java - 为什么方法中的局部变量会提前被垃圾收集？

Question

这是一个多线程测试代码，每次创建一个newSingleThreadExecutor，另外两个线程不断触发gc。HotSpot java8(1.8.0_221)下会出现线程池已关闭的错误。

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();
        }
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    System.gc();
                }
            }
        }).start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    System.gc();
                }
            }
        }).start();
    }


    public Future<String> submit() {
        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;
    }

}

运行一段时间后会报错：

Exception in thread "Thread-2" 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]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2063)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:830)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1379)
at java.util.concurrent.Executors$DelegatedExecutorService.execute(Executors.java:668)
at ThreadPoolTest.submit(ThreadPoolTest.java:54)
at ThreadPoolTest$1.run(ThreadPoolTest.java:12)
at java.lang.Thread.run(Thread.java:748)

Executors.newSingleThreadExecutor 创建一个自动关闭的线程池（java.util.concurrent.Executors.FinalizableDelegatedExecutorService），所以它应该只在对象被收集之前执行关闭。

但是从错误日志来看，executorService被提前关闭了。和栈帧弹出之前一样，executorService 的finalized 方法提前执行了。

为什么会这样？

标签： javagarbage-collectionexecutorservicestack-frame