Java线程池实现原理

参数配置

核心池大小、最大池大小

 /**
     * Core pool size is the minimum number of workers to keep alive
     * (and not allow to time out etc) unless allowCoreThreadTimeOut
     * is set, in which case the minimum is zero.
     */
    private volatile int corePoolSize;

    /**
     * Maximum pool size. Note that the actual maximum is internally
     * bounded by CAPACITY.
     */
        private volatile int maximumPoolSize;
 /**
     * Timeout in nanoseconds for idle threads waiting for work.
     * Threads use this timeout when there are more than corePoolSize
     * present or if allowCoreThreadTimeOut. Otherwise they wait
     * forever for new work.
     */
    private volatile long keepAliveTime;

    /**
     * If false (default), core threads stay alive even when idle.
     * If true, core threads use keepAliveTime to time out waiting
     * for work.
     */
    private volatile boolean allowCoreThreadTimeOut;

ThreadPoolExecutor根据这两个参数自动分配池大小。当线程数>corePoolSize 时，多于corePoolSize 的线程在超过keepAliveTime时间后，会终止执行。
另外如果设置了allowCoreThreadTimeOut为true，核心线程在空闲时间超过keepAliveTime 后终止执行。

线程分配流程：

• 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.

---

1. 如果有新请求提交到线程池，且运行中的线程数

2. 如果corePoolSize<=运行线程数,请求被放入队列。

3. 如果corePoolSize<运行线程

4. 如果运行线程达到maximumPoolSize且队列已满，将会执行rejectedExecution方法。

执行过程示意图

队列实现策略

1. SynchronousQueue：如果线程池中没有可用线程，将会创建一个新的线程。maximumPoolSize无限大，以防止提交的任务达到maximumPoolSize大小后被拒绝执行。

2. LinkedBlockingQueue：基于链表结构的阻塞队列，如果corePoolSize全部线程用完，在队列中等待，不会创建新线程，适合彼此独立的任务。

3. ArrayBlockingQueue：配合有限的maximumPoolSizes，防止资源用尽，但难以协调控制。队列的大小和maximumPoolSizes需要权衡。

线程池的饱和策略

1.AbortPolicy
默认实现，直接抛出异常

    /**
     * A handler for rejected tasks that throws a
     * {@code RejectedExecutionException}.
     */
    public static class AbortPolicy implements RejectedExecutionHandler {
        /**
         * Creates an {@code AbortPolicy}.
         */
        public AbortPolicy() { }

        /**
         * Always throws RejectedExecutionException.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         * @throws RejectedExecutionException always
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            throw new RejectedExecutionException("Task " + r.toString() +
                                                 " rejected from " +
                                                 e.toString());
        }
    }

2.CallerRunsPolicy
executor调用线程直接执行被拒绝的任务（Runnable），除非调用线程executor被关闭，此时任务被丢弃。

 /**
     * A handler for rejected tasks that runs the rejected task
     * directly in the calling thread of the {@code execute} method,
     * unless the executor has been shut down, in which case the task
     * is discarded.
     */
    public static class CallerRunsPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code CallerRunsPolicy}.
         */
        public CallerRunsPolicy() { }

        /**
         * Executes task r in the caller's thread, unless the executor
         * has been shut down, in which case the task is discarded.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                r.run();
            }
        }
    }

3.DiscardPolicy
直接丢弃被拒绝的任务,不做任何处理.

  /**
     * A handler for rejected tasks that silently discards the
     * rejected task.
     */
    public static class DiscardPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code DiscardPolicy}.
         */
        public DiscardPolicy() { }

        /**
         * Does nothing, which has the effect of discarding task r.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        }
    }

4.DiscardOldestPolicy
从队列中丢弃被拒绝的请求，executor调用线程执行被拒绝的任务（Runnable），除非调用线程executor被关闭，此时任务被丢弃。

 /**
     * A handler for rejected tasks that discards the oldest unhandled
     * request and then retries {@code execute}, unless the executor
     * is shut down, in which case the task is discarded.
     */
    public static class DiscardOldestPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code DiscardOldestPolicy} for the given executor.
         */
        public DiscardOldestPolicy() { }

        /**
         * Obtains and ignores the next task that the executor
         * would otherwise execute, if one is immediately available,
         * and then retries execution of task r, unless the executor
         * is shut down, in which case task r is instead discarded.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                e.getQueue().poll();
                e.execute(r);
            }
        }
    }