Reflection and annotation

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.

Exercise 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().

Show example 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());
    }
}

Test yourself

Complete the annotation definition

@interface Logged {
    String ____();
}

Show answer

@interface Logged {
    String value();
}

What does getMethod("describe") return?
- A Method object
- A String
- An int
- A Class
Show answer

A Method object — getMethod returns a Method object.
Match the reflection concepts

@interface

Annotation definition
getMethod

Looks up by name
invoke

Calls the method
getAnnotation

Reads the annotation

Next lesson: Putting it together

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