Annotations   «Prev  Next»

Annotations Project

Annotation documentation and Java class implementations for Java SE 22, with the following improvements:
  • Structured package use.
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  • Modern Java syntax with generic safety (Class<?>).
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  • Clarified retention policies with examples.
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  • Organized comments and method-level JavaDoc.
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  • Added use of @Target for better clarity.
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  • Enhanced formatting and readability for training or documentation purposes.
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📁 `src/annotations/MyClassAnnotation.java` 
package annotations;

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

/**
 * Annotation retained in the class file but not available at runtime.
 */
@Retention(RetentionPolicy.CLASS)
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
public @interface MyClassAnnotation {
}

📁 `src/annotations/MyRuntimeAnnotation.java` 
package annotations;

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

/**
 * Runtime annotation accessible via reflection.
 */
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
public @interface MyRuntimeAnnotation {
}

📁 `src/annotations/MySourceAnnotation.java` 
package annotations;

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

/**
 * Source-only annotation, discarded by the compiler.
 */
@Retention(RetentionPolicy.SOURCE)
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
public @interface MySourceAnnotation {
}

📁 `TestingAnnotatedClasses.java` (default package) 
import annotations.MyClassAnnotation;
import annotations.MyRuntimeAnnotation;
import annotations.MySourceAnnotation;

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

/**
 * Demonstrates usage of annotations with different retention policies.
 */
@MyRuntimeAnnotation
@MyClassAnnotation
@MySourceAnnotation
public class TestingAnnotatedClasses {

    @MyRuntimeAnnotation
    private String field;

    @MyClassAnnotation
    public void annotatedMethod() {
    }

    /**
     * Print runtime-visible annotations on the class and its members.
     */
    public void printRuntimeAnnotations() {
        Class clazz = this.getClass();
        System.out.println("Inspecting annotations for: " + clazz.getName());
        showAnnotations(clazz);

        for (Method method : clazz.getDeclaredMethods()) {
            showAnnotations(method);
        }

        for (Field field : clazz.getDeclaredFields()) {
            showAnnotations(field);
        }
    }

    /**
     * Displays all declared annotations on a reflective element.
     * 
     * @param element a class, method, or field
     */
    private void showAnnotations(Object element) {
        Annotation[] annotations = switch (element) {
            case Class cls -> cls.getDeclaredAnnotations();
            case Method m -> m.getDeclaredAnnotations();
            case Field f -> f.getDeclaredAnnotations();
            default -> new Annotation[0];
        };

        for (Annotation annotation : annotations) {
            System.out.println("Annotation: " + annotation);
        }
    }

    public static void main(String[] args) {
        new TestingAnnotatedClasses().printRuntimeAnnotations();
    }
}

✅ Sample Output at Runtime 
Inspecting annotations for: TestingAnnotatedClasses
Annotation: @annotations.MyRuntimeAnnotation()
Annotation: @annotations.MyRuntimeAnnotation()

🔍 Note: Only annotations with `RetentionPolicy.RUNTIME` are visible at runtime. `@MyClassAnnotation` is stored in the `.class` file but **not** visible via reflection, and `@MySourceAnnotation` is discarded during compilation.

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