ThreadPoolExecutor源码学习以及观雄哥大佬博客有感

朱雀 2023-07-15 02:50 111阅读 0赞

### 文章目录 ###

*  前言
 *  ThreadPookExecutor部分源码
 *  观后感
 *   *  重写线程池
     *  重写队列

# 前言 #

今天看了下雄哥的[Java如何让线程池满后再放队列][Java]，虽然说看完实现有点骚，但是感叹到我，这招曲线救国很强。

# ThreadPookExecutor部分源码 #

**execute(Runnable command)**

看下下面中文注释

public void execute(Runnable command) {
            if (command == null)
                throw new NullPointerException();
            /*
             * Proceed in 3 steps:
             *
             * 1. If fewer than corePoolSize threads are running, try to
             * start a new thread with the given command as its first
             * task.  The call to addWorker atomically checks runState and
             * workerCount, and so prevents false alarms that would add
             * threads when it shouldn't, by returning false.
             *
             * 2. If a task can be successfully queued, then we still need
             * to double-check whether we should have added a thread
             * (because existing ones died since last checking) or that
             * the pool shut down since entry into this method. So we
             * recheck state and if necessary roll back the enqueuing if
             * stopped, or start a new thread if there are none.
             *
             * 3. If we cannot queue task, then we try to add a new
             * thread.  If it fails, we know we are shut down or saturated
             * and so reject the task.
             */
             //AtomicInteger进行计数
            int c = ctl.get();
            //线程数小于核心线程数，就是新建线程，丢到线程池里面addWorker().然后里面会对计数进行+1
            if (workerCountOf(c) < corePoolSize) {
                if (addWorker(command, true))
                    return;
                c = ctl.get();
            }
            //如果线程池还在跑，尝试丢到队列里头，通过判断offer方法来判断是否队列已经满了，这个也是为啥无界队列会导致cpu爆的原因，一直往队列插入。
            if (isRunning(c) && workQueue.offer(command)) {
                int recheck = ctl.get();
                if (! isRunning(recheck) && remove(command))
                    reject(command);
                else if (workerCountOf(recheck) == 0)
                    addWorker(null, false);
            }
            //如果到达最大线程数的话，会抛出异常RejectedExecutionException
            else if (!addWorker(command, false))
                reject(command);
        }

RejectedExecutionException是不是有点熟悉，当任务太多的时候，线程池会抛出这个异常也可以理解为线程池的拒绝策略。

# 观后感 #

看了雄哥的那篇博客，是先达到最大线程数再往队列里头插~

我们来看下是怎么实现的

## 重写线程池 ##

public class EagerThreadPoolExecutor extends ThreadPoolExecutor {
    
        /**
         * 定义一个成员变量，用于记录当前线程池中已提交的任务数量
         */
        private final AtomicInteger submittedTaskCount = new AtomicInteger(0);
    
        public EagerThreadPoolExecutor(int corePoolSize,
                                       int maximumPoolSize,
                                       long keepAliveTime,
                                       TimeUnit unit, TaskQueue<Runnable> workQueue,
                                       ThreadFactory threadFactory,
                                       RejectedExecutionHandler handler) {
            super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, handler);
        }
    
    
        public int getSubmittedTaskCount() {
            return submittedTaskCount.get();
        }
    
        @Override
        protected void afterExecute(Runnable r, Throwable t) {
            // ThreadPoolExecutor的勾子方法，在task执行完后需要将池中已提交的任务数 - 1
            submittedTaskCount.decrementAndGet();
        }
    
        @Override
        public void execute(Runnable command) {
            if (command == null) {
                throw new NullPointerException();
            }
            // do not increment in method beforeExecute!
            // 将池中已提交的任务数 + 1
            submittedTaskCount.incrementAndGet();
            try {
                super.execute(command);
            } catch (RejectedExecutionException rx) {
                // retry to offer the task into queue.
                final TaskQueue queue = (TaskQueue) super.getQueue();
                try {
                    if (!queue.retryOffer(command, 0, TimeUnit.MILLISECONDS)) {
                        submittedTaskCount.decrementAndGet();
                        throw new RejectedExecutionException("Queue capacity is full.", rx);
                    }
                } catch (InterruptedException x) {
                    submittedTaskCount.decrementAndGet();
                    throw new RejectedExecutionException(x);
                }
            } catch (Throwable t) {
                // decrease any way
                submittedTaskCount.decrementAndGet();
                throw t;
            }
        }
    }

**主要代码**

super.**execute**(command);这就是曲线救国的方法，通过执行原线程池的执行方法，那么之前ThreadPoolExecute的执行顺序怎么办？

这里涉及上面源码解读里头的workQueue.offer(command)，**如果它为false的时候，表示插入不了**，那么在在线程数超过核心线程数，没有超过最大数量时，会抛出RejectedExecutionException异常。

那么雄哥的实现是捕获异常，塞到队列里头。当它为false的时候，表示队列也满了，抛出RejectedExecutionException异常。

我感觉这操作很骚，但是又很强，哈哈，牛逼牛逼~

那么这里涉及队列的offer返回false的操作（**看上面加粗的字的解释，为啥需要返回false**）

## 重写队列 ##

public class TaskQueue<R extends Runnable> extends LinkedBlockingQueue<Runnable> {
    
        private static final long serialVersionUID = -2635853580887179627L;
        
        // 自定义的线程池类，继承自ThreadPoolExecutor
        private EagerThreadPoolExecutor executor;
    
        public TaskQueue(int capacity) {
            super(capacity);
        }
    
        public void setExecutor(EagerThreadPoolExecutor exec) {
            executor = exec;
        }
    
        // offer方法的含义是：将任务提交到队列中，返回值为true/false，分别代表提交成功/提交失败
        @Override
        public boolean offer(Runnable runnable) {
            if (executor == null) {
                throw new RejectedExecutionException("The task queue does not have executor!");
            }
            // 线程池的当前线程数
            int currentPoolThreadSize = executor.getPoolSize();
            if (executor.getSubmittedTaskCount() < currentPoolThreadSize) {
                // 已提交的任务数量小于当前线程数，意味着线程池中有空闲线程，直接扔进队列里，让线程去处理
                return super.offer(runnable);
            }
    
            // return false to let executor create new worker.
            if (currentPoolThreadSize < executor.getMaximumPoolSize()) {
                // 重点： 当前线程数小于 最大线程数 ，返回false，暗含入队失败，让线程池去创建新的线程
                return false;
            }
    
            // 重点: 代码运行到此处，说明当前线程数 >= 最大线程数，需要真正的提交到队列中
            return super.offer(runnable);
        }
    
        public boolean retryOffer(Runnable o, long timeout, TimeUnit unit) throws InterruptedException {
            if (executor.isShutdown()) {
                throw new RejectedExecutionException("Executor is shutdown!");
            }
            return super.offer(o, timeout, unit);
        }
    }

**重点代码**

// return false to let executor create new worker.
            if (currentPoolThreadSize < executor.getMaximumPoolSize()) {
                // 重点： 当前线程数小于 最大线程数 ，返回false，暗含入队失败，让线程池去创建新的线程
                return false;
            }

[Java]: https://www.jianshu.com/p/62043956bfb6?utm_campaign=hugo&utm_medium=reader_share&utm_content=note&utm_source=weixin-friends