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ICSM Computer

ICSM Computer

24-Apr-2025

In Java, you can create your very own annotations using the @interface keyword. These custom annotations may be applied to classes, methods, fields, or other elements, and you can retrieve and system them at runtime the use of mirrored image.

1. Basic Syntax

public @interface MyAnnotation {
    String value();  // a required element
}

This defines an annotation named MyAnnotation that has a single required element called value.

2. Adding Metadata: @Target and @Retention

You should define where the annotation can be applied and how long it should be retained.

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)  // Available at runtime via reflection
@Target(ElementType.METHOD)          // Can be applied to methods
public @interface MyAnnotation {
    String value();
    int count() default 1;           // Optional element with default value
}
  • @Retention(RetentionPolicy.RUNTIME) — Makes the annotation available during runtime.
  • @Target(ElementType.METHOD) — Limits annotation use to methods.

3. Using the Custom Annotation

public class MyService {
    @MyAnnotation(value = "Run this", count = 3)
    public void doSomething() {
        System.out.println("Doing something...");
    }

    @MyAnnotation(value = "Another method")
    public void anotherMethod() {
        System.out.println("Another thing...");
    }
}

4. Accessing Custom Annotations Using Reflection

import java.lang.reflect.Method;

public class AnnotationReader {
    public static void main(String[] args) {
        Class<MyService> clazz = MyService.class;

        for (Method method : clazz.getDeclaredMethods()) {
            if (method.isAnnotationPresent(MyAnnotation.class)) {
                MyAnnotation annotation = method.getAnnotation(MyAnnotation.class);
                System.out.println("Method: " + method.getName());
                System.out.println("  Value: " + annotation.value());
                System.out.println("  Count: " + annotation.count());
            }
        }
    }
}

5. Common @Target Values

ElementType Description
TYPE Class, interface, enum
FIELD Member variables
METHOD Methods
PARAMETER Parameters
CONSTRUCTOR Constructors
LOCAL_VARIABLE Local variables

Summary

  • Use @interface to define your annotation.
  • Use @Retention to specify when the annotation is available.
  • Use @Target to limit where the annotation can be used.
  • Use reflection to read annotations at runtime.
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