Java annotation
From Wikipedia, the free encyclopedia
In computer programming, a Java annotation is a way of adding metadata to Java source code that is available to the programmer at run-time. Many times it is used as an alternative to XML technology.
Java annotations can be added to program elements such as classes, methods, fields, parameters, local variables, and packages. Unlike tags added to Java documentation and processed with tools such as XDoclet, Java annotations are completely accessible to the programmer while the software is running using reflection.
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[edit] History of Java annotation
Java annotations were introduced to the Java Community Process as JSR-175 in 2002 and approved in September 2004. Annotations became available with the JDK version 1.5. The feature was added to the Java language through the specification.
[edit] How Java annotations are processed
When Java source code is compiled, the Java compiler stores annotation metadata in the class files. Later, the JVM or other programs can look for the metadata to determine how to interact with the program elements or change their behavior.
[edit] Syntax of annotation
Declaring an annotation is a variation on tags that have been added to comment sections in the past.
Annotations take the form of an interface declaration with an @ preceding it and optionally marked with meta-annotations, as shown below:
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
In the above example both Retention and Target are examples of annotations.
[edit] Pros of using Java annotations
Annotations allow the programmer to declare in their source code how the software should behave. It is an example of how declarative programming constructs can be added to a procedural language.
[edit] Cons of using Java annotations
- Adding metadata to a run time causes additional memory overhead.
- There are few standards that dictate what metadata tags should be used
