Java 执行器无法对任务进行排队

我需要一个 Java 执行器，如果正在处理其他任务，它会拒绝任务。我想不可能操纵工作队列大小。

有人可能会奇怪，为什么我首先需要一个具有这种特征的执行者。我需要能够轻松更改策略并允许非零队列大小。

有任何想法吗？

Use a 线程池执行器 http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ThreadPoolExecutor.html with a 同步队列 http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/SynchronousQueue.html（复制自这个答案 https://stackoverflow.com/a/10186825/3080094).

它似乎有效：

import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.Semaphore;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class SingleTaskExecutor {

public static void main(String[] args) {

    try {
        new SingleTaskExecutor().runTest();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

public void runTest() throws Exception {

    ThreadPoolExecutor tp = new ThreadPoolExecutor(1, 1,
            60L, TimeUnit.SECONDS,
            new SynchronousQueue<Runnable>());

    tp.setRejectedExecutionHandler(new RejectedTaskHandler());

    final Semaphore oneTaskDone = new Semaphore(0);
    tp.execute(new Runnable() {
        @Override public void run() { 
            System.out.println("Sleeping");
            try { Thread.sleep(300); } catch (Exception e) { e.printStackTrace();} 
            System.out.println("Done sleeping");
            oneTaskDone.release();
        }
    });
    tp.execute(new Runnable() {
        @Override public void run() { System.out.println("Never happends"); }
        @Override public String toString() { return "Rejected Runnable"; }
    });
    oneTaskDone.acquire();
    tp.execute(new Runnable() {
        @Override public void run() { System.out.println("Running"); }
    });
    tp.shutdown();
    tp.awaitTermination(100, TimeUnit.MILLISECONDS);
    System.out.println("Finished");
}

static class RejectedTaskHandler implements RejectedExecutionHandler {

    @Override public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        System.out.println("Task rejected: " + r);
    }

}

}