Custom Annotations

Spring Boot provides a wide range of built-in annotations that help simplify and streamline the development of applications. However, there may be scenarios where you need to create your own custom annotations to encapsulate specific behavior or to add additional metadata to your code. In this section, we will explore how to create and use custom annotations in Spring Boot.

Creating Custom Annotations

To create a custom annotation in Spring Boot, you need to define a new annotation type using the `@interface` keyword. Here’s an example:

“`java
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.TYPE)
public @interface CustomAnnotation {

String value() default “”;

int priority() default 0;
}
“`

In this example, we define a custom annotation called `CustomAnnotation`. It has two elements: `value` and `priority`. The `@Retention` annotation specifies that the annotation should be retained at runtime, and the `@Target` annotation specifies that the annotation can be applied to a type (class, interface, enum).

Using Custom Annotations

Once you have defined your custom annotation, you can use it to annotate your classes, methods, or fields. Here’s an example:

“`java
@CustomAnnotation(value = “CustomAnnotationExample”, priority = 1)
public class CustomAnnotationExample {

@CustomAnnotation(value = “CustomAnnotationMethod”, priority = 2)
public void customMethod() {
// Method implementation goes here
}

@CustomAnnotation(priority = 3)
private String customField;

// …
}
“`

In this example, we apply the `CustomAnnotation` to the `CustomAnnotationExample` class, the `customMethod` method, and the `customField` field. We can also specify values for the annotation elements, such as the `value` and `priority`.

Processing Custom Annotations

To process custom annotations, you can use reflection or custom annotation processors. Reflection allows you to inspect the annotated elements at runtime and retrieve the values of the annotation elements. Here’s an example of how you can process custom annotations using reflection:

“`java
public class AnnotationProcessor {

public static void processAnnotations(Class<?> clazz) {
if (clazz.isAnnotationPresent(CustomAnnotation.class)) {
CustomAnnotation annotation = clazz.getAnnotation(CustomAnnotation.class);
String value = annotation.value();
int priority = annotation.priority();
// Process annotation values
}

// Process annotated methods and fields
// …
}
}
“`

In this example, the `processAnnotations` method takes a `Class` object as an argument and checks if it is annotated with `CustomAnnotation`. If it is, it retrieves the annotation instance and extracts the values of the annotation elements.

Custom annotations can be used to add additional metadata or behavior to your code, making it more expressive and easier to understand. They allow you to define your own domain-specific annotations and leverage them in your Spring Boot applications.