一、说明
RejectedExecutionHandler
- 当线程池已经被关闭，或者任务数超过maximumPoolSize+workQueue时执行拒绝策略
ThreadPoolExecutor.AbortPolicy默认拒绝策略，丢弃任务并抛出RejectedExecutionException异常
ThreadPoolExecutor.DiscardPolicy直接丢弃任务，但不抛出异常
ThreadPoolExecutor.DiscardOldestPolicy丢弃任务队列最先加入的任务，再执行execute方法把新任务加入队列执行
ThreadPoolExecutor.CallerRunsPolicy：由创建了线程池的线程来执行被拒绝的任务
二、理解
AbortPolicy
默认拒绝策略，丢弃任务并抛出RejectedExecutionException异常
private static final RejectedExecutionHandler defaultHandler = new AbortPolicy();
public static class AbortPolicy implements RejectedExecutionHandler {r public AbortPolicy() { } public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { throw new RejectedExecutionException("Task " + r.toString() + " rejected from " + e.toString()); } }
DiscardPolicy
直接丢弃任务，但不抛出异常
public static class DiscardPolicy implements RejectedExecutionHandler { public DiscardPolicy() { } public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { }`在这里插入代码片` }
DiscardOldestPolicy
丢弃队列中等待最久的任务，再把新任务添加进去执行，从任务队列弹出最先加入的任务，空出一个位置，然后再次执行execute方法把任务加入队列
public static class DiscardOldestPolicy implements RejectedExecutionHandler { public DiscardOldestPolicy() { } public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { if (!e.isShutdown()) { e.getQueue().poll(); e.execute(r); } } }
CallerRunsPolicy
会调用当前线程池的所在的线程去执行被拒绝的任务
public static class CallerRunsPolicy implements RejectedExecutionHandler { public CallerRunsPolicy() { } public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { if (!e.isShutdown()) { r.run(); } } }
三、实现
1.AbortPolicy
创建 ThreadPoolExecutorTest类，默认使用ThreadPoolExecutor.AbortPolicy拒绝策略，队列是ArrayBlockingQueue，设置核心线程数最大值为1，线程池线程数最大值为2，最大等待时间为5秒，等待队列值为2
public class RejectedExecutionHandlerTest { public static void main(String[] args) throws InterruptedException { // 1.创建自定义线程池 ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2, 4, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<>(2), Executors.defaultThreadFactory(), new ThreadPoolExecutor.AbortPolicy()); // 2.创建线程任务 for (int i = 1; i <= 6; i++) { // 3.执行任务 System.out.println("执行第"+i+"个任务"); threadPoolExecutor.execute(new runnable("任务"+i)); System.out.println("当前核心线程数" + threadPoolExecutor.getCorePoolSize()); System.out.println("当前线程池线程数" + threadPoolExecutor.getPoolSize()); // 4.迭代器获取等待队列 Iterator iterator = threadPoolExecutor.getQueue().iterator(); System.out.print("当前等待队列 "); while (iterator.hasNext()){ runnable thread = (runnable) iterator.next(); System.out.print(thread.name + "\t"); } System.out.print("\n"); System.out.println("--------"); } Thread.sleep(10000); System.out.println("----休眠10秒后----"); System.out.println("当前核心线程数" + threadPoolExecutor.getCorePoolSize()); System.out.println("当前线程池线程数" + threadPoolExecutor.getPoolSize()); System.out.println("当前队列任务数" + threadPoolExecutor.getQueue().size()); // 5.关闭线程池 threadPoolExecutor.shutdown(); } // 实现Runnable static class runnable implements Runnable{ // 设置任务名 String name; public runnable(String setName) { this.name = setName; } @Override public void run() { try { System.out.println("线程:"+Thread.currentThread().getName() +" 执行: "+name); } catch (Exception e) { e.printStackTrace(); } } } }
当线程数达到corePoolSize后，若有新任务加入，则直接进入任务队列等待，超出队列的任务会创建新的线程来执行
一共有1个核心，当线程数超过corePoolSize+workQueue时，将创建非核心线程，核心线程默认情况下不会被回收，不受时间限制，而超时的非核心线程将被回收
但如果再执行1个任务，线程数超过maximumPoolSize+workQueue，再提交任务将被丢弃并抛出RejectedExecutionException异常
2.DiscardPolicy
创建5个任务，让被线程池拒绝的任务直接丢弃，不会抛异常也不会执行
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 2, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<>(2), Executors.defaultThreadFactory(), new ThreadPoolExecutor.DiscardPolicy());
任务5不会执行，恶意不会抛出异常，超时的非核心线程将被回收
3.DiscardOldestPolicy
丢弃任务队列最先加入的任务，再执行execute方法把新任务加入队列执行
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 2, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<>(2), Executors.defaultThreadFactory(), new ThreadPoolExecutor.DiscardOldestPolicy());
添加任务5时，线程数已经超过maximumPoolSize+workQueue，抛弃最先加入队列的任务2并且不执行，再将任务5加进队列中执行
4.CallerRunsPolicy
会调用当前线程池的所在的线程去执行被拒绝的任务
// 1.创建自定义线程池 ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 2, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<>(2), Executors.defaultThreadFactory(), new ThreadPoolExecutor.CallerRunsPolicy());
主线程执行任务1，空闲线程执行任务4，此时队列中有任务2和任务3
添加任务5时，线程数已经超过maximumPoolSize+workQueue，任务5直接调用当前线程池的所在的线程main去执行，这时主线程被阻塞了
当任务5执行完成时，最先的两个任务已经完成了，主线程去执行任务2和任务3，添加任务6也可以直接执行
超时的非核心线程将被回收
5.自定义拒绝执行策略
当线程数已经超过maximumPoolSize+workQueue时，调用新线程去执行任务
static class MyRejectedExecutionHandler implements RejectedExecutionHandler { @Override public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) { new Thread(r, "新线程"+(new Random().nextInt(4) + 1)).start(); } }
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 2, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<>(2), Executors.defaultThreadFactory(), new MyRejectedExecutionHandler());
