Reflection and annotation
- Java course
- Advanced
- Lesson 81/82
- 6 min
Write configuration with reflection and annotations.
Annotation with @interface
In this lesson you will define a custom annotation. @interface creates a marker type. String value() declares a single property.
@interface Logged {
String value();
}You mark a method with @Logged("audit"). The value can later be read with reflection. This is a clean way to add metadata to code.
Calling methods via reflection
With reflection you can find and call a class's methods at runtime. getMethod("describe") looks up by name; invoke calls it on an instance.
Method m = EventService.class.getMethod("describe");
String result = (String) m.invoke(service);
Logged logged = m.getAnnotation(Logged.class);
System.out.println(logged.value());Here the @Logged annotation value audit is read. This shows the annotation-based marking and reading flow.
Practice Print describe and the @Logged value via reflection
The EventService class has a describe method marked with @Logged("audit"). Look up the method with Method m = EventService.class.getMethod("describe");. Call it with m.invoke(service) and print the result. Get the annotation with m.getAnnotation(Logged.class) and print value().
Starter code
import java.util.ArrayList;
class Event {
String type;
int severity;
Event(String type, int severity) {
this.type = type;
this.severity = severity;
}
}
@interface Logged {
String value();
}
class EventService {
@Logged("audit")
public String describe() {
return "event-service";
}
}
public class Main {
public static void main(String[] args) throws Exception {
// Call EventService.describe via reflection
// Print the @Logged annotation value
}
}Sample solution
import java.util.ArrayList;
import java.lang.reflect.*;
class Event {
String type;
int severity;
Event(String type, int severity) {
this.type = type;
this.severity = severity;
}
}
@interface Logged {
String value();
}
class EventService {
@Logged("audit")
public String describe() {
return "event-service";
}
}
public class Main {
public static void main(String[] args) throws Exception {
EventService service = new EventService();
Method m = EventService.class.getMethod("describe");
String result = (String) m.invoke(service);
Logged logged = m.getAnnotation(Logged.class);
System.out.println(result);
System.out.println(logged.value());
}
}Expected output
event-service audit
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the annotation definition
@interface Logged {
String ____();
}@interface Logged {
String value();
}Explanation: value() is the single property of the annotation.
2 What does getMethod("describe") return?
- A Method object Correct answer
- A String
- An int
- A Class
Explanation: getMethod returns a Method object.
3 Match the reflection concepts
| Term | Meaning |
|---|---|
@interface |
Annotation definition |
getMethod |
Looks up by name |
invoke |
Calls the method |
getAnnotation |
Reads the annotation |