双冒号运算符 (::) 和（有效）final [重复]

Question

所以我一直在检查将Closeable 接口与ExecutorService 结合使用的可能性。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想在尝试块中初始化服务：

@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，eclipse有点困惑，但如果有异常我相信它会被压制，那么我已经尝试过：

@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，但代码块成功执行（创建了新线程，但在 close() 阶段我得到了异常）。我注意到，当我声明时：

Closeable closeable = es::shutdown

es 必须是（有效地）最终的，这个问题只发生在双冒号运算符上。即下面一个不编译：

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，但可以：

@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，这不是错误。需要在捕获时知道要关闭的变量/参数的值。

当您获得 NPE 时，这是因为您在 es = null 时声明了匿名内部类（或 lambda 或方法引用）。这就是捕获的价值：看不到未来的分配。

Why can method reference use non-final variables?

为了阻止你犯这个错误，有一个限制“X 必须是（有效地）最终的”，这意味着如果你尝试更改你试图捕获的变量 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成这样：

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