java线程池
是什么
线程池是一种多线程处理形式，处理过程中将任务添加到队列，然后在创建线程后自动启动这些任务。
解决问题
- 降低资源的消耗（重复利用）
- 提高响应速度 （并行执行）
- 提高线程的可关联性
组成部分
corePoolSize 线程池的基本大小
maximumPoolSize 线程池最大数量
keepAliveTime 线程活动保持的时间
-
taskQueue
ArrayBlockingQueue 基于数据数据结构的有界阻塞队列
LinkedBlockingQueue 链表的阻塞队列
SynchronousQueue 不存储元素的阻塞队列
PriorityBlockingQueue 优先级的无限队列
ThreadFactory 创建线程工厂
-
RejectExecutionHandler
AbortPolicy 直接抛异常
CallerRunsPolicy 只用调用者所在线程来运行任务
DiscardPolicy 丢弃队列里最近一个任务，并执行当前任务
DiscardOldestPolicy 不处理，丢失掉
如何使用
- 创建线程池
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
- 执行
public void execute(Runnable command) {
.....
}
工作流程
执行流程
屏幕快照 2017-12-24 下午2.33.52.png
转载《java并行编程的艺术》一张图（这张图非常详细）
592290-20161214224720245-477638873.png
- 提交一个新任务
- 判断核心线程池是否执行任务，如果不是，则创建新线程来执行任务
- 如果线程池都在执行，则判断队列是否满，如果没有满，则将任务放到队列中
- 如果队列满了，并且没有线程池线程个数小于最大个数，则创建新的线程执行任务
- 如果队列满了，并且也达到了线程池的最大个数，则走线程池的拒绝策略
其他
线程池的监控
目标：方便查看线程池的状态
-
参数
taskCount：线程池需要执行的任务数量
completedTaskCount:线程池在运行过程中已完成的任务数量
largestPoolSize线程池里曾经创建过最大的线程数量
getPoolSize 线程池的线程数量
getActiveCount 获取活动的线程数
-
自定义实现线程池监控
- 通过继承线程池来自定义线程池，然后重写beforeExecute、afterExecute、terminated方法
附
思维导图
java线程池.png
参考资料
《java并行编程的艺术》
