JAVA基础学习之-ThreadPoolExecutor的实现原理

2025-01-08 技术教程

池技术是性能优化的重要手段：连接池，线程池已经是开发中的标配了。面试中这个知识点也是高频问题。抽空学习了Java的ThreadPoolExecutor, 把学习的思路记录一下。

由于线程的创建和销毁都是系统层面的操作，涉及到系统资源的占用和回收，所以创建线程是一个重量级的操作。为了提升性能，就引入了线程池；即线程复用。Java不仅提供了线程池，还提供了线程池的操作工具类。我们由浅入深了解一下。

import java.util.concurrent.Executor;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class ThreadDemo { static class Worker implements Runnable{ public void run(){ System.out.println("run work "+Thread.currentThread().getName() ); } } public static void main(String[] args) { Worker w1 = new Worker(); ExecutorService service = Executors.newFixedThreadPool(10); service.submit(w1); service.shutdown(); }}

看Executors的源码，发现其使用的是ThreadPoolExecutor。研究一下ThreadPoolExecutor, 发现其默认的参数Executors.defaultThreadFactory(), defaultHandler。线程池的创建工厂默认如下:

public Thread newThread(Runnable r) { Thread t = new Thread(group, r, namePrefix + threadNumber.getAndIncrement(), 0); if (t.isDaemon()) t.setDaemon(false); if (t.getPriority() != Thread.NORM_PRIORITY) t.setPriority(Thread.NORM_PRIORITY); return t; }

也就是自定义了一下线程的名字，将线程归到了同一个组。
线程池的defaultHandler如下:

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()); } }

也就是说，当提交的任务超过线程池的容量，那么就会抛出RejectedExecutionException异常。但是使用Executors会发现，并没有抛出异常。这是因为Executors创建BlockingQueue时没有指定队列的容量。

换言之，线程池能容纳的任务数量最多为maximumPoolSize + queueSize。比如线程池如下new ThreadPoolExecutor(10, 11, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(5)); 则最大任务数量为16个，超过16个就会抛出异常。

线程池中线程数量有多少呢？先运行如下的代码:

import java.util.concurrent.*;import java.util.concurrent.locks.ReentrantLock;public class ThreadDemo { static class Worker implements Runnable{ public void run(){ try { Thread.sleep(1000); System.out.println("done work "+Thread.currentThread().getName() ); } catch (InterruptedException e) { e.printStackTrace(); } } } public static void main(String[] args) throws InterruptedException { Worker w1 = new Worker(); ThreadPoolExecutor executor = new ThreadPoolExecutor(3, 4, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(5)); for(int i=0;i<9;i++) { executor.submit(w1); } executor.shutdown(); }}

可以发现最多开启了4个线程。这4个线程就对应了4个Worker的实例。
看worker的源码可以发现，它兼备AQS和Runnable两个特性。我们只关注它Runnable的特性。

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(); } }

在这个线程中不断从队列中获取任务，然后执行。 worker中反复出现的ctl又是什么呢？

ctl是两个变量组合，一个32位的int, 高3位用于控制线程池的状态，低29位用于记录线程池启动线程的数量。
所以有这么几个方法

// Packing and unpacking ctl private static int runStateOf(int c) { return c & ~CAPACITY; } private static int workerCountOf(int c) { return c & CAPACITY; } private static int ctlOf(int rs, int wc) { return rs | wc; }

整个线程池的核心，就worker和ctl的理解。有点复杂，主要是集中了:

1. AQS2. BlockingQueue

这也是为什么我建议先学AQS,后学线程池的实现原理。