线程池的RejectedExecutionHandler(拒绝策略)

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  • CallerRunsPolicy
    该策略既不抛弃任务,也不抛出异常。就是任务添加到线程池失败,那么主线程会自己去执行该任务
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();
            }
        }
    }
  • AbortPolicy
          该策略是默认的拒绝策略,该策略将抛出未检查的RejectedExecutionException,我们可以捕获这个异常,然后根据需求做相应的处理
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());
        }
    }
  • DiscardPolicy
          当新提交的任务无法保存到队列中等待执行时,该策略会直接丢掉这个任务并且不会有任何异常,源码如下,实际就是对线程不执行操作
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) {
        }
    }
  • DiscardOldestPolicy
          字面意思就是丢弃最旧的,该策略会抛弃下一个将被执行的任务,然后尝试重新提交新的任务(如果工作队列是一个优先级队列,那么“抛弃最旧”,就意味着丢弃优先级最高的任务,因此最好不要将此策略和优先级队列放在一起使用)
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);
            }
        }
    }

从以上4中拒绝策略看,我们想自定义拒绝策略只需要实现RejectedExecutionHandler接口,并且实现rejectedExecution方法即可。