为什么null不是编译时常量？ [英] Why isn't null a compile time constant?

问题描述

因此，如果我有一个静态最终对象CONSTANT = null ，出于某种原因我会在另一段代码中引用它，例如 doSomething（CONSTANT ），它不会在编译期间内嵌到代码中。所以编译后不是 doSomething（null），而是 doSomething（CONSTANT）。

So if I have a static final Object CONSTANT = null, for some reason if I reference that in another piece of code like doSomething(CONSTANT), it won't be in-lined onto the code during compilation. So instead of being doSomething(null) after being compiled, it would be doSomething(CONSTANT).

推荐答案

您的 CONSTANT 不是编译时常量，因为JLS 表示不是。可以在常量表达式中使用的唯一类型是基本类型和 String 。

Your CONSTANT is not a compile time constant because the JLS says it is not. The only types that can be used in constant expressions are the primitive types and String.

它的意义是 Object 实例（通常）具有语义上重要的对象标识，可将其与其他 Object 实例区分开来。此对象标识不能在类文件中编码...或者至少，它不能使用当前的类文件格式进行编码。 （如果可以，会有各种其他问题......）

The sense of it is that an Object instance (in general) has a semantically significant object identity that distinguishes it from other Object instances. This Object identity cannot be encoded in a class file ... or at least, it can't be encoded with the current classfile formats. (And if it could, there would be all sorts of other problems ...)

值 null 可以（理论上）作为一个特例处理，除了没有太多意义。具体来说，您不能在语言学角度需要（或有利）编译时间常数的任何上下文中使用 null 。例如：

The value null could (in theory) be handled as a special case, except that there is not a great deal of point. Specifically, you can't use null in any of the contexts where a "compile time constant" is required (or advantageous) from the linguistic perspective. For instance:

• 你不能将 null 作为 case 表达。


• 由于 == 对于引用类型不是常量表达式，你可以不能将它用于Java条件编译习惯用法，如果，则使用常量表达式作为条件的。 （除了 null == null 不是一个有用的条件......）



• You can't have null as a case expression.
• Since == for reference types is not a constant expression, you can't use it for the Java "conditional compilation" idiom involving an if with a constant expression as a condition. (And besides null == null is not a useful condition ...)

就内联而言，虽然常量不能在字节码中内联（因为关于常量表达式是什么的JLS规则），JIT编译器的优化器将被允许这样做，并且可能实际上是这样做的......如果有明显的性能优势。

As far as inlining is concerned, while the "constant" cannot be inlined in the bytecodes (because of the JLS rules about what a "constant expression" is), the JIT compiler's optimizer would be permitted to do this, and may actually do it ... if there are tangible performance benefits.

参考：

• JLS 15.28 - 常量表达式

• JLS 15.28 - Constant Expressions

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