Retention Policy

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

Java defines three such policies:SOURCE, CLASS, and RUNTIME.

SOURCE is retained only in the source file and is discarded during compilation.
CLASS is stored in the .class file during compilation. It is not available through the JVM during run time.
RUNTIME is stored in the .class file and is available through the JVM during run time.

A retention policy is specified for an annotation by using one of Java's built-in annotations:

@Retention.

Its general form is shown here:


@Retention(retention-policy)

retention-policy must be one of SOURCE, CLASS, and RUNTIME.

The default policy is CLASS.


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

@Retention(RetentionPolicy.RUNTIME)
@interface MyAnno {
  String str();

  int val();
}

Obtaining Annotations at Run Time by Use of Reflection


import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Method;

// An annotation type declaration. 
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnno {
  String str();

  int val();
}

public class Main {
  @MyAnno(str = "Annotation Example", val = 100)
  public static void myMeth() {
    Main ob = new Main();
    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();
  }
}

Obtaining All Annotations

You can obtain all annotations that have RUNTIME retention that are associated with an item by calling getAnnotations( ) on that item. It has this general form:


Annotation[ ] getAnnotations( )


import java.lang.annotation.Annotation;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Method;

@Retention(RetentionPolicy.RUNTIME)
@interface MyAnno {
  String str();

  int val();
}

@Retention(RetentionPolicy.RUNTIME)
@interface What {
  String description();
}

@What(description = "An annotation")
@MyAnno(str = "Meta2", val = 99)
public class Main {
  @What(description = "test method")
  @MyAnno(str = "Testing", val = 100)
  public static void myMeth() throws Exception {
    Main ob = new Main();
    Annotation annos[] = ob.getClass().getAnnotations();
    System.out.println("All annotations for Meta2:");
    for (Annotation a : annos) {
      System.out.println(a);
    }
    Method m = ob.getClass().getMethod("myMeth");
    annos = m.getAnnotations();
    for (Annotation a : annos) {
      System.out.println(a);
    }

  }

  public static void main(String args[]) throws Exception {
    myMeth();
  }
}

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