Java Review - 线程池使用FutureTask的小坑

import java.util.concurrent.*;

/**
 * @author 小工匠
 * @version 1.0
 * @description: TODO
 * @date 2021/11/21 0:11
 * @mark: show me the code , change the world
 */
public class FutureTest {


    //  1 线程池单个线程，队列大小为1  - 初始化线程池
    private final static ThreadPoolExecutor tpe = new ThreadPoolExecutor(1, 1, 1, TimeUnit.MINUTES,
            new ArrayBlockingQueue<Runnable>(1),
            new ThreadPoolExecutor.DiscardPolicy());


    public static void main(String[] args) throws ExecutionException, InterruptedException {

        // 2 添加你任务1
        Future futureOne = tpe.submit(() -> {
            System.out.println("开始处理业务1");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("业务1执行结束");
            return "Result1";
        });

        // 3 添加你任务2
        Future futureTwo = tpe.submit(() -> {
            System.out.println("开始处理业务2");
            System.out.println("业务2执行结束");
            return "Result2";
        });

        // 4  添加任务3
        Future futureThree = null;
        try {
            futureThree = tpe.submit(() -> System.out.println("开始处理业务3"));
        } catch (Exception e) {
            System.out.print(e.getLocalizedMessage());
        }




        // 5 等待任务1执行完毕
        System.out.println("任务1返回结果： " + futureOne.get());
        // 6 等待任务2执行完毕
        System.out.println("任务2返回结果：  " + futureTwo.get());
        // 7 等待任务3执行完毕
        System.out.println("任务3返回结果：  " + futureThree==null?null:futureThree.get());



        //关闭线程池,阻塞知道所有任务执行完毕
        tpe.shutdown();

    }
}

开始处理业务1
Task java.util.concurrent.FutureTask@27bc2616 rejected from java.util.concurrent.ThreadPoolExecutor@3941a79c[Running, pool size = 1, active threads = 1, queued tasks = 1, completed tasks = 0]业务1执行结束
任务1返回结果： Result1
开始处理业务2
业务2执行结束
任务2返回结果：  Result2
Exception in thread "main" java.lang.NullPointerException
	at com.artisan.bfzm.chapter11.FutureTest.main(FutureTest.java:58)

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        // 1 装饰Runnable对象为Future对象
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        // 6 返回Future对象
        return ftask;
    }

   /**
     * 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.
         */	

		// 2 如果线程个数小于核心线程数量，则新增线程处理
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        // 3. 如果线程都在工作且当前线程个数已经达到核心线程数，就把任务放入队列
        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);
        }
        // 4 尝试新增处理线程
        else if (!addWorker(command, false))
            // 5 新增失败则触发拒绝策略
            reject(command);
    }

    /**
     * A handler for rejected tasks that silently discards the
     * rejected task.
     */
    public static class DiscardPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code DiscardPolicy}.
         */
        public DiscardPolicy() { }

        /**
         * Does nothing, which has the effect of discarding task r.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        }
    }

   protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new FutureTask<T>(callable);
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     * @param  callable the callable task
     * @throws NullPointerException if the callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

   public V get() throws InterruptedException, ExecutionException {
        int s = state;
        //当前状态值 <= COMPLETING需要等待，否则调用report返回 
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

    /**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    @SuppressWarnings("unchecked")
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        // 状态值为NORMAL的时候正常返回
        if (s == NORMAL)
            return (V)x;
       // 状态值大于等于CANCELLED的时候抛出异常
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

    /**
     * Invokes the rejected execution handler for the given command.
     * Package-protected for use by ScheduledThreadPoolExecutor.
     */
    final void reject(Runnable command) {
        handler.rejectedExecution(command, this);
    }

import java.util.concurrent.FutureTask;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;

/**
 * @author 小工匠
 * @version 1.0
 * @description: TODO
 * @date 2021/11/21 1:40
 * @mark: show me the code , change the world
 */
public class MyRejectedExecutionHandler implements RejectedExecutionHandler {
    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
            if (!executor.isShutdown()){
                if (null != r && r instanceof FutureTask) {
                    ((FutureTask) r).cancel(true);
                }
            }
    }
}