  // 请求不能被入队（队列满）时，如果线程数未超过maximum，线程将被创建。否则将被拒绝

　　　　排队的几种策略：

• • Direct handoffs直接传递：A good default choice for a work queue is a SynchronousQueue that hands off tasks to threads without otherwise holding them.

• • Unbounded queues：

  • Bounded queues：