@Retention is a meta-annotation, meaning it’s an annotation used to annotate other annotations.
It tells the compiler and JVM how long your annotation should be retained (kept).

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✅ Why is @Retention Important?

Not all annotations need to be available at runtime — some are only needed during compilation, others only during class file generation. @Retention defines where the annotation will be “visible” or “kept”.

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🔗 Possible Retention Policies

Retention	Meaning	Example Use Case
RetentionPolicy.SOURCE	Annotation exists only in source code, removed during compilation	@Override (compiler-only checks)
RetentionPolicy.CLASS	Annotation exists in .class file, but not available at runtime	Internal tools (bytecode enhancement)
RetentionPolicy.RUNTIME	Annotation exists in .class file and available via reflection at runtime	Frameworks like Spring, JPA (dynamic processing)

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📚 Example — Custom Annotation with Retention

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

@Retention(RetentionPolicy.RUNTIME)
public @interface LogExecutionTime {
}

✅ This makes @LogExecutionTime available at runtime, so you can read it using reflection.

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🚀 Example Usage in a Class

public class Service {

    @LogExecutionTime
    public void process() {
        // some work
    }
}

🔎 Reading Annotation with Reflection

import java.lang.reflect.Method;

public class AnnotationProcessor {
    public static void main(String[] args) throws Exception {
        Method method = Service.class.getMethod("process");
        if (method.isAnnotationPresent(LogExecutionTime.class)) {
            System.out.println("@LogExecutionTime is present");
        }
    }
}

✅ This works because LogExecutionTime is retained at runtime.

@Retention(RetentionPolicy.SOURCE)
public @interface DeveloperNote {
    String value();
}

This annotation disappears after compilation.It’s useful for tools like Checkstyle, ErrorProne, or code generators.You cannot read @DeveloperNote at runtime — it’s gone after compilation.

🔥 Summary Table

Retention	Kept in Source?	Kept in Class File?	Available at Runtime?	Example Annotation
SOURCE	✔️ Yes	❌ No	❌ No	@Override, @SuppressWarnings
CLASS	✔️ Yes	✔️ Yes	❌ No	Internal processing tools
RUNTIME	✔️ Yes	✔️ Yes	✔️ Yes	@Entity, @Service, @Test

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🎯 When to Use What

Case	Recommended Retention
Compiler checks only (no runtime need)	SOURCE
Annotation only for internal tooling, not for runtime reflection	CLASS
Annotation used by frameworks at runtime (like Spring, JPA, JUnit)	RUNTIME

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💡 Pro Tip for Interviews

✅ Always say:

“The @Retention annotation controls how long an annotation is kept — source, class file, or runtime. Most framework annotations (like @Entity or @Service) use RUNTIME, so they can be processed using reflection at runtime.”

✅ Also mention:

• @Override uses SOURCE — compiler only.
• JPA annotations like @Entity use RUNTIME — required for reflection at runtime.