ThreadPoolExecutor（下篇）

上篇写到了ThreadPoolExecutor构造方法前4个参数int corePoolSize、int maximumPoolSize,、long keepAliveTime、TimeUnit unit与工作流程机制，现在来看看后3个参数BlockingQueue workQueue、ThreadFactory threadFactory、RejectedExecutionHandler handler的含义与用法。

参数详解

workQueue：阻塞任务队列，用于保存任务以及为工作线程提供待执行的任务

block queue有以下几种实现：

• ArrayBlockingQueue : 有界的数组队列
• LinkedBlockingQueue : 可支持有界/无界的队列，使用链表实现
• PriorityBlockingQueue : 优先队列，可以针对任务排序
• SynchronousQueue : 队列长度为1的队列，和Array有点区别就是：client thread提交到block queue会是一个阻塞过程，直到有一个worker thread连接上来poll task。
  
  threadFactory：线程工厂，线程生成器
  
  handler：当提交任务数超过maxmumPoolSize+workQueue之和时，任务会交给RejectedExecutionHandler来处理；ThreadPoolExecutor内部有实现4个拒绝策略，默认为AbortPolicy策略：
• CallerRunsPolicy：由调用execute方法提交任务的线程来执行这个任务
• AbortPolicy：抛出异常RejectedExecutionException拒绝提交任务
• DiscardPolicy：直接抛弃任务，不做任何处理
• DiscardOldestPolicy：去除任务队列中的第一个任务，重新提交。

首先若正在运行的线程数量大于或等于 maximumPoolSize时：

public static void main(String[] args) {
        ExecutorService executor = new ThreadPoolExecutor(2, 3, 1, TimeUnit.HOURS,
                new LinkedBlockingQueue<>(1));
        List<Future<String>> resultList = new ArrayList<Future<String>>();
        for (int i = 0; i < 10 ; i++) {
            Future<String> future = executor.submit(new MyWorker(i));
            resultList.add(future);
        }
        System.out.println("所有线程已经运行完成");
        for (Future<String> future:resultList) {
            try {
                System.out.println(future.get());
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }
        }
    }
    static class MyWorker implements Callable<String> {
        private int i;
        public MyWorker(int i) {
            this.i = i;
        }
        @Override
        public String call() {
            try {
                System.out.println(i);
                //模拟执行
                Thread.sleep(500);
                String result = "序号" + i+ "_当前线程==" + Thread.currentThread().getName();
                return result;
            } catch (Exception e) {
                e.printStackTrace();
            }
            return "error";
        }
    }

对此种情况进行处理，自定义拒绝处理类，实现RejectedExecutionHandler接口（和new ThreadPoolExecutor.CallerRunsPolicy()功能一致）

ExecutorService executor = new ThreadPoolExecutor(2, 3, 1, TimeUnit.HOURS,
                new LinkedBlockingQueue<>(1), new ThreadFactory() {
            @Override
            public Thread newThread(Runnable r) {
                return null;
            }
        }, new MyRejectedExecutionHandler());
static class MyRejectedExecutionHandler implements RejectedExecutionHandler {
        @Override
        public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
            if (!executor.isShutdown()) {
                r.run();
            }
        }
    }

新增ThreadFactory自定义类：

static class MyThreadFactory implements ThreadFactory {
        private final AtomicInteger mThreadNum = new AtomicInteger(1);
        @Override
        public Thread newThread(Runnable r) {
            Thread t = new Thread(r, "my-thread-" + mThreadNum.getAndIncrement());
            System.out.println(t.getName() + " has been created");
            return t;
        }
    }