Java in a Nutshell, 5th Edition

Annotations

Annotations provide a way to associate arbitrary information or metadata with program elements. Syntactically, annotations are used like modifiers and can be applied to the declarations of packages, types, constructors, methods, fields, parameters, and local variables. The information stored in an annotation takes the form of name = value pairs, whose type is specified by the annotation type. The annotation type is a kind of interface that also serves to provide access to the annotation through the Java Reflection API.

Annotations can be used to associate any kind of information you want with a program element. The only fundamental rule is that an annotation cannot affect the way the program runs: the code must run identically even if you add or remove annotations. Another way to say this is that the Java interpreter ignores annotations (although it does make “runtime-visible” annotations available for reflective access through the Java Reflection API). Since the Java VM ignores annotations, an annotation type is not useful unless accompanied by a tool that can do something with the information stored in annotations of that type. In this chapter we’ll cover standard annotation and meta-annotation types like Override and Target. The tool that accompanies these types is the Java compiler, which must process them in certain ways (as we’ll describe later in this section).

It is easy to imagine any number of other uses for annotations.^[7] A local variable might be annotated ...