Creating and Processing Custom Annotations in Advanced Java

Custom annotations in Java allow developers to define metadata for classes, methods, fields, or other program elements. This metadata can be processed at compile-time or runtime to enhance functionality or enforce specific behaviors. This article demonstrates how to create and process custom annotations in Advanced Java.

Step-by-Step Guide

Step 1: Define a Custom Annotation

To define a custom annotation, use the @interface keyword. Annotations can include elements (attributes) with default values.

Example:

            import java.lang.annotation.*;

            // Define a custom annotation
            @Retention(RetentionPolicy.RUNTIME) // Make annotation available at runtime
            @Target(ElementType.METHOD)         // Restrict usage to methods
            public @interface MyAnnotation {
                String value() default "Default Value"; // Annotation element with default
                int priority() default 1;              // Another element
            }
        

Step 2: Apply the Custom Annotation

Use the custom annotation on methods, fields, or classes as specified in the @Target.

Example:

            public class MyClass {
                @MyAnnotation(value = "Custom Annotation Example", priority = 5)
                public void annotatedMethod() {
                    System.out.println("This method is annotated with @MyAnnotation.");
                }

                @MyAnnotation // Using default values
                public void anotherAnnotatedMethod() {
                    System.out.println("This method uses default annotation values.");
                }
            }
        

Step 3: Process the Custom Annotation

Use reflection to process the custom annotation at runtime. This involves inspecting classes, methods, or fields for the annotation and retrieving its values.

Example:

            import java.lang.reflect.Method;

            public class AnnotationProcessor {
                public static void main(String[] args) {
                    try {
                        // Get the Class object
                        Class clazz = MyClass.class;

                        // Loop through all methods
                        for (Method method : clazz.getDeclaredMethods()) {
                            // Check if the method has @MyAnnotation
                            if (method.isAnnotationPresent(MyAnnotation.class)) {
                                // Get the annotation
                                MyAnnotation annotation = method.getAnnotation(MyAnnotation.class);

                                // Print annotation details
                                System.out.println("Method: " + method.getName());
                                System.out.println("Value: " + annotation.value());
                                System.out.println("Priority: " + annotation.priority());
                            }
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        

Step 4: Advanced Use Cases

Custom annotations can be used for more advanced tasks such as enforcing validation rules, injecting dependencies, or controlling behavior.

Example: Validation Annotation

Create an annotation for validating input fields.

            import java.lang.annotation.*;

            @Retention(RetentionPolicy.RUNTIME)
            @Target(ElementType.FIELD)
            public @interface NotNull {
            }

            class User {
                @NotNull
                private String name;

                public User(String name) {
                    this.name = name;
                }

                public String getName() {
                    return name;
                }
            }

            public class ValidationProcessor {
                public static void main(String[] args) {
                    try {
                        User user = new User(null); // Invalid user
                        Class clazz = user.getClass();

                        for (var field : clazz.getDeclaredFields()) {
                            if (field.isAnnotationPresent(NotNull.class)) {
                                field.setAccessible(true);
                                Object value = field.get(user);

                                if (value == null) {
                                    System.out.println("Field " + field.getName() + " cannot be null!");
                                }
                            }
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        

Example: Custom Logging Annotation

Implement a custom annotation to log method execution.

            import java.lang.annotation.*;
            import java.lang.reflect.*;

            @Retention(RetentionPolicy.RUNTIME)
            @Target(ElementType.METHOD)
            public @interface LogExecution {
            }

            class Service {
                @LogExecution
                public void performTask() {
                    System.out.println("Task performed!");
                }
            }

            public class LoggingProcessor {
                public static void main(String[] args) {
                    try {
                        Service service = new Service();
                        Method[] methods = service.getClass().getDeclaredMethods();

                        for (Method method : methods) {
                            if (method.isAnnotationPresent(LogExecution.class)) {
                                System.out.println("Executing method: " + method.getName());
                                method.invoke(service); // Invoke the method
                            }
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        

Step 5: Best Practices

• Always use @Retention and @Target to specify the annotation's scope and availability.
• Keep annotations lightweight and focused on specific functionality.
• Avoid using custom annotations for trivial tasks to maintain code clarity.

Conclusion

Custom annotations in Java provide a powerful mechanism for adding metadata to code and enabling dynamic behaviors at runtime. By understanding how to define, apply, and process annotations, developers can create flexible and reusable solutions.