Introduction to Java Annotations annotation

annotation definition and use

Annotations are defined as:

public @interface AnnotationName{}

Using annotations

@ AnnotationNamepublicvoidfunc(){}

Java provides 4 meta annotations

@Retention: The stage of the reservation.
@Target: The object of the annotation adornment, can say class, method, member variable, package.
@Documented: Whether it was written by Javadoc.
@Inherited: Whether annotations can be inherited.

Custom annotations

Member variable definitions for annotations:

public @interface AnnotationName{    default"abc";    intdefault33;    boolean flag();}

Where the default decoration represents the defaults.

• Note: Annotation modifies classes, methods, and member variables after they do not take effect on their own and do not have any effect on the program. Only tool classes that provide processing for these annotations are useful.

Instance

Defining annotation Classes

import java.lang.annotation.*;@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.METHOD)public @interface Testable {    booleandefaultfalse;    String name();}

A class that is decorated with annotations

 Public classFoo { Public Static void M1() {System. out. println ("I am M1"); } @Testable (isskiped =false, name ="Methond2") Public Static void m2() {System. out. println ("I am M2"); } @Testable (name ="Methond3") Public Static void M3() {System. out. println ("I am M3"); } @Testable (isskiped =true, name ="Methond4") Public Static void M4() {System. out. println ("I am M4"); }}

Tool classes for handling annotations

Import java.lang.reflect.*; Public classtestannotation { Public Static void Main(string[] args) throws SecurityException, ClassNotFoundException, Illegalaccessexception, Illegalarg Umentexception, InvocationTargetException { for(Method M:class.forname ("Foo"). GetMethods ()) {if(M.isannotationpresent (Testable.class)) {System. out. Print (M.getname () +"is Annotation element, so invoke result is:"); M.invoke (NULL); }Else{System. out. println (M.getname () +"is not Annotation element"); }        }    }}

Output:

M1 is  notAnnotation ELEMENTM2 isAnnotation element, so invokeresult  is: I am M2M3 isAnnotation element, so invokeresult  is: I am M3M4 isAnnotation element, so invokeresult  is: I am m4wait is  notAnnotation elementwait is  notAnnotation elementwait is  notAnnotation elementequals  is  notAnnotation elementtostring is  notAnnotation Elementhashcode is  notAnnotation Elementgetclass is  notAnnotation elementNotify  is  notAnnotation Elementnotifyall is  notAnnotation element

Introduction to Java Annotations annotation