Java Tutorial/Reflection/Annotation

Get annotation by Annotation class

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Method;
@Retention(RetentionPolicy.RUNTIME)
@interface MySingle {
  int value(); // this variable name must be value
}
class Single {
  @MySingle(100)
  public static void myMeth() {
    Single ob = new Single();
    try {
      Method m = ob.getClass().getMethod("myMeth");
      MySingle anno = m.getAnnotation(MySingle.class);
      System.out.println(anno.value()); // displays 100
    } catch (NoSuchMethodException exc) {
      System.out.println("Method Not Found.");
    }
  }
  public static void main(String args[]) {
    myMeth();
  }
}

Get annotation by type

import java.lang.annotation.Annotation;
import javax.ejb.Entity;
public class GetAnnotation {
    public static void main(String[] args) {
        Annotation[] as = Person.class.getAnnotations();
        for(int i=0;i<as.length;i++){
            System.out.println(as[i].annotationType());        
        }
        Annotation a =Person.class.getAnnotation(Entity.class);
        System.out.println(a);  
        Annotation[] das = Person.class.getDeclaredAnnotations();
        for(int i=0;i<das.length;i++){
            System.out.println(das[i].annotationType());        
        }        
    }
}

///

import javax.ejb.Entity;
import javax.ejb.AccessType;
import javax.ejb.Id;
import javax.ejb.GeneratorType;
import java.io.Serializable;
@Entity(access = AccessType.FIELD)
public class Person implements Serializable {
  @Id(generate = GeneratorType.AUTO)
  Integer id;
  String name;
}

Show all annotations for a class and a method.

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 test class")
@MyAnno(str = "Meta2", val = 99)
class Meta2 {
  @What(description = "An annotation test method")
  @MyAnno(str = "Testing", val = 100)
  public static void myMeth() {
    Meta2 ob = new Meta2();
    try {
      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();
      System.out.println("All annotations for myMeth:");
      for (Annotation a : annos)
        System.out.println(a);
    } catch (NoSuchMethodException exc) {
      System.out.println("Method Not Found.");
    }
  }
  public static void main(String args[]) {
    myMeth();
  }
}

Use reflection to display the annotation associated with a method.

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Method;
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnno {
  String str();
  int val();
}
class Meta {
  @MyAnno(str = "Annotation Example", val = 100)
  public static void myMeth() {
    Meta ob = new Meta();
    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();
  }
}