重写线程池 execute 方法导致线程池“失效” 问题

亦凉 2023-09-28 20:24 13阅读 0赞

## 一、背景 ##

今天群里有个同学遇到一个看似很奇怪的问题，自定义 `ThreadPoolTaskExecutor` 子类，重写了 execute 方法，通过 execute 方法来执行任务时打印当前线程，日志显示任务一直在调用者线程里执行 (其实并不是)，似乎线程池失效了。

## 二、场景复现 ##

自定义 `ThreadPoolTaskExecutor` 子类

import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
    
    public class ThreadPoolTaskExecutorImpl extends ThreadPoolTaskExecutor {
    
        @Override
        public void execute(Runnable command) {
            System.out.println("当前线程" + Thread.currentThread().getName());
    
            super.execute(command);
        }
    
    }
    复制代码

编写测试代码：

import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    
    public class ThreadPoolDemo {
    
        public static void main(String[] args) throws InterruptedException {
            // 构造线程池
            ThreadPoolTaskExecutorImpl executor = new ThreadPoolTaskExecutorImpl();
            executor.initialize();
            executor.setCorePoolSize(10);
            executor.setMaxPoolSize(20);
            executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
            executor.setThreadNamePrefix("[线程池的线程]");
    
            // 执行任务
             for (int i = 0; i < 5; i++) {
                executor.execute(() -> {
                    System.out.println("测试");
                });
            }
    
    
            TimeUnit.SECONDS.sleep(3);
    
            executor.shutdown();
        }
    }
    复制代码

执行结果：

> 当前线程main 当前线程main 测试 测试 当前线程main 测试 当前线程main 当前线程main 测试 测试

由此断定：自定义的线程池失效，在 execute 方法中获取当前线程时，并没有出现我们定义的线程名称前缀的线程，仍然使用 main 线程来执行任务。  
但是，真的是这样吗？

## 三、分析 ##

由于很多同学没有认真思考过多线程的本质，会想当然地认为线程池的 `execute`方法的所有代码都是在线程池创建的线程中执行，可是真的是这样吗？

我们知道在没有使用新线程的情况下，程序会使用当前线程（main 线程）顺序执行。

![format_png][]

因此，在 `org.example.thread.ThreadPoolTaskExecutorImpl#execute` 中，打印的 “当前线程” 的代码仍然是在 main 方法中执行的。

![format_png 1][]

进入 `super.execute` 方法

@Override
    	public void execute(Runnable task) {
    		Executor executor = getThreadPoolExecutor();
    		try {
    			executor.execute(task);
    		}
    		catch (RejectedExecutionException ex) {
    			throw new TaskRejectedException("Executor [" + executor + "] did not accept task: " + task, ex);
    		}
    	}
    复制代码

这里的 `executor.execute` 方法实际上是 `java.util.concurrent.ThreadPoolExecutor#execute`

/**
         * Executes the given task sometime in the future.  The task
         * may execute in a new thread or in an existing pooled thread.
         *
         * If the task cannot be submitted for execution, either because this
         * executor has been shutdown or because its capacity has been reached,
         * the task is handled by the current {@code RejectedExecutionHandler}.
         *
         * @param command the task to execute
         * @throws RejectedExecutionException at discretion of
         *         {@code RejectedExecutionHandler}, if the task
         *         cannot be accepted for execution
         * @throws NullPointerException if {@code command} is null
         */
        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.
             */
            int c = ctl.get();
            if (workerCountOf(c) < corePoolSize) {
                if (addWorker(command, true))
                    return;
                c = ctl.get();
            }
            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);
            }
            else if (!addWorker(command, false))
                reject(command);
        }
    复制代码

小于核心线程数，会执行到 \`addWorker ，此时才真正创建新的线程去执行任务：

/**
         * Checks if a new worker can be added with respect to current
         * pool state and the given bound (either core or maximum). If so,
         * the worker count is adjusted accordingly, and, if possible, a
         * new worker is created and started, running firstTask as its
         * first task. This method returns false if the pool is stopped or
         * eligible to shut down. It also returns false if the thread
         * factory fails to create a thread when asked.  If the thread
         * creation fails, either due to the thread factory returning
         * null, or due to an exception (typically OutOfMemoryError in
         * Thread.start()), we roll back cleanly.
         *
         * @param firstTask the task the new thread should run first (or
         * null if none). Workers are created with an initial first task
         * (in method execute()) to bypass queuing when there are fewer
         * than corePoolSize threads (in which case we always start one),
         * or when the queue is full (in which case we must bypass queue).
         * Initially idle threads are usually created via
         * prestartCoreThread or to replace other dying workers.
         *
         * @param core if true use corePoolSize as bound, else
         * maximumPoolSize. (A boolean indicator is used here rather than a
         * value to ensure reads of fresh values after checking other pool
         * state).
         * @return true if successful
         */
        private boolean addWorker(Runnable firstTask, boolean core) {
            
            // 省略部分代码
    
            boolean workerStarted = false;
            boolean workerAdded = false;
            Worker w = null;
            try {
                //【关键1】 创建新的线程（run 方法）
                w = new Worker(firstTask);
                final Thread t = w.thread;
                if (t != null) {
                    final ReentrantLock mainLock = this.mainLock;
                    mainLock.lock();
                    try {
                        // Recheck while holding lock.
                        // Back out on ThreadFactory failure or if
                        // shut down before lock acquired.
                        int rs = runStateOf(ctl.get());
    
                        if (rs < SHUTDOWN ||
                            (rs == SHUTDOWN && firstTask == null)) {
                            if (t.isAlive()) // precheck that t is startable
                                throw new IllegalThreadStateException();
                            workers.add(w);
                            int s = workers.size();
                            if (s > largestPoolSize)
                                largestPoolSize = s;
                            workerAdded = true;
                        }
                    } finally {
                        mainLock.unlock();
                    }
                    if (workerAdded) {
                        //【关键2】启动创建的线程
                        t.start();
                        workerStarted = true;
                    }
                }
            } finally {
                if (! workerStarted)
                    addWorkerFailed(w);
            }
            return workerStarted;
        }
    复制代码

我们再看下 `new Worker` 时做了什么。

private final class Worker
            extends AbstractQueuedSynchronizer
            implements Runnable
        {
         
    
            /** Thread this worker is running in.  Null if factory fails. */
            final Thread thread;
            
            /** Initial task to run.  Possibly null. */
            Runnable firstTask;
            
            /** Per-thread task counter */
            volatile long completedTasks;
    
            /**
             * Creates with given first task and thread from ThreadFactory.
             * @param firstTask the first task (null if none)
             */
            Worker(Runnable firstTask) {
                setState(-1); // inhibit interrupts until runWorker
                this.firstTask = firstTask;
                //【关键】 使用线程工厂创建线程
                this.thread = getThreadFactory().newThread(this);
            }
            
            //【关键】 重写 Runnable 的 run 方法，里面就封装了外面传入的 Runnable
            /** Delegates main run loop to outer runWorker  */
            public void run() {
                runWorker(this);
            }
            
            
           final void runWorker(Worker w) {
            Thread wt = Thread.currentThread();
            Runnable task = w.firstTask;
            w.firstTask = null;
            w.unlock(); // allow interrupts
            boolean completedAbruptly = true;
            try {
                while (task != null || (task = getTask()) != null) {
                    w.lock();
                    // If pool is stopping, ensure thread is interrupted;
                    // if not, ensure thread is not interrupted.  This
                    // requires a recheck in second case to deal with
                    // shutdownNow race while clearing interrupt
                    if ((runStateAtLeast(ctl.get(), STOP) ||
                         (Thread.interrupted() &&
                          runStateAtLeast(ctl.get(), STOP))) &&
                        !wt.isInterrupted())
                        wt.interrupt();
                    try {
                        beforeExecute(wt, task);
                        Throwable thrown = null;
                        try {
                            task.run();
                        } catch (RuntimeException x) {
                            thrown = x; throw x;
                        } catch (Error x) {
                            thrown = x; throw x;
                        } catch (Throwable x) {
                            thrown = x; throw new Error(x);
                        } finally {
                            afterExecute(task, thrown);
                        }
                    } finally {
                        task = null;
                        w.completedTasks++;
                        w.unlock();
                    }
                }
                completedAbruptly = false;
            } finally {
                processWorkerExit(w, completedAbruptly);
            }
        }
    
            
    
    // 省略其他
    
    }
    复制代码

因此  
（1）自定义的 `ThreadPoolTaskExecutorImpl` 类里重写的 execute 方法里打印的当前线程实际还是调用者线程。  
（2）外面传入的 Runnable 参数最终会在 Worker 现成的 run 方法中执行到。

## 四、解决之道 ##

我们可以使用包装器模式处理下即可：

import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
    
    public class ThreadPoolTaskExecutorImpl extends ThreadPoolTaskExecutor {
    
        @Override
        public void execute(Runnable command) {
        // 当前在父线程里
    
            super.execute(() -> {
                // 这里在子线程里执行
                
                // 可以在任务前打印下当前线程名称，线程池的状态等信息
                System.out.println("当前线程" + Thread.currentThread().getName());
                
                // 原始的任务
                command.run();
            });
        }
    
    }
    
    复制代码

测试代码：

import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    
    public class ThreadPoolDemo {
    
        public static void main(String[] args) throws InterruptedException {
            // 构造线程池
            ThreadPoolTaskExecutorImpl executor = new ThreadPoolTaskExecutorImpl();
            executor.initialize();
            executor.setCorePoolSize(10);
            executor.setMaxPoolSize(20);
            executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
            executor.setThreadNamePrefix("[线程池的线程]");
    
            // 执行任务
            for (int i = 0; i < 5; i++) {
                executor.execute(() -> {
                    System.out.println(Thread.currentThread().getName() + "-> 测试");
                });
            }
    
            TimeUnit.SECONDS.sleep(3);
    
            executor.shutdown();
        }
    }
    复制代码

输出结果：

> 当前线程\[线程池的线程\]2 当前线程\[线程池的线程\]1 \[线程池的线程\]1-> 测试 \[线程池的线程\]2-> 测试 当前线程\[线程池的线程\]3 \[线程池的线程\]3-> 测试 当前线程\[线程池的线程\]4 \[线程池的线程\]4-> 测试 当前线程\[线程池的线程\]5 \[线程池的线程\]5-> 测试

## 五、启示 ##

### 5.1 关于提问 ###

该同学提问非常模糊，甚至“反复修改问题”，最终给出关键代码截图，才真正理解问题是什么。  
大家请教别人时，尽量能将问题转化为别人容易理解的表达方式。  
大家请教别人时，一定自己先搞清楚问题究竟是什么，而不需要别人一再追问下，才不断逼近真实的问题。  
大家请教别人时，最好能够有源码或者关键信息截图等。

### 5.2 现象与本质 ###

我们使用线程池时，总是观察到我们传入的 Runnable 是在线程池中的线程执行的，我们是使用 execute 方法来执行的，但这并不意味着 execute 方法的所有步骤都是在线程池中的线程里执行的。  
学习某个技术时，要真正理解技术的本质，而不是表象。  
如调用线程的 start 方法才真正启动线程，在重写的 execute 方法第一行压根就没有创建新的线程，怎么会在新的线程里执行呢？  
在实际开发和验证问题时，多进行代码调试，掌握高级的调试技巧，如调试时表达式计算、条件断点、移除栈帧回退调用等。

[format_png]: https://img-blog.csdnimg.cn/img_convert/f6eb3d185fd7510f2521adbaeb328262.webp?x-oss-process=image/format,png
[format_png 1]: https://img-blog.csdnimg.cn/img_convert/b185cc8173418d568f3dd11735553133.webp?x-oss-process=image/format,png