如何在Java语言解析器中正确标记'@'符号？

Question

I'm working on a Java source code parser, following the lexical and syntactic specifications.

I'm stuck on annotations, though; the relevant rules are:

Annotation:
    @ QualifiedIdentifier [ ( [AnnotationElement] ) ]

AnnotationTypeDeclaration:
    @ interface Identifier AnnotationTypeBody

but I don't see the @ symbol mentioned anywhere in the lexical spec. What is the correct rule for tokenizing @ , or where can I find such a rule?

I realize that I could easily add a token definition for this, and it might even work --- sometimes. But I'd prefer to do it correctly according to the spec if possible.

Answer 1

In 9.6. Annotation Types , it says:

Note that the at-sign (@) and the keyword interface are two distinct tokens. Technically it is possible to separate them with whitespace, but this is discouraged as a matter of style.

Answer 2

You are correct, in the sense that '@' is not mentioned as either an operator or as punctuation in the "3. Lexical Structure" section of the JLS.

However, it is used as a symbol in the JLS 9.6, and also the "18. Collected Syntax" section, so it is obvious that your lexer should treat it as one.

This is just a minor editorial issue, and not something that should cause you any concern in implementing a Java parser.