线程池的一个BUG，被我发现了

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java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@a5acd19 rejected from java.util.concurrent.ThreadPoolExecutor@30890a38[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 0]

public class ThreadPoolTest {

    public static void main(String[] args) {
        final ThreadPoolTest threadPoolTest = new ThreadPoolTest();
        for (int i = 0; i < 8; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    while (true) {

                        Future<String> future = threadPoolTest.submit();
                        try {
                            String s = future.get();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        } catch (ExecutionException e) {
                            e.printStackTrace();
                        } catch (Error e) {
                            e.printStackTrace();
                        }
                    }
                }
            }).start();
        }

        //子线程不停gc，模拟偶发的gc
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    System.gc();
                }
            }
        }).start();
    }

    /**
     * 异步执行任务
     * @return
     */
    public Future<String> submit() {
        //关键点，通过Executors.newSingleThreadExecutor创建一个单线程的线程池
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        FutureTask<String> futureTask = new FutureTask(new Callable() {
            @Override
            public Object call() throws Exception {
                Thread.sleep(50);
                return System.currentTimeMillis() + "";
            }
        });
        executorService.execute(futureTask);
        return futureTask;
    }

}

分析&疑问

public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                    0L, TimeUnit.MILLISECONDS,
                    new LinkedBlockingQueue<Runnable>()));
}

A reachable object is any object that can be accessed in any potential continuing computation from any live thread.

Optimizing transformations of a program can be designed that reduce the number of objects that are reachable to be less than those which would naively be considered reachable. For example, a Java compiler or code generator may choose to set a variable or parameter that will no longer be used to null to cause the storage for such an object to be potentially reclaimable sooner.

class A {
    @Override protected void finalize() {
        System.out.println(this + " was finalized!");
    }

    public static void main(String[] args) throws InterruptedException {
        A a = new A();
        System.out.println("Created " + a);
        for (int i = 0; i < 1_000_000_000; i++) {
            if (i % 1_000_00 == 0)
                System.gc();
        }
        System.out.println("done.");
    }
}

//打印结果
Created A@1be6f5c3
A@1be6f5c3 was finalized!//finalize方法输出
done.

...
System.out.println(a);

//打印结果
Created A@1be6f5c3
done.
A@1be6f5c3

A a = new A();
System.out.println("Created " + a);
a = null;//手动置null
for (int i = 0; i < 1_000_000_000; i++) {
    if (i % 1_000_00 == 0)
        System.gc();
}
System.out.println("done.");
System.out.println(a);

//打印结果
Created A@1be6f5c3
A@1be6f5c3 was finalized!
done.
null

//入口函数
public class FinalizedTest {
    public static void main(String[] args) {
        final FinalizedTest finalizedTest = new FinalizedTest();
        for (int i = 0; i < 8; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    while (true) {
                        TFutureTask future = finalizedTest.submit();
                    }
                }
            }).start();
        }
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    System.gc();
                }
            }
        }).start();
    }
    public TFutureTask submit(){
        TExecutorService TExecutorService = Executors.create();
        TExecutorService.execute();
        return null;
    }
}

//Executors.java，模拟juc的Executors
public class Executors {
    /**
     * 模拟Executors.createSingleExecutor
     * @return
     */
    public static TExecutorService create(){
        return new FinalizableDelegatedTExecutorService(new TThreadPoolExecutor());
    }

    static class FinalizableDelegatedTExecutorService extends DelegatedTExecutorService {

        FinalizableDelegatedTExecutorService(TExecutorService executor) {
            super(executor);
        }

        /**
         * 析构函数中执行shutdown，修改线程池状态
         * @throws Throwable
         */
        @Override
        protected void finalize() throws Throwable {
            super.shutdown();
        }
    }

    static class DelegatedTExecutorService extends TExecutorService {

        protected TExecutorService e;

        public DelegatedTExecutorService(TExecutorService executor) {
            this.e = executor;
        }

        @Override
        public void execute() {
            e.execute();
        }

        @Override
        public void shutdown() {
            e.shutdown();
        }
    }
}

//TThreadPoolExecutor.java，模拟juc的ThreadPoolExecutor
public class TThreadPoolExecutor extends TExecutorService {

    /**
     * 线程池状态，false：未关闭，true已关闭
     */
    private AtomicBoolean ctl = new AtomicBoolean();

    @Override
    public void execute() {
        //启动一个新线程，模拟ThreadPoolExecutor.execute
        new Thread(new Runnable() {
            @Override
            public void run() {

            }
        }).start();
        //模拟ThreadPoolExecutor，启动新建线程后，循环检查线程池状态，验证是否会在finalize中shutdown
        //如果线程池被提前shutdown，则抛出异常
        for (int i = 0; i < 1_000_000; i++) {
            if(ctl.get()){
                throw new RuntimeException("reject!!!["+ctl.get()+"]");
            }
        }
    }

    @Override
    public void shutdown() {
        ctl.compareAndSet(false,true);
    }
}

Exception in thread "Thread-1" java.lang.RuntimeException: reject!!![true]

//TThreadPoolExecutor.java，修改后的execute方法
public void execute() {
    try {
        //显式的sleep 1 ns，主动切换线程
        TimeUnit.NANOSECONDS.sleep(1);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    //模拟ThreadPoolExecutor，启动新建线程后，循环检查线程池状态，验证是否会在finalize中shutdown
    //如果线程池被提前shutdown，则抛出异常
    for (int i = 0; i < 1_000_000; i++) {
        if(ctl.get()){
            throw new RuntimeException("reject!!!["+ctl.get()+"]");
        }
    }
}

Exception in thread "Thread-3" java.lang.RuntimeException: reject!!![true]

总结

JUC  Executors.FinalizableDelegatedExecutorService
public void execute(Runnable command) {
    try {
        e.execute(command);
    } finally { reachabilityFence(this); }
}