Annotations und Reflection

<?xml version="1.0" encoding="UTF-8"?>
        <project xmlns="http://maven.apache.org/POM/4.0.0"
                 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
            <modelVersion>4.0.0</modelVersion>
            <parent>
                <groupId>at.aydin.academy</groupId>
                <artifactId>14annotationsreflection-pair</artifactId>
                <version>1.0.0</version>
                <relativePath>../pom.xml</relativePath>
            </parent>
            <artifactId>annotations-reflection-modern</artifactId>
            <packaging>jar</packaging>
            <name>Annotations Reflection Modern</name>

            <dependencies>

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <scope>test</scope>
</dependency>

            </dependencies>
            <build>
                <plugins>
                    <plugin>
                        <groupId>org.apache.maven.plugins</groupId>
                        <artifactId>maven-compiler-plugin</artifactId>
                    </plugin>

                </plugins>
            </build>
        </project>

package at.aydin.lab.annotations.modern;

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public
@interface
Command {
    String value();
}

package at.aydin.lab.annotations.modern;

import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;
import java.util.Optional;

// Design Pattern: Command + Metadata Registry
// Zweck: Annotationen machen Namen explizit und entkoppeln sie von Methodenkonventionen.
public final class CommandRegistry {
    private record Handler(Object target, Method method) {}

    private final Map<String, Handler> handlers = new HashMap<>();

    public CommandRegistry(Object... targets) {
        for (Object target : targets) {
            for (Method method : target.getClass().getDeclaredMethods()) {
                Command command = method.getAnnotation(Command.class);
                if (command != null) {
                    handlers.put(command.value(), new Handler(target, method));
                }
            }
        }
    }

    public String execute(String command, String argument) {
        Handler handler = Optional.ofNullable(handlers.get(command))
                .orElseThrow(() -> new IllegalArgumentException(
                        "Unbekanntes Kommando: " + command));
        try {
            return (String) handler.method().invoke(handler.target(), argument);
        } catch (ReflectiveOperationException exception) {
            throw new IllegalStateException(exception);
        }
    }
}

package at.aydin.lab.annotations.modern;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

class CommandRegistryTest {
    @Test
    void usesExplicitAnnotation() {
        assertEquals("Hallo Test", new CommandRegistry(new ModernCommands()).execute("hello", "Test"));
    }
}

package at.aydin.lab.annotations.modern;

public final class ModernCommands {
    @Command("hello")
    public String greet(String name) {
        return "Hallo " + name;
    }

    @Command("bye")
    public String farewell(String name) {
        return "Tschüss " + name;
    }
}