tl  tr
  Home | Tutorials | Articles | Videos | Products | Tools | Search
Open Source | Tag Cloud | Follow Us | Bookmark | Contact   
 
 Java > Annotations > Class and Runtime Retention Policy

Class and Runtime Retention Policy 

Annotations with Retention Policy CLASS and RUNTIME are accessible from class byte code. We need to use a byte code manipulation library to access the Annotations available in byte code. This example uses ASM to fetch Annotations from class byte code.

Annotations with Retention Policy CLASS,

File Name  :  
com/bethecoder/tutorials/annotations/retention2/Author.java 
Author  :  Sudhakar KV
Email  :  kvenkatasudhakar@gmail.com
   
package com.bethecoder.tutorials.annotations.retention2;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Permitted types for annotation attributes
 
 * 1. primitive type
 * 2. String
 * 3. Class
 * 4. annotation 
 * 5. enumeration 
 * 6. 1-dimensional arrays
 */
@Retention(RetentionPolicy.CLASS)
@Target(ElementType.TYPE)
public @interface Author {
  String name() default "admin";
  String creationDate();
}
   

Annotations with Retention Policy RUNTIME,

File Name  :  
com/bethecoder/tutorials/annotations/retention2/VersionHistory.java 
Author  :  Sudhakar KV
Email  :  kvenkatasudhakar@gmail.com
   
package com.bethecoder.tutorials.annotations.retention2;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Permitted types for annotation attributes
 
 * 1. primitive type
 * 2. String
 * 3. Class
 * 4. annotation 
 * 5. enumeration 
 * 6. 1-dimensional arrays
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface VersionHistory {
  String [] value();
}
   

Its usage is shown below,

File Name  :  
com/bethecoder/tutorials/annotations/retention2/Source.java 
Author  :  Sudhakar KV
Email  :  kvenkatasudhakar@gmail.com
   
package com.bethecoder.tutorials.annotations.retention2;

@Author(name="BTC", creationDate="01/01/2500")
@VersionHistory({ "1.0""2.0""3.0" })
public class Source {

  private int fieldOne;
  private String fieldTwo;
  
  public Source(int fieldOne, String fieldTwo) {
    super();
    this.fieldOne = fieldOne;
    this.fieldTwo = fieldTwo;
  }

  public int getFieldOne() {
    return fieldOne;
  }
  
  public void setFieldOne(int fieldOne) {
    this.fieldOne = fieldOne;
  }
  
  public String getFieldTwo() {
    return fieldTwo;
  }
  
  public void setFieldTwo(String fieldTwo) {
    this.fieldTwo = fieldTwo;
  }
  
}
   

The ASM Tree API code to access Annotations is shown below,

File Name  :  
com/bethecoder/tutorials/annotations/retention2/Test.java 
Author  :  Sudhakar KV
Email  :  kvenkatasudhakar@gmail.com
   
package com.bethecoder.tutorials.annotations.retention2;

import java.util.List;

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.tree.AnnotationNode;
import org.objectweb.asm.tree.ClassNode;

public class Test {

  /**
   @param args
   */
  public static void main(String[] argsthrows Exception {
    //ASM Tree API
    ClassNode cn = new ClassNode();
    
    //Read the class byte code from stream
    ClassReader cr = new ClassReader(
        ClassLoader.getSystemResourceAsStream(
            "com/bethecoder/tutorials/annotations/retention2/Source.class"));
    cr.accept(cn, 0);
    
    System.out.println("Class Name : " + cn.name);
    System.out.println("Source File : " + cn.sourceFile);
  
    //Annotations with RetentionPolicy.CLASS
    //@Retention(RetentionPolicy.CLASS)
    System.out.println("\n==Invisible Annotations==");
    printAnnotation(cn.invisibleAnnotations);

    //Annotations with RetentionPolicy.RUNTIME
    //@Retention(RetentionPolicy.RUNTIME)
    System.out.println("\n==Visible Annotations==");
    printAnnotation(cn.visibleAnnotations);
  }
  
  private static void printAnnotation(List annotationList) {
    if (annotationList != null && !annotationList.isEmpty()) {
      AnnotationNode anNode = null;
      for (Object annotation : annotationList) {
        anNode = (AnnotationNodeannotation;
        System.out.println("Annotation Descriptor : " + anNode.desc);
        System.out.println("Annotation attribute pairs : " + anNode.values);
      }
    else {
      System.out.println("No annotations found..");
    }
  }

}
   

It gives the following output,
Class Name : com/bethecoder/tutorials/annotations/retention2/Source
Source File : Source.java

==Invisible Annotations==
Annotation Descriptor : Lcom/bethecoder/tutorials/annotations/retention2/Author;
Annotation attribute pairs : [name, BTC, creationDate, 01/01/2500]

==Visible Annotations==
Annotation Descriptor : Lcom/bethecoder/tutorials/annotations/retention2/VersionHistory;
Annotation attribute pairs : [value, [1.0, 2.0, 3.0]]


                    Print

   



Books Recommendation

 
 
bl  br