Tags: annotation

What is Annotations?
Annotations, a form of metadata, provide data about a program that is not part of the program it self. Annotations has no direct effect on the operation of the code they annotate.

Annotation Syntax

public @interface AnnotationName {
 String elementName();
 
 /* Element with default value */
 String elementDefault() default "HelloWorld";
}

Annotations used in example
We've used @Retention, @Target and @Documented annotations to understand basics of Annotations.

@Retention
Indicates how long annotations with the annotated type are to be retained.

• RetentionPolicy.SOURCE – The marked annotation is retained only in the source level and is ignored by the compiler.
• RetentionPolicy.CLASS – The marked annotation is retained by the compiler at compile time, but is ignored by the Java Virtual Machine (JVM).
• RetentionPolicy.RUNTIME – The marked annotation is retained by the JVM so it can be used by the runtime environment.

@Target
Indicates the contexts in which an annotation type is applicable.

• ElementType.ANNOTATION_TYPE can be applied to an annotation type.
• ElementType.CONSTRUCTOR can be applied to a constructor.
• ElementType.FIELD can be applied to a field or property.
• ElementType.LOCAL_VARIABLE can be applied to a local variable.
• ElementType.METHOD can be applied to a method-level annotation.
• ElementType.PACKAGE can be applied to a package declaration.
• ElementType.PARAMETER can be applied to the parameters of a method.
• ElementType.TYPE can be applied to any element of a class.

@Documented
Indicates that annotations with a type are to be documented by javadoc and similar tools by default.

Source code (Default.java)

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
 * Annotation to hold default value of field and method.
 */
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.METHOD})
public @interface Default {
 String value();
}

Source code (Author.java)

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
 * Annotation to hold the Author details.
 */
@Documented
/* Un-comment following two annotation in order to access it at Runtime in 'main' method of AnnotationExample.java */
// @Retention(RetentionPolicy.RUNTIME)
// @Target({ElementType.FIELD, ElementType.METHOD})
public @interface Author {
 /* 'company' element is optional because we provided default constant value. */
 String company() default "javaQuery";
 
 /* Compulsory elements of Author Annotation */
 String name();
 String date();
 String description();
}

Source code

import java.lang.reflect.Field;
import java.lang.reflect.Method;
/**
 * Example of How to access annotations in Java
 * @author javaQuery
 * @date 13th October, 2015
 */
public class AnnotationExample {
 /* Variable/field with @Default annotation */
 @Default(value = "vicky.thakor@javaquery.com")
 private String email;
 /* Method with two @Author and @Default Annotation */
 @Author(name = "Vicky Thakor", date = "13th October, 2015", description = "Method prints welcome message")
 @Default("Hello World!")
 public String welcomeMessage() {
 return "Hello JavaQuery";
 }
 public static void main(String[] args) {
 System.out.println("Inspect class fields");
 System.out.println("++++++++++++++++++++++++++++++++++++++++");
 
 /* Get all declared fields of class */
 Field[] fields = AnnotationExample.class.getDeclaredFields();
 for (Field field : fields) {
 /* Check particular annotation is present or not */
 if (field.isAnnotationPresent(Default.class)) {
 Default getDefault = field.getAnnotation(Default.class);
 /* Get value of annotation */
 System.out.println("> Default value on field '" + field.getName() + "': " + getDefault.value());
 }
 }
 System.out.println("\n");
 System.out.println("Inspect class methods");
 System.out.println("++++++++++++++++++++++++++++++++++++++++");
 Method[] methods = AnnotationExample.class.getDeclaredMethods();
 for (Method method : methods) {
 /* Check particular annotation is present or not */
 /* In order to work following if uncomment @Retention and @Target in Author.java */
 if (method.isAnnotationPresent(Author.class)) {
 Author getAuthor = method.getAnnotation(Author.class);
 /* Get value of annotation */
 System.out.println("> Author name on method '" + method.getName() + "': " + getAuthor.name());
 } else {
 System.out.println("> Author annotation is not available on method '" + method.getName()+"'");
 }
 /* Check particular annotation is present or not */
 if (method.isAnnotationPresent(Default.class)) {
 Default getDefault = method.getAnnotation(Default.class);
 /* Get value of annotation */
 System.out.println("> Default value on method '" + method.getName() + "': " + getDefault.value());
 }
 System.out.println("--------------------------------------");
 }
 }
}

Output

Inspect class fields
++++++++++++++++++++++++++++++++++++++++
> Default value on field 'email': vicky.thakor@javaquery.com
Inspect class methods
++++++++++++++++++++++++++++++++++++++++
> Author annotation is not available on method 'main'
--------------------------------------
> Author annotation is not available on method 'welcomeMessage'
> Default value on method 'welcomeMessage': Hello World!
--------------------------------------

Tags: annotation

(追記) (追記ここまで)

annotation