Java自定义拒绝策略与线程池

1、拒绝策略

ThreadPoolExcetor 的最后一个参数指定了拒绝策略，JDK提供了四种拒绝策略：
AbortPolicy 策略、CallerRunsPolicy策略、 DiscardOledestPolicy策略、DiscardPolicy策略。

AbortPolicy策略：该策略会直接抛出异常，阻止系统正常工作。

CallerRunsPolicy 策略：只要线程池未关闭，该策略直接在调用者线程中，运行当前的被丢弃的任务。

DiscardOleddestPolicy策略： 该策略将丢弃最老的一个请求，也就是即将被执行的任务，并尝试再次提交当前任务。

DiscardPolicy策略：该策略默默的丢弃无法处理的任务，不予任何处理。

这种策略和上一种，实际上正好是相反的策略，一种是丢弃最新的任务，一个是丢弃最老的任务。

下面看一下源码的具体实现

 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());
        }
    }


和上面的解释完全一样，就是抛出一个异常。



  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();
            }
        }
    }




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) {
        }
    }


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);
            }
        }
    }

2、自定义线程创建 ThreadFactory

线程池中创建的线程就是通过ThreadFactory来实现的。

Thread newThread(Runnable r);

下面是一个线程池中自定义创建线程的示例

ExecutorService es = new ThreadPoolExecutor(5, 5, 0L, TimeUnit.MILLISECONDS, new SynchronousQueue(), new ThreadFactory() {

            public Thread newThread(Runnable r) {
                Thread t = new Thread(r);
                t.setDaemon(true);
                System.out.println("create" + t);
                return null;
            }
        });

自定义创建线程，以及自定义拒绝策略

    ExecutorService es2 = new ThreadPoolExecutor(5, 5, 0L, TimeUnit.MILLISECONDS, new SynchronousQueue(), new ThreadFactory() {

            public Thread newThread(Runnable r) {
                Thread t = new Thread(r);
                t.setDaemon(true);
                System.out.println("create" + t);
                return null;
            }

        }, new RejectedExecutionHandler() {

            public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
                 System.out.println(r.toString()+" is discard");

            }

        });