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Archive for May, 2009

Java Annotations provide Java developers full potential of doing things in more easy and nice way. I will not go explaining Java annotations pros, cons, and the definitions; you can find out here http://java.sun.com/docs/books/tutorial/java/javaOO/annotations.html

instead I will add some piece which I hardly found: how can I write my own Annotation class and make use of them?! here you go:

I will take a simple Beans that Document who wrote some Java class and who reviewed it.

Requirements

  1. Author, Data Type: String
  2. Reviewers, Data Type: Array of String

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

@Retention(RetentionPolicy.RUNTIME)

public @interface Documentation {

String author();

String [] reviewers() default {};

}

what have we done? we created a file called Documentation.java that is an interface, and because it’s an annotation it’s written like this @interface

we have add two methods signatures:

  1. author: returns a String.
  2. reviewers: returns an array of string, that is an optional element to mention, so we provided a default value of empty string array (I know you noticed that 😉

a question: what does@Retention(RetentionPolicy.RUNTIME) do? well, it indicates to the JVM that this annotation is intended to be used at runtime, simply.

now, it’s time to introduce the famous Hello World class 😀

import java.lang.annotation.Annotation;

@Documentation(author=”Alex”, reviewers={“Divid”, “Osama”})

public class HelloWorld {

public static void main(String [] args){

System.out.println(“checking documentation for class HelloWorld.java”);

Annotation annotation = HelloWorld.class.getAnnotation(Documentation.class);

if (annotation == null) {

System.out.println(“No Documentation was found for class HelloWorld, exit”);

System.exit(0);

}

Documentation doc = (Documentation) annotation;

System.out.println(“author: ” + doc.author());

for (String reviewer : doc.reviewers()) {

System.out.println(“reviewer: ” + reviewer);

}

System.exit(0);

}

}

what have we done? we created a Java Class called HelloWorld, that uses @Documentation annotation.

we have extracted the informtion provided for this class in @Documentation annotation element.

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