Annotations und Reflection

package at.aydin.lab.annotations.legacy;

import java.lang.reflect.*;

// Design Pattern: Registry
// Zweck: Historische Registrierung über String-Konventionen und Reflection.
public final class LegacyCommandRegistry {
    private final Object target;
    public LegacyCommandRegistry(Object target) {
        this.target = target;
    }

    public String execute(String command, String argument) {
        try {
            Method m = target.getClass().getMethod("command_" + command, String.class);
            return (String) m.invoke(target, argument);
        } catch (ReflectiveOperationException ex) {
            throw new IllegalArgumentException("Unbekanntes Kommando: " + command, ex);
        }
    }
}

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 java.lang.annotation.*;

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

package at.aydin.lab.annotations.legacy;

public final class LegacyCommands {
    public String command_hello(String name) {
        return "Hallo " + name;
    }

    public String command_bye(String name) {
        return "Tschüss " + name;
    }
}

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;
    }
}