ThreadPoolExecutor源码历险-拒绝策略静态内部类

jdk提供的线程池拒绝策略都实现了同一个接口：RejectedExecutionHandler

DiscardPolicy 丢弃任务

 public static class DiscardPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code DiscardPolicy}.
         */
        public DiscardPolicy() { }

        /**
          * 什么也不做就是丢弃任务了
         * 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) {
        }
    }

CallerRunsPolicy 由调用exectue方法的线程处理该任务

 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();
            }
        }
    }

DiscardOldestPolicy 丢弃队头任务（最老的那个），重新尝试执行任务

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()) {
               // 获取工作队列workQueue，并且删除队头任务
               e.getQueue().poll();
               // 重新执行任务
               e.execute(r);
           }
       }
   }

拒绝任务，抛出一个RejectedExecutionException异常

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());
       }
   }