Java annotation

In computer programming, a Java annotation is a way of adding metadata to Java source code that can also be 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.

History

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. A provisional interface for compile-time annotation processing was provided by the apt tool in JDK version 1.5, and was formalized through JSR-269 and integrated into the javac compiler in version 1.6.

Processing

When Java source code is compiled, annotations can be processed by compiler plug-ins called annotation processors. Processors can produce informational messages or create additional Java source files or resources, which in turn may be compiled and processed, but processors cannot modify the annotated code itself. The Java compiler conditionally stores annotation metadata in the class files if the annotation has a RetentionPolicy of CLASS or RUNTIME. Later, the JVM or other programs can look for the metadata to determine how to interact with the program elements or change their behavior.

Syntax

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.

Impact and Perception

Pros

Declarative programming: 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 an object oriented language.

Cons

Performance: Adding metadata to a run time causes additional memory overhead.
Standards lack: There are few standards that dictate what metadata tags should be used.