Understanding Annotations in Java

Introduction

This article discusses the way of creating annotations in Java. It also shows how to apply annotations to other declarations. Finally it discusses how to obtain annotation information at runtime using Reflection.

Background

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.

Using the code

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, orRUNTIME.

• 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.

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// 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 theAnnotation 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:

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// Applying annotation to the class
@MyAnnotation(author="Azim",date="22/10/2011,23/10/2011")
public class Test
{
    // 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:

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public static void showAnnotations()
// Function to show annotation information
{
    Test test=new Test(); // 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());
    }
}

In the above code, I have queried annotations applied to the class Test as well as the methodtestMethod().

To obtain annotation information for the class, the following statement is used:

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MyAnnotation annotation1=(MyAnnotation)c.getAnnotation(MyAnnotation.class);

To obtain annotation information for the method, the following statement is used:

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MyAnnotation annotation2=m.getAnnotation(MyAnnotation.class);

The annotation information is printed by using annotation objects.

Points of interest

The Java program is compiled on the command line as follows:

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javac Test.java

and executed as follows:

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java Test