combine metadata with source-code files,instead of keeping it in external documents
They provide information that you need to fully describe your program,but that cannot be expressed in Java.
javal.lang
@Override
@Deprecated
@SuppressWarnings
Basic syntax
Syntactically,annotations are used in much the same way a modifiers.
Defining annotations
compile to class files
@Target defines where you can apply this annotation(a method or a field,for example)
@Retention defines whether the annotations are available in the source code(SOUCE),in the class files(CLASS),or at run time(RUNTIME).
Meta-annotations
@Target
@Retention
@Documented
@Inherited
Writing annotation processors
public <T extends Annotation> java.lang.reflect.Method.getAnnotation(Class<T> annotationClass)
public Method[] getDeclaredMethods() throws SecurityException
Annotation elements
Here is a list of the allowed types for annotation elements:
All primitives(int,float,boolean etc.)
String
Class
enums
Annotations
Arrays of any of the above
Default value constraints
elements must either have default values or values provided by the class that uses the annotation.
none of the non-primitive type elements are allowed to take null as a value.
Generating external files
If you define an element on an annotation with the name value,then as long as it is the only element type specified you don't need to use the name-value pair syntax:you can just specify the value in parentheses.
Alternative solutions
when using multiple annotations,you cannot use the same annotation twice.
how not to use the rather long-winded name-value pair form for this nested annotation,respecifying the element name and the @interface name
Annotations don't support inheritance
Implementing the processor
public Annotation[] java.lang.reflect.Field.getDeclaredAnnotations()