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How to get annotations at runtime using java reflection?


Description:

  • Incase if any annotation is specifies its retention policy as RUNTIME, then those annotations can be queried at runtime by any java program.
  • Below example code shows how to query an annotation values during runtime using java reflection.


Code:
package com.java2novice.annotations;

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

@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation{
	
	String key();
	String value();
}

public class MyAnnotationTest {

	@MyAnnotation(key="site", value="java2novice.com")
	public void myAnnotationTestMethod(){
		
		try {
			Class<? extends MyAnnotationTest> cls = this.getClass();
			Method mth = cls.getMethod("myAnnotationTestMethod");
			MyAnnotation myAnno = mth.getAnnotation(MyAnnotation.class);
			System.out.println("key: "+myAnno.key());
			System.out.println("value: "+myAnno.value());
		} catch (SecurityException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (NoSuchMethodException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
	
	public static void main(String a[]){
		
		MyAnnotationTest mat = new MyAnnotationTest();
		mat.myAnnotationTestMethod();
	}
}

Output:
key: site
value: java2novice.com
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  2. What is Retention policy in java annotations?
  3. How to get annotations at runtime using java reflection?
  4. How to get all annotations from a class?
  5. How to assign default values to custom annotations?
  6. What is marker annotation and how to process it at runtime?
  7. Built-in Java Annotations: @Deprecated example
  8. Built-in Java Annotations: @Override example
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  10. What is single member annotation and how to process it?
Knowledge Centre
What is abstract class or abstract method?
We cannot create instance for an abstract class. We can able to create instance for its subclass only. By specifying abstract keyword just before class, we can make a class as abstract class.

public abstract class MyAbstractClass{

}

Abstract class may or may not contains abstract methods. Abstract method is just method signature, it does not containes any implementation. Its subclass must provide implementation for abstract methods. Abstract methods are looks like as given below:

public abstract int getLength();
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-- Winston Churchill

About Author

I'm Nataraja Gootooru, programmer by profession and passionate about technologies. All examples given here are as simple as possible to help beginners. The source code is compiled and tested in my dev environment.

If you come across any mistakes or bugs, please email me to [email protected] or you can comment on the page.

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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java is registered trademark of Oracle.
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