Annotation In Java

What is Annotation?

Annotation is a new feature introduced by J2SE 5 that allows programmers to embed additional information called metadata into a Java source file. Annotations do not alter the execution of a program but the information embedded using annotations can be used by various tools during development and deployment.

How to create Annotation:

Creating an annotation is similar to creating an interface. But the annotation declaration is preceded by an @ symbol. The annotation declaration itself is annotated with the @Retention annotation. The @Retention annotation is used to specify the retention policy, which can be SOURCE, CLASS, or RUNTIME.

• RetentionPolicy.SOURCE retains an annotation only in the source file and discards it during compilation.
• RetentionPolicy.CLASS stores the annotation in the .class file but does not make it available during runtime.
• RetentionPolicy.RUNTIME stores the annotation in the .class file and also makes it available during runtime.

Ex:

// Specifying runtime retention policy
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation
{
    String author();    // Annotation member
    String date();      // Annotation member
}

An annotation cannot have the extends clause. However, annotations implicitly

extend the Annotation interface. The body of an annotation consists 
of method declarations without body. These methods work like fields.

In the above example, I have created two members, author and date to represent information about the creator, and date of writing of the class and method.

Once an annotation is created, it can be applied to classes, methods, fields, parameters, enums, and so on. While applying an annotation to a declaration, you must provide values for its members as follows:

Ex:

// Applying annotation to the class
@MyAnnotation(author="Azim",date="22/10/2011,23/10/2011")
public class TestAnnotation
{
    // Applying annotation to the method
    @MyAnnotation(author="Azim",date="22/10/2011")
    public static void testMethod()
    {
        System.out.println("Welcome to Java");
        System.out.println("This is an example of Annotations");
    }
    public static void main(String args[])
    {
        testMethod();
        showAnnotations();
    }

Annotations can be queried at runtime using Reflection as follows:

Ex:

public static void showAnnotations()
// Function to show annotation information
{
    TestAnnotation test=new TestAnnotation(); // Instantiating Test class
    try
    {
        Class c=test.getClass(); // Getting Class reference
        Method m=c.getMethod("testMethod"); // Getting Method reference
        // Getting Class annotation
        MyAnnotation annotation1=
          (MyAnnotation)c.getAnnotation(MyAnnotation.class);
        // Getting Method annotation
        MyAnnotation annotation2=m.getAnnotation(MyAnnotation.class);
        // Displaying annotation information
        System.out.println("Author of the class: "+annotation1.author());
        // Displaying annotation information
        System.out.println("Date of Writing the class: "+annotation1.date());
        // Displaying annotation information
        System.out.println("Author of the method: "+annotation2.author());
        // Displaying annotation information
        System.out.println("Date of Writing the method: "+annotation2.date());
    }
    catch(NoSuchMethodException ex)
    {
        System.out.println("Invalid Method..."+ex.getMessage());
    }
}