Question

所以我一直在检查使用Closeable与ExecutorService接口的可能性。然后我看到Lukas回答here这只是使用方法参考：

ExecutorService service = Executors.newSingleThreadExecutor();
try (Closeable close = service::shutdown) {

}

所以我一直在挖掘并希望在try块中初始化服务：

@Test(expected = RejectedExecutionException.class)
    public void singleThreadTest() throws Exception {

        ExecutorService es = null;

        try (Closeable closeable = (es = Executors.newSingleThreadExecutor())::shutdown){

            es.execute(()->System.out.println("executed " + Thread.currentThread().getId()));
        } 
        System.out.println("executed " + Thread.currentThread().getId());

        System.out.println(es.isShutdown());

        es.execute(()->{});

    }

这部分相当不错，有点困惑，但如果有异常，我会相信，然后我试过了：

@Test(expected = RejectedExecutionException.class)
    public void singleThreadTest() throws Exception {

        ExecutorService es = null;

        try (Closeable closeable = es::shutdown){

            es = Executors.newSingleThreadExecutor();
            es.execute(()->System.out.println("executed " + Thread.currentThread().getId()));
        } 
        System.out.println("executed " + Thread.currentThread().getId());

        System.out.println(es.isShutdown());

        es.execute(()->{});

    }

对于这个代码块，我得到NullPointerException但代码块已成功执行（新线程已创建，但在close（）阶段我遇到异常）。我注意到，当我宣布：

Closeable closeable = es::shutdown

es必须（有效）最终，这个问题只发生在双冒号运算符中。即低于一个不编译：

try (Closeable closeable = new Closeable() {

            @Override
            public void close() throws IOException {
                es.shutdown();

            }
        }){

这是一个错误吗？这是什么原因？ *方法参考impl？ * IDE * JVM（oracle - debian）？

跟进：

@Test(expected = RejectedExecutionException.class)
    public void singleThreadTest() throws Exception {

        ExecutorService es = null;


        es = Executors.newSingleThreadExecutor();
        es = Executors.newSingleThreadExecutor();
        es = null;
        try (Closeable closeable = ()->es.shutdown();){

            es = Executors.newSingleThreadExecutor();
            es.execute(()->System.out.println("executed " + Thread.currentThread().getId()));
        } 
        System.out.println("executed " + Thread.currentThread().getId());

        System.out.println(es.isShutdown());

        es.execute(()->{});

    }

此代码无法编译，但确实如此：

@Test(expected = RejectedExecutionException.class)
    public void singleThreadTest() throws Exception {

        ExecutorService es = null;


        es = Executors.newSingleThreadExecutor();
        es = Executors.newSingleThreadExecutor();
        es = null;
        try (Closeable closeable = es::shutdown;){

            es = Executors.newSingleThreadExecutor();
            es.execute(()->System.out.println("executed " + Thread.currentThread().getId()));
        } 
        System.out.println("executed " + Thread.currentThread().getId());

        System.out.println(es.isShutdown());

        es.execute(()->{});

    }

Answer 1

不，这不是一个错误。您要关闭的变量/参数的值需要在捕获点知道。

当你得到一个NPE时，因为你在es = null时声明了匿名内部类（或lambda或方法引用）。这就是被捕获的价值：未来的任务没有被看到。

Why can method reference use non-final variables?

为了阻止你犯这个错误，有一个限制“X必须（有效）最终”，这意味着如果你试图改变你想要捕获的变量X的值，它将无法编译。

Why are only final variables accessible in anonymous class?

基本上，您需要将代码重新排列为以下内容：

final ExecutorService es = Executors.newSingleThreadExecutor();
try (Closeable closeable = es::shutdown) {
    // ...

双冒号运算符（：:)和（有效）最终

1 个答案: