Annotations via reflection
- Java course
- Advanced
- Lesson 35/82
- 5 min
Learn to read annotations with reflection.
Reading annotations with getAnnotation
To read an annotation at runtime you use the getAnnotation method of the Class object. This works only for annotations with RUNTIME retention.
Example: Note n = Sample.class.getAnnotation(Note.class). Then n.value() accesses its element. If the annotation is absent the result is null; so it is safe to first check with isAnnotationPresent(Note.class). This mechanism lets frameworks discover classes automatically.
Safe checks with isAnnotationPresent
isAnnotationPresent returns a boolean for whether an annotation is present. Using it before getAnnotation gives safe access without a null check.
Example: if (c.isAnnotationPresent(Note.class)) { ... }. This pattern is common in frameworks: behavior changes based on whether a class carries a certain annotation. Checking several annotation types needs a separate call for each.
Practice Read the Note annotation with reflection
Define an @interface Note with String value(). Add @Retention(RetentionPolicy.RUNTIME) and @Target(ElementType.TYPE) above it. Apply @Note("hello") to the Sample class. In main, read the value with getAnnotation and print it with System.out.println.
Starter code
// Define the Note annotation (RUNTIME, TYPE)
// Apply Note to the Sample class
// Read the value in main and print itSample solution
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.annotation.ElementType;
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface Note { String value(); }
@Note("hello")
class Sample { }
public class NoteReader {
public static void main(String[] args) {
Note n = Sample.class.getAnnotation(Note.class);
System.out.println(n.value());
}
}Expected output
hello
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the reflection methods
| Term | Meaning |
|---|---|
getAnnotation |
Returns a single annotation |
isAnnotationPresent |
Presence check |
getAnnotations |
Array of all annotations |
value() |
Element access |
2 Complete the getAnnotation call
Note n = Sample.class.getAnnotation(____.class);Note n = Sample.class.getAnnotation(Note.class);Explanation: The annotation class object is passed as the parameter.
3 What does getAnnotation return when the annotation is absent?
- An empty array
- false
- null Correct answer
- Throws IllegalAccessException
Explanation: It returns null; that is why checking is safe.