java - java语言规范上下文中的 "binary name of a class"是什么？

Question

在阅读类加载器时，我对二进制名称的概念有所了解，但我不太了解它。

你能解释一下java类的二进制名称是什么，为什么包+类名不够(我猜是因为内部类，但这是唯一的原因)？
谢谢

Answer 1

内部类不是唯一的原因；本地类、匿名类和类型变量也有二进制名称。
来自 Java 语言规范 ( §13.1 ):

The class or interface must be named by its binary name, which must meet the following constraints:

• The binary name of a top level type (§7.6) is its canonical name (§6.7).

• The binary name of a member type (§8.5, §9.5) consists of the binary name of its immediately enclosing type, followed by $, followed by the simple name of the member.

• The binary name of a local class (§14.3) consists of the binary name of its immediately enclosing type, followed by $, followed by a non-empty sequence of digits, followed by the simple name of the local class.

• The binary name of an anonymous class (§15.9.5) consists of the binary name of its immediately enclosing type, followed by $, followed by a non-empty sequence of digits.

• The binary name of a type variable declared by a generic class or interface (§8.1.2, §9.1.2) is the binary name of its immediately enclosing type, followed by $, followed by the simple name of the type variable.

• The binary name of a type variable declared by a generic method (§8.4.4) is the binary name of the type declaring the method, followed by $, followed by the descriptor of the method (JVMS §4.3.3), followed by $, followed by the simple name of the type variable.

• The binary name of a type variable declared by a generic constructor (§8.8.4) is the binary name of the type declaring the constructor, followed by $, followed by the descriptor of the constructor (JVMS §4.3.3), followed by $, followed by the simple name of the type variable.

至于二进制名称的目的，这在同一部分中给出:

A reference to another class or interface type must be symbolic, using the binary name of the type.

也就是说，在编译的字节码中，类是通过它们的二进制名称引用的，而不仅仅是它们的规范名称。
一个很好的理由是两个类可以具有相同的规范名称:例如，规范名称 A.B可以是一个名为 B 的类在名为 A 的包中，或名为 B 的类声明为名为 A 的类的内部类在默认包中。这两个类的二进制名称将是 A.B和 A$B分别。
另一个原因是某些类根本没有规范名称——例如，“本地类没有规范名称”。 ( §6.7 )。