匿名内部类显示不正确的修饰符 [英] Anonymous-Inner classes showing incorrect modifier

问题描述

据我了解，以下代码应已将true打印为输出.

To my understanding, the following code should have printed true as output.

但是，当我运行这段代码时，它正在打印false.

However, when I ran this code it is printing false.

从匿名类15.9.5. :

匿名类始终是隐式终结的

An anonymous class is always implicitly final

public class Test {
    public static void main(String args[]) {
        Object o = new Object() {
        };
        System.out.println("Annonymous class is final: " + Modifier.isFinal(o.getClass().getModifiers()));
    }
}

为什么这段代码的行为如此?

Why this code is behaving like this ?

推荐答案

请注意，自那时以来，该特定部分的JLS中的措词已发生了显着变化.现在(JLS 11)显示为:

Note that the wording in the JLS of that particular section has changed significantly since then. It now (JLS 11) reads:

15.9.5.匿名类声明:

匿名类永远不会是最终的类( _{匿名类不是final的事实与强制转换有关，尤其是强制强制转换转换允许强制转换运算符(}

_{The fact that an anonymous class is not final is relevant in casting, in particular the narrowing reference conversion allowed for the cast operator (§5.5). It is also of interest in subclassing, in that it is impossible to declare a subclass of an anonymous class, despite an anonymous class being non-final, because an anonymous class cannot be named by an extends clause (§8.1.4).}

这种措辞更改是在JLS 9中引入的.匿名类的语义和问题中方法的行为基本上保持不变，目的是避免确切地引起该问题的混淆.

This change in wording was introduced in JLS 9. The semantics of anonymous classes and the behavior of the methods in the question remained mostly unchanged, the intention was to avoid exactly the kind of confusion this question is about.

导致更改的票说:

从1.3开始，javac的长期行为在大多数情况下不是 ，而是将类视为最终"类.为了解决这种不一致问题，应更改规范以准确反映参考实现.

Longstanding behavior of javac, since 1.3, has been, for the most part, not to treat the classes as 'final'. To address this inconsistency, the specification should be changed to accurately reflect the reference implementation.

具体地说，匿名类几乎不会在设置ACC_FINAL标志的情况下生成.我们不能在不影响某些序列化客户端的情况下改变这种长期的行为(这是允许的，但是不必要地破坏性).而且，如果没有类文件对语言的修饰符进行编码，我们将无法忠实地实现Class.getModifers(它承诺提供"Java语言修饰符").

Specifically, anonymous classes are almost never generated with the ACC_FINAL flag set. We can't change this longstanding behavior without impacting some serialization clients (this would be permissible, but is unnecessarily disruptive). And we can't faithfully implement Class.getModifers (which promises to provide the "Java language modifiers") without the class files encoding the language's modifiers.

但是，更改 did 实际上在某种程度上改变了语义，并且在此票证中也对此进行了记录，作为可接受的影响:

However, the change did actually change semantics to some degree, and this was also documented in this ticket as an acceptable impact:

此更改会影响法律程序集，因为它允许某些强制转换在当前规范下被视为非法(请参见

The change impacts the set of legal programs, in that it allows some casts that would be considered illegal under the current specification (see JDK-6219964). But, after searching for mentions of 'final' classes in JLS, I don't anticipate any other impact, meaning that this is a source-compatible fix.

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