自定义线程池拒绝策略
一、Executors提供四种线程池配置方案
1、构造一个固定线程数目的线程池，核心线程数与最大线程数相同，同时使用了一个无界LinkedBlockingQueue存放阻塞任务，因此多余的任务将存在阻塞队列，不会由RejectedExecutionHandler处理
//固定线程数目的线程池源码 public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); } //无界LinkedBlockingQueue源码 默认是Integer.MAX_VALUE大小，也可以指定大小 public LinkedBlockingQueue() { this(Integer.MAX_VALUE); } public LinkedBlockingQueue(int capacity) { if (capacity <= 0) throw new IllegalArgumentException(); this.capacity = capacity; last = head = new Node<E>(null); }
2、构造一个缓冲功能的线程池，配置核心线程数corePoolSize=0，最大线程数maximumPoolSize=Integer.MAX_VALUE，keepAliveTime=60s,以及一个无容量的阻塞队列 SynchronousQueue，因此任务提交之后，将会创建新的线程执行；线程空闲超过60s将会销毁
//缓冲功能的线程池源码 public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); }
3、构造一个只支持一个线程的线程池，配置corePoolSize=maximumPoolSize=1，无界阻塞队列LinkedBlockingQueue；保证任务由一个线程串行执行
public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); }
4、构造有定时功能的线程池，配置corePoolSize，无界延迟阻塞队列DelayedWorkQueue；有意思的是：maximumPoolSize=Integer.MAX_VALUE，由于DelayedWorkQueue是无界队列，所以这个值是没有意义的
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) { return new ScheduledThreadPoolExecutor(corePoolSize); } public static ScheduledExecutorService newScheduledThreadPool( int corePoolSize, ThreadFactory threadFactory) { return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory); } public ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory) { super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS, new DelayedWorkQueue(), threadFactory); }
注意：阿里巴巴开发手册强调在日常编写代码时不要使用Executors显示的去创建线程池。
二 线程池执行流程
ThreadPoolExecutor最核心的构造方法源码
public ThreadPoolExecutor(int corePoolSize, //核心线程数 int maximumPoolSize, //最大线程数 long keepAliveTime, //线程池中超过核心线程数目的空闲线程最大存活时间 TimeUnit unit, //时间单位 BlockingQueue<Runnable> workQueue,//阻塞任务队列 ThreadFactory threadFactory, //新建线程工厂 RejectedExecutionHandler handler //拒绝策略 //当提交任务数超过最大线程数+队列(maxmumPoolSize+workQueue)之和时， //任务会交给RejectedExecutionHandler来处理 ) { if (corePoolSize < 0 || maximumPoolSize <= 0 || maximumPoolSize < corePoolSize || keepAliveTime < 0) throw new IllegalArgumentException(); if (workQueue == null || threadFactory == null || handler == null) throw new NullPointerException(); this.acc = System.getSecurityManager() == null ? null : AccessController.getContext(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler; }
//四种拒绝策略
RejectedExecutionHandler rejected = null;
rejected = new ThreadPoolExecutor.AbortPolicy();//默认，队列满了丢任务抛出异常
rejected = new ThreadPoolExecutor.DiscardPolicy();//队列满了丢任务不异常
rejected = new ThreadPoolExecutor.DiscardOldestPolicy();//将最早进入队列的任务删，之后再尝试加入队列
rejected = new ThreadPoolExecutor.CallerRunsPolicy();//如果添加到线程池失败，那么主线程会自己去执行该任务
自定义线程池和自定义拒绝策略
import java.util.concurrent.*; import java.util.concurrent.atomic.AtomicInteger; public class MyThreadPoolExecutor { private ThreadPoolExecutor pool = null; public void init() { pool = new ThreadPoolExecutor( 1, 3, 5, TimeUnit.MINUTES, new ArrayBlockingQueue<Runnable>(5), new CustomThreadFactory(), new CustomRejectedExecutionHandler()); } public void destory() { if(pool != null) { pool.shutdownNow(); } } public ExecutorService getMyThreadPoolExecutor() { return this.pool; } /** * 自定义创建线程工厂 */ private class CustomThreadFactory implements ThreadFactory { private AtomicInteger count = new AtomicInteger(0); @Override public Thread newThread(Runnable r) { Thread t = new Thread(r); String threadName = MyThreadPoolExecutor.class.getSimpleName() + count.addAndGet(1); //System.out.println(threadName); t.setName(threadName); return t; } } /** * 自定义拒绝策略 */ private class CustomRejectedExecutionHandler implements RejectedExecutionHandler { @Override public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) { try { // 核心改造点，由blockingqueue的offer改成put阻塞方法 executor.getQueue().put(r); } catch (InterruptedException e) { e.printStackTrace(); } } } // 测试构造的线程池 public static void main(String[] args) { MyThreadPoolExecutor exec = new MyThreadPoolExecutor(); // 1.初始化 exec.init(); ExecutorService pool = exec.getMyThreadPoolExecutor(); MyTask task = new MyTask(); for(int i=0; i<1000; i++) { pool.execute(task); } try { Thread.sleep(5000); // 2.销毁----留点时间给线程池执行,因为任务没有提交执行完,如果销毁线程池,任务也就无法执行了 exec.destory(); } catch (InterruptedException e) { e.printStackTrace(); } } } class MyTask implements Runnable{ public AtomicInteger i = new AtomicInteger(0); @Override public void run() { System.out.println("第"+i.getAndIncrement()+"次执行任务"); } }
执行结果：
//excute执行源码 public void execute(Runnable command) { if (command == null) throw new NullPointerException(); 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)) // 进入拒绝机制， 我们把runnable任务拿出来，重新用阻塞操作put，来实现提交阻塞功能 reject(command); }
阻塞队列常见操作方法：
总结：
1、用ThreadPoolExecutor自定义线程池，看线程是的用途，如果任务量不大，可以用无界队列，如果任务量非常大，要用有界队列，防止OOM
2、如果任务量很大，还要求每个任务都处理成功，要对提交的任务进行阻塞提交，重写拒绝机制，改为阻塞提交。保证不抛弃一个任务
3、最大线程数一般设为2N+1最好，N是CPU核数，可以通过Runtime.getRuntime().availableProcessors();来获取
4、核心线程数，看应用，如果是任务，一天跑一次，设置为0，合适，因为跑完就停掉了，如果是常用线程池，看任务量，是保留一个核心还是几个核心线程数
转载：https://blog.csdn.net/weixin_44777693/article/details/104569711
