In the Java computer programming language, an annotation is a form of syntactic metadata that can be added to Java source code.. Classes, methods, variables, parameters and packages may be annotated. Like Javadoc tags, Java annotations can be read from source files. Unlike Javadoc tags, Java annotations can also be embedded in and read from class files generated by the compiler. This allows annotations to be retained by Java VM at run-time and read via reflection.
The Java platform has various ad-hoc annotation mechanisms—for example, the transient modifier, or the @deprecated javadoc tag. JSR-175 introduced the general-purpose annotation (also known as metadata) facility to the Java Community Process in 2002; it gained approval in September 2004.
Annotations became available in the language itself beginning with version 1.5 of the JDK. The apt tool provided a provisional interface for compile-time annotation processing in JDK version 1.5; JSR-269 formalized this, and it became integrated into the javac compiler in version 1.6.
This example demonstrates the use of the @Override annotation. It instructs the compiler to check parent classes for matching methods. In this case, an error is generated because the gettype() method of class Cat doesn't in fact override getType() of class Animal like is desired. If the @Override annotation was absent, a new method of name gettype() would be created in class Cat.
Annotation type declarations are similar to normal interface declarations. An at-sign (@) precedes the interface keyword. Each method declaration defines an element of the annotation type. Method declarations must not have any parameters or a throws clause. Return types are restricted to primitives, String, Class, enums, annotations, and arrays of the preceding types. Methods can have default values.
Annotations may include an optional list of key-value pairs:
Annotations themselves may be annotated to indicate where and when they can be used:
The compiler reserves a set of special annotations (including @Deprecated, @Override and @SuppressWarnings) for syntactic purposes.
Annotations are often used by frameworks as a way of conveniently applying behaviours to user-defined classes and methods that must otherwise be declared in an external source (such as an XML configuration file) or programmatically (with API calls). The following, for example, is an annotated JPA data class:
The annotations are not method calls and will not, by themselves, do anything. Rather, the class object is passed to the JPA implementation at run-time, which then extracts the annotations to generate an object-relational mapping.
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, and also 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.
In addition to processing an annotation using an annotation processor, a Java programmer can write their own code that uses reflections to process the annotation. Java SE 5 supports a new interface that is defined in the java.lang.reflect package. This package contains the interface called AnnotatedElement that is implemented by the Java reflection classes including Class, Constructor, Field, Method, and Package. The implementations of this interface are used to represent an annotated element of the program currently running in the Java Virtual Machine. This interface allows annotations to be read reflectively.
The AnnotatedElement interface provides access to annotations having RUNTIME retention. This access is provided by the getAnnotation, getAnnotations, and isAnnotationPresent methods. Because annotation types are compiled and stored in byte code files just like classes, the annotations returned by these methods can be queried just like any regular Java object. A complete example of processing an annotation is provided below: