Java annotations

Before, JDK 1.5 XML was used to access metadata. Annotations have replaced it since java 1.5

Spring 2.X was XML based and Spring 3 was Annotation based

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
                           http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="myBean" class="com.example.MyBean">
        <property name="message" value="Hello, Spring!"/>
    </bean>

</beans>

With Spring 3.x and later, you can use annotations for configuration, which simplifies the setup.

With just one line of Annotation, the xml document was eliminated

@Component
public class MyBean {
    @Value("Hello, Spring!")
    private String message;

    public String getMessage() {
        return message;
    }
}

In Java, annotations have two main types of syntax:

• declaration and
• utilization.

Declaration Syntax

Annotations are defined using the @interface keyword.

@Retention(RetentionPolicy.RUNTIME) // Retention policy: available at runtime
@Target(ElementType.METHOD) // Target: can be applied to methods
public @interface MyCustomAnnotation {
    String description() default "No description"; // Default value for the annotation
}

@Retention

• Runtime level : RetentionPolicy.RUNTIME): Specifies that the annotation should be retained at runtime and be available for reflection.
• Source level - RetentionPolicy.SOURCE
• Classlevel - RetentionPolicy.CLASS

@Target

• Method level - ElementType.METHOD Specifies that this annotation can only be applied to methods.
• Class Level - ElementType.TYPE
• Field level - ElementType.FIELD
• Package level - ElementType.PACKAGE
• local variable level - ElementType.LOCAL_VARIABLE

description() default "No description":

• An element of the annotation with a default value.

Utilization Syntax

This is where annotation in the code is used. Apply annotation to various Java elements (such as methods, classes, fields, etc.) according to its @Target specification.

Marker Annotation : @Override

Single Valued Annotation : @SuppressWarning("Unchecked")

Multivalued Annotation : below custom code

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MultiValueAnnotation {
    String name();
    int value();
    String[] tags() default {};  // Default empty array
}

Usage

@MultiValueAnnotation(
    name = "SampleAnnotation",
    value = 100,
    tags = {"example", "annotation"}
)
public class MyClass {
    // Class implementation
}

Useful Custom Annotation

Annotation to mark code that is duplicated and be moved to a common place

/**
 * Annotation to mark classes or methods that can be moved to a common library.
 */
@Retention(RetentionPolicy.SOURCE)//discarded by the compiler.
@Target({ElementType.TYPE, ElementType.METHOD})
public @interface MoveToCommonLibrary {
    String description() default "";
}

@ReviewRequired custom annotation for keepping a watch on the tech debt items

@Target({ElementType.LOCAL_VARIABLE, ElementType.FIELD,ElementType.TYPE, ElementType.METHOD, ElementType.PACKAGE})
@Retention(RetentionPolicy.SOURCE) //discarded by the compiler.
public @interface ReviewRequired {
    String description() default "";

    /**
     * The reviewers assigned to this class/method.
     * @return the list of reviewers
     */
    String[] reviewers() default {"nitin"}; // Using an array for compatibility
}