Executors.newCachedThreadPool 源码解析
Executors 还有个常用静态方法newCachedThreadPool(),来构造线程池
今天我们其源码实现,探一探究竟
//底层还是调用ThreadPoolExecutor,不过参数有变化
//corePoolSize 竟然为0,maximumPoolSize为默认的最大值
//当任务队列满时,就会判断maximumPoolSize大小
//keepAliveTime 空闲线程的最大等待时间,,60s后立马销毁线程了
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
SynchronousQueue
注意这个队列
A {@linkplain BlockingQueue blocking queue} in which each insert
* operation must wait for a corresponding remove operation by another
* thread, and vice versa. A synchronous queue does not have any
* internal capacity, not even a capacity of one.
- SynchronousQueue,实际上它不是一个真正的队列,因为它不会为队列中元素维护存储空间。与其他队列不同的是,它维护一组线程,这些线程在等待着把元素加入或移出队列。
- 在使用SynchronousQueue作为工作队列的前提下,客户端代码向线程池提交任务时,而线程池中又没有空闲的线程能够从SynchronousQueue队列实例中取一个任务,那么相应的offer方法调用就会失败(即任务没有被存入工作队列)。此时,ThreadPoolExecutor会新建一个新的工作者线程用于对这个入队列失败的任务进行处理(假设此时线程池的大小还未达到其最大线程池大小maximumPoolSize)。
newFixedThreadPool 和 newCachedThreadPool最大差别就是 队列,线程回收的时间
newFixedThreadPool 应用场景
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 <tt>execute</tt> 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.
newFixedThreadPool 线程池的数量是不确定的,可以无限大。
它比较适合处理执行时间比较小的任务