Java Annotations & A Real World Spring Example

Friday, 13 January 2012

Java Annotations & A Real World Spring Example

An "annotation" is a type of programming language definition and used as a “marker”. They can be thought as comment lines which programming language engine can understand. They don’t directly affect program execution but affect indirecly if wanted.

Definition

An annotation is defined with @interface keyword and is similar with an interface. It has attributes which are defined like interface methods. Attributes can have default values. Let’s define an annotation named “Page”, which defines UI pages of an application:

public @interface Page {
    int    id();
    String url();
    String icon() default "[none]";
    String name(); default "[none]";
}

Usage

Annotations are widely used to inform compiler or compile-time/runtime/deployment-time processing.
Usage of an annotation is simpler:

@Page(id=1, url=”studentView”, icon=“icons/student.png”, name=”Students”)
public class StudentWindow extends Window { … }

Annotations can also be defined for methods and attributes:

@AnAnnotation
public String getElementName() {…}

@AnAnnotation(type=”manager”, score=3)
public int income;

Examples

1) Reflection/code generation:

Methods having a specific annotation can be processed at runtime:

public @interface MyAnnotation { ... }
public class TestClass {
    @MyAnnotation
    public static method1() { ... }
    @MyAnnotation
    public static method2() { ... }
    @MyAnnotation
    public static method3() { ... }
}

public static void main(String[] args) {
    for (Method method : Class.forName("TestClass").getMethods()) {
        if (method.isAnnotationPresent(MyAnnotation.class)) {
            // do what you want
        }
    }
}

2) Spring bean configuration (this section requires Spring bean configuration knowledge):

Let’s use our “Page” annotation again:

package com.cmp.annotation;
public @interface Page {
    int    id();
    String url();
    String icon() default "[none]";
    String name(); default "[none]";
}

Say that we have a few classes having @Page annotation in a package:

@Page(id=1, url=”studentView”, icon=“icons/student.png”, name=”Students”)
public class StudentWindow extends Window { … }

If we define a bean configuration as below in a Spring application-context.xml file, Spring will create class instances “which has @Page annotation” placed in “given package”.

<context:component-scan base-package="com.cmp.ui" annotation-config="true">
<context:include-filter type="annotation" expression="com.cmp.annotation.Page"/>
</context:component-scan>