Can you explain what the RetentionPolicy annotation means in Java and how it affects the lifecycle of an annotation?

The @Retention annotation in Java specifies how long annotations are retained and available at runtime. It takes a value from the RetentionPolicy enum, which can be:

• SOURCE — the annotation is only available in source code and is discarded by the compiler, not included in the bytecode.
• CLASS — the annotation is stored in the bytecode but not available at runtime (default).
• RUNTIME — the annotation is stored in the bytecode and available at runtime via reflection.

For example, if an annotation is marked with @Retention(RetentionPolicy.RUNTIME), it can be accessed through reflection during program execution, which is useful for frameworks and libraries that analyze annotations dynamically.

Example:

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

@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
    String value();
}

@MyAnnotation("example")
public class Test {}

// At runtime, you can get the annotation like this:
MyAnnotation ann = Test.class.getAnnotation(MyAnnotation.class);
if (ann != null) {
    System.out.println(ann.value());
}