Java Annotations

Introduction

Annotations in Java are a form of metadata that provide information about the program but do not have any direct effect on the code execution. They are used to give additional information to the compiler, code analyzers, and frameworks. Annotations can be applied to declarations of classes, fields, methods, parameters, and more.

Table of Contents

  1. What are Annotations?
  2. Built-in Annotations
  3. Custom Annotations
  4. Meta-annotations
  5. Real-World Analogy
  6. Examples
  7. Conclusion

1. What are Annotations?

Annotations are special markers in the code that provide metadata. They can be used for various purposes such as compiler instructions, runtime processing, and code generation.

Key Points:

  • Annotations do not change the action of the compiled program.
  • They can be applied to various elements like classes, methods, variables, parameters, etc.
  • They can be retained at runtime, source code, or class file level.

2. Built-in Annotations

Java provides several built-in annotations that serve different purposes. Some of the commonly used built-in annotations are:

@Override

The @Override annotation indicates that a method is intended to override a method in a superclass.

@Override
public void run() {
    // method implementation
}

@Deprecated

The @Deprecated annotation indicates that a method, class, or field is outdated and should not be used.

@Deprecated
public void oldMethod() {
    // method implementation
}

@SuppressWarnings

The @SuppressWarnings annotation tells the compiler to suppress specific warnings that it would otherwise generate.

@SuppressWarnings("unchecked")
public void method() {
    // method implementation
}

@FunctionalInterface

The @FunctionalInterface annotation indicates that an interface is intended to be a functional interface, which means it has exactly one abstract method.

@FunctionalInterface
public interface MyFunctionalInterface {
    void abstractMethod();
}

@SafeVarargs

The @SafeVarargs annotation indicates that a method does not perform potentially unsafe operations on its varargs parameter.

@SafeVarargs
public final void safeMethod(List<String>... lists) {
    // method implementation
}

@Retention

The @Retention annotation specifies how long annotations with the annotated type are to be retained.

@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
    // elements
}

@Target

The @Target annotation specifies the kinds of program elements to which an annotation type is applicable.

@Target(ElementType.METHOD)
public @interface MyAnnotation {
    // elements
}

3. Custom Annotations

You can create your own custom annotations in Java. Custom annotations are defined using the @interface keyword.

Example: Custom Annotation

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

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MyAnnotation {
    String value();
    int count() default 1;
}

Using Custom Annotation

public class Test {
    @MyAnnotation(value = "Test Method", count = 3)
    public void testMethod() {
        // method implementation
    }

    public static void main(String[] args) {
        Test test = new Test();
        test.testMethod();
    }
}

4. Meta-annotations

Meta-annotations are annotations that apply to other annotations. Java provides several meta-annotations to define the behavior of custom annotations.

@Retention

Specifies how long annotations are to be retained.

  • RetentionPolicy.SOURCE: Annotations are retained only in the source code and are discarded during compilation.
  • RetentionPolicy.CLASS: Annotations are retained in the class file but are not available at runtime.
  • RetentionPolicy.RUNTIME: Annotations are retained at runtime and can be accessed through reflection.

@Target

Specifies the kinds of program elements to which an annotation type is applicable.

  • ElementType.TYPE: Applicable to any element of a class.
  • ElementType.FIELD: Applicable to fields.
  • ElementType.METHOD: Applicable to methods.
  • ElementType.PARAMETER: Applicable to parameters.
  • ElementType.CONSTRUCTOR: Applicable to constructors.
  • ElementType.LOCAL_VARIABLE: Applicable to local variables.
  • ElementType.ANNOTATION_TYPE: Applicable to annotation types.
  • ElementType.PACKAGE: Applicable to packages.

@Inherited

Indicates that an annotation type is automatically inherited.

@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MyInheritedAnnotation {
    // elements
}

@Documented

Indicates that elements annotated with this annotation should be documented by Javadoc and similar tools.

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MyDocumentedAnnotation {
    // elements
}

5. Real-World Analogy

Consider a scenario where you have a library system:

  • A book might have annotations like @Deprecated to indicate that it is an old edition.
  • A library interface might have an annotation @FunctionalInterface to indicate it has only one method to be implemented.
  • Custom annotations like @LibraryInfo might be used to add metadata about the library, such as the name and location.

6. Examples

Example: Custom Annotation with Meta-annotations

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

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface MyCustomAnnotation {
    String value();
}

public class Example {
    @MyCustomAnnotation("Example Method")
    public void exampleMethod() {
        System.out.println("This is an example method.");
    }

    public static void main(String[] args) {
        Example example = new Example();
        example.exampleMethod();
    }
}

Output:

This is an example method.

Example: Accessing Annotations Using Reflection

import java.lang.reflect.Method;

public class ReflectionExample {
    @MyCustomAnnotation("Reflection Method")
    public void reflectionMethod() {
        System.out.println("This is a reflection method.");
    }

    public static void main(String[] args) {
        try {
            Method method = ReflectionExample.class.getMethod("reflectionMethod");
            MyCustomAnnotation annotation = method.getAnnotation(MyCustomAnnotation.class);
            System.out.println("Annotation value: " + annotation.value());
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
    }
}

Output:

Annotation value: Reflection Method
This is a reflection method.

7. Conclusion

Annotations in Java are a powerful way to add metadata to your code. They provide valuable information to the compiler, code analyzers, and frameworks without affecting the program’s logic. Understanding built-in annotations, custom annotations, and meta-annotations is essential for writing maintainable and flexible Java applications. With the ability to create custom annotations, you can tailor annotations to fit your specific needs and enhance the readability and functionality of your code.

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