{"id":833,"date":"2012-01-13T16:32:00","date_gmt":"2012-01-13T16:32:00","guid":{"rendered":"http:\/\/www.javacodegeeks.com\/2012\/10\/java-annotations-a-real-world-spring-example.html"},"modified":"2012-10-21T22:46:25","modified_gmt":"2012-10-21T22:46:25","slug":"java-annotations-real-world-spring","status":"publish","type":"post","link":"https:\/\/www.javacodegeeks.com\/2012\/01\/java-annotations-real-world-spring.html","title":{"rendered":"Java Annotations & A Real World Spring Example"},"content":{"rendered":"
An “annotation” is a type of programming language definition and used as a \u201cmarker\u201d. They can be thought as comment lines which programming language engine can understand. They don\u2019t directly affect program execution but affect indirecly if wanted.<\/p>\n

Definition<\/strong><\/p>\n

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\u2019s define an annotation named \u201cPage\u201d, which defines UI pages of an application:<\/p>\n

public @interface Page {\r\n    int    id();\r\n    String url();\r\n    String icon() default \"[none]\";\r\n    String name(); default \"[none]\";\r\n}<\/pre>\n
Usage<\/strong><\/p>\n
Annotations are widely used to inform compiler or compile-time\/runtime\/deployment-time processing.
\nUsage of an annotation is simpler:<\/p>\n
@Page(id=1, url=\u201dstudentView\u201d, icon=\u201cicons\/student.png\u201d, name=\u201dStudents\u201d)\r\npublic class StudentWindow extends Window { \u2026 }<\/pre>\n
Annotations can also be defined for methods and attributes:<\/p>\n
@AnAnnotation\r\npublic String getElementName() {\u2026}\r\n\r\n@AnAnnotation(type=\u201dmanager\u201d, score=3)\r\npublic int income;\r\n<\/pre>\n
Examples<\/strong><\/p>\n
1) <\/strong>Reflection\/code generation:
 
<\/div> <\/div><\/p>\n
Methods having a specific annotation can be processed at runtime:<\/p>\n
public @interface MyAnnotation { ... }\r\npublic class TestClass {\r\n    @MyAnnotation\r\n    public static method1() { ... }\r\n    @MyAnnotation\r\n    public static method2() { ... }\r\n    @MyAnnotation\r\n    public static method3() { ... }\r\n}\r\n\r\npublic static void main(String[] args) {\r\n    for (Method method : Class.forName(\"TestClass\").getMethods()) {\r\n        if (method.isAnnotationPresent(MyAnnotation.class)) {\r\n            \/\/ do what you want\r\n        }\r\n    }\r\n}\r\n<\/pre>\n
2)<\/strong> Spring bean configuration (this section requires Spring bean configuration knowledge):<\/p>\n
Let\u2019s use our \u201cPage\u201d annotation again:<\/p>\n
package com.cmp.annotation;\r\npublic @interface Page {\r\n    int    id();\r\n    String url();\r\n    String icon() default \"[none]\";\r\n    String name(); default \"[none]\";\r\n}<\/pre>\n
Say that we have a few classes having @Page annotation in a package:<\/p>\n
@Page(id=1, url=\u201dstudentView\u201d, icon=\u201cicons\/student.png\u201d, name=\u201dStudents\u201d)\r\npublic class StudentWindow extends Window { \u2026 }\r\n<\/pre>\n
If we define a bean configuration as below in a Spring application-context.xml file, Spring will create class instances \u201cwhich has @Page annotation\u201d placed in \u201cgiven package\u201d.<\/p>\n
<context:component-scan base-package=\"com.cmp.ui\" annotation-config=\"true\">\r\n<context:include-filter type=\"annotation\" expression=\"com.cmp.annotation.Page\"\/>\r\n<\/context:component-scan>\r\n<\/pre>\n
So, we have been enforced Spring to instantiate only a selection of classes at runtime.<\/p>\n
For more detailed info about annotations, please refer to:<\/p>\n
http:\/\/docs.oracle.com\/javase\/1.5.0\/docs\/guide\/language\/annotations.html<\/a>
\nhttp:\/\/docs.oracle.com\/javase\/tutorial\/java\/javaOO\/annotations.html<\/a><\/p>\n
Reference: <\/i><\/strong>Java Annotations & A Real World Spring Example<\/a> from our JCG partner<\/a> Cagdas Basaraner at the CodeBalance<\/a> blog.<\/p>\n
Related Articles :<\/i><\/strong><\/p>\n