How to describe Java code with annotations

You’ve probably encountered situations where you need to associate metadata (data that describes other data) with classes, methods, and/or other application elements. For example, your programming team might need to identify unfinished classes in a large application. For each unfinished class, the metadata would likely include the name of the developer responsible for finishing the class and the class’s expected completion date.

Before Java 5, comments were the only flexible mechanism that Java had to offer for associating metadata with application elements. However, comments are a poor choice. Because the compiler ignores them, comments are not available at runtime. And even if they were available, the text would have to be parsed to obtain crucial data items. Without standardizing how the data items are specified, these data items might prove impossible to parse.

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