Annotations via reflection

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.

Exercise 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.

Show example 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());
    }
}

Test yourself

Match the reflection methods

getAnnotation

Returns a single annotation
isAnnotationPresent

Presence check
getAnnotations

Array of all annotations
value()

Element access

Complete the getAnnotation call

Note n = Sample.class.getAnnotation(____.class);

Show answer

Note n = Sample.class.getAnnotation(Note.class);

What does getAnnotation return when the annotation is absent?
- An empty array
- false
- null
- Throws IllegalAccessException
Show answer

null — It returns null; that is why checking is safe.

Next lesson: Summary - Annotation

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