Annotation in java is a marker which associates information with a program element, but has no effect at runtime.

Annotation Interfaces

An annotation interface is a specialized kind of interface, precedes the keyword interface by sign @

annotation interface is never generic
the direct superinterface type of annotation interface is always java.lang.annotation.Annotation

Annotation Interface Elements

Element declaration:

{AnnotationInterfaceElementModifier} UnannType Identifier ( ) [Dims] [DefaultValue] ;

@interface RequestForEnhancement {
    int    id();       // No default - must be specified in 
                       // each annotation
    String synopsis(); // No default - must be specified in 
                       // each annotation
    String engineer()  default "[unassigned]";
    String date()      default "[unimplemented]";
    String[] targets() default {};
}

Annotation

An annotation denotes a specific instance of an annotation interface and usually provides values for the element of that interface.

Normal Annotation

An normal annotation specifies the name of an annotation interface and optionally a list of comma separated element-value pairs.Each pair contains element value that is associated with the element of the annotation interface.

NormalAnnotation:
    @ TypeName ( [ElementValuePairList] )
ElementValuePairList:
    ElementValuePair {, ElementValuePair}
ElementValuePair:
    Identifier = ElementValue
ElementValue:
    ConditionalExpression
    ElementValueArrayInitializer
    Annotation
ElementValueArrayInitializer:
    { [ElementValueList] [,] }
ElementValueList:
    ElementValue {, ElementValue}

Marker Annotation

An marker annotation is a shorthand designed for use with marker annotation interfaces.

MarkerAnnotation:
@ TypeName

shorthand for 

@TypeName()

Single Element Annotation

Single element annotation is a shorthand designed for use with single-element annotation interfaces

SingleElementAnnotation:
@ TypeName ( ElementValue )

short hand for

@TypeName(value = ElementValue)

Annotation Usage

Annotations have a number of usages, among them :

Information for compiler - used by compiler to detect errors and suppress warnings.
Compile-time and deployment-time processing - Software tools can process annotation information to generate code, XML files and so forth.
Runtime processing - Some annotations are available to be examined at runtime.

JAVA Annotation