Annotations in Java

Annotations in Java, provide a mean to describe classes, fields and methods. Essentially, they are a form of metadata added to a Java source file, they can’t affect the semantics of a program directly. However, annotations can be read at run-time using Reflection & this process is known as Introspection. Then it could be used to modify classes, fields or methods.

This feature, is often exploited by Libraries & SDKs (hibernate, JUnit, Spring Framework) to simplify or reduce the amount of code that a programmer would unless do in order to work with these Libraries or SDKs.Therefore, it’s fair to say Annotations and Reflection work hand-in hand in Java.

We also get to limit the availability of an annotation to either compile-time or runtime. Below is a simple example on creating a custom annotation

01. Driver.java

package io.hamzeen;
import java.lang.annotation.Annotation;
public class Driver {
    public static void main(String[] args) {
        Class<TestAlpha> obj = TestAlpha.class;
        if (obj.isAnnotationPresent(IssueInfo.class)) {
            Annotation annotation = obj.getAnnotation(IssueInfo.class);
            IssueInfo testerInfo = (IssueInfo) annotation;
            System.out.printf("%nType: %s", testerInfo.type());
            System.out.printf("%nReporter: %s", testerInfo.reporter());
            System.out.printf("%nCreated On: %s%n%n",
                    testerInfo.created());
        }
    }
}

02. TestAlpha.java

package io.hamzeen;
import io.hamzeen.IssueInfo;
import io.hamzeen.IssueInfo.Type;
@IssueInfo(type = Type.IMPROVEMENT, reporter = "Hamzeen. H.")
public class TestAlpha {
}

03. IssueInfo.java

package io.hamzeen;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
 * @author Hamzeen. H.
 * @created 10/01/2015
 * 
 * IssueInfo annotation definition
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface IssueInfo {
    public enum Type {
        BUG, IMPROVEMENT, FEATURE
    }
    Type type() default Type.BUG;
    String reporter() default "Vimesh";
    String created() default "10/01/2015";
}

P.S. This post is based on my stack-overflow answer at, http://stackoverflow.com/questions/1372876/how-and-where-are-annotations-used-in-java/32903945#32903945