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JAVA EXAMPLE PROGRAMS

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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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Knowledge Centre
Default value of a local variables?
The local variables are not initialized to any default values. We should not use local variables with out initialization. Even the java compiler throws error.
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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java is registered trademark of Oracle.
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