饱和策略源码分析

前言

Github：https://github.com/yihonglei/thinking-in-concurrent

一 Abort策略

默认策略，新任务提交时直接抛出未检查的异常RejectedExecutionException，该异常可由调用者捕获。

new ThreadPoolExecutor.AbortPolicy()

/**
 * 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());
    }
}

二 CallerRuns策略

为调节机制，既不抛弃任务也不抛出异常，而是将某些任务回退到调用者。不会在线程池的线程中执行新的任务，

而是在调用exector的线程中运行新的任务，即谁调谁负责执行。

new ThreadPoolExecutor.CallerRunsPolicy()

/**
 * 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();
        }
    }
}

三 Discard策略

新提交的任务被抛弃。

new ThreadPoolExecutor.DiscardPolicy()

/**
 * 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) {
    }
}

四 DiscardOldest策略

队列的是“队头”的任务，然后尝试提交新的任务。对头任务被丢弃（不适合工作队列为优先队列场景）

new ThreadPoolExecutor.DiscardOldestPolicy()

/**
 * 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);
        }
    }
}

五 自定义饱和处理策略

import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;
/**
 * 循环处理,当队列有空位时,该任务进入队列，等待线程池处理
 */
public class CustomRejectedExecutionHandler implements RejectedExecutionHandler {
  public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
    try {
      executor.getQueue().put(r);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
}