一、JAVA中的线程池
线程池的实现原理及流程如下图所示：
如上图所示，当一个线程提交到线程池时（execute()或submit()），先判断核心线程数（corePoolSize）是否已满，如果未满，则直接创建线程执行任务；如果已满，则判断队列（BlockingQueue）是否已满，如果未满，则将线程添加到队列中；如果已满，则判断线程池（maximumPoolSize）是否已满，如果未满，则创建线程池执行任务；如果线程池已满，则交给饱和策略（RejectedExecutionHandler.rejectExcution()）来处理。
可以看下线程池ThreadPoolExecutor的全参构造函数源码：
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.acc = System.getSecurityManager() == null ? null : AccessController.getContext(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler; }