Thursday, February 24, 2011

Java Annotations

A few days ago in our project we needed that the field which we are using should not be displayed to the user but relevant title should be displayed. This title will never change as this property is one of the important properties of that class.
There I thought that Java Annotations might be useful for me in this case, now for those who don't know about annotations I will explain a little about them.


Java Annotations is the feature provided by Java with the help of which we can add data to our class, method or fields.
Now there are three different ways of annotations that we can use.

Types
Source  - These types of annotations are not present in class file, they are only present in the source file and they are discarded by the compiler.
Class     - These types of annotations are present in class file and source file, they are not loaded by JVM.
Runtime - These types of annotations are present in class file and source file and they are also loaded by JVM which we can use at runtime.

How to create Annotations


Writing Java Annotations is very easy. Its the same way we create an interface but just with @ sign preceding to interface.
Here is an example.

// We want that our annotation should be compiled and should be loaded by JVM.
@Retention(RetentionPolicy.RUNTIME)
public @interface DisplayName{
     String value();
}

Note the declaration of the variable value, we don't need to write getter or setter method for the variables in the annotation, that is managed by Java.

How to use Annotations
Now as we have our annotation ready, we can use it.
Following example shows how can we apply annotations.

@DisplayName("Center")
public class Centers{
     private Integer center_id;
  
     @DisplayName("Center Name")
     private String centerName;

     // other code
}

This is how we can use our annotations, now we can access them in our Java code. Following is an example how we can use annotations.

public class AnTest{
     public static void main(String args[]){
          Center c= new Center();
          System.out.println(c.getClass().getAnnotation(DisplayName.class).value());
     }
}

This will print the value for DisplayName annotation assigned to Centers class.
Annotations are really useful, with which we can add metadata to our fields, classes and methods.
More information about annotations
Annotations
http://download.oracle.com/javase/tutorial/java/javaOO/annotations.html
http://en.wikipedia.org/wiki/Java_annotation

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