Java Tutorial/Language/Annotations Create

Creating Annotations

Annotation is created based on the interface.

1. Adding "@" before the keyword interface to declare an annotation type.
2. All annotations consist only method declarations.
3. These methods act much like fields.

Our first annotation type:

   <source lang="java">

// A simple annotation type. @interface MyAnnotation {

 String stringValue();
 int intValue();

}</source>

default values in an annotation.

   <source lang="java">

import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.reflect.Method; @Retention(RetentionPolicy.RUNTIME) @interface MyAnno {

 String str() default "Testing";
 int val() default 9000;

} class Meta3 {

 @MyAnno()
 public static void myMeth() {
   Meta3 ob = new Meta3();
   try {
     Class c = ob.getClass();
     Method m = c.getMethod("myMeth");
     MyAnno anno = m.getAnnotation(MyAnno.class);
     System.out.println(anno.str() + " " + anno.val());
   } catch (NoSuchMethodException exc) {
     System.out.println("Method Not Found.");
   }
 }
 public static void main(String args[]) {
   myMeth();
 }

}</source>

Define new annotation type

1. All annotation types automatically extend the Annotation interface.
2. Annotation is a super-interface of all annotations.
3. It overrides hashCode( ), equals( ), and toString() defined by Object.
4. It defines annotationType( ), which returns a Class object that represents the invoking annotation.
5. When you apply an annotation, you give values to its members.

   <source lang="java">

// A simple annotation type. @interface MyAnnotation {

 String stringValue();
 int intValue();

} public class MainClass {

 // Annotate a method.
 @MyAnnotation(stringValue = "Annotation Example", intValue = 100)
 public static void myMethod() {
 }

}</source>

Specifying a Retention Policy

A retention policy determines at what point an annotation is discarded.

1. SOURCE: annotation retained only in the source file and is discarded during compilation.
2. CLASS: annotation stored in the .class file during compilation, not available in the run time.
3. RUNTIME: annotation stored in the .class file and available in the run time.
4. They are defined java.lang.annotation.RetentionPolicy enumeration.

A retention policy is specified using Java"s built-in annotations: @Retention.

   <source lang="java">

@Retention(retention-policy)</source>