JVM 아키텍처와 클래스 로딩 완벽 가이드

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public class LoadingExample {
    // Preparation 단계: value = 0 (기본값)
    // Initialization 단계: value = 42
    static int value = 42;

    static {
        System.out.println("클래스가 초기화되었습니다. value = " + value);
    }
}

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public class ClassLoaderHierarchy {
    public static void main(String[] args) {
        // Application ClassLoader
        ClassLoader appLoader = ClassLoaderHierarchy.class.getClassLoader();
        System.out.println("App ClassLoader: " + appLoader);

        // Platform(Extension) ClassLoader
        ClassLoader platformLoader = appLoader.getParent();
        System.out.println("Platform ClassLoader: " + platformLoader);

        // Bootstrap ClassLoader (네이티브 구현이므로 null 반환)
        ClassLoader bootstrapLoader = platformLoader.getParent();
        System.out.println("Bootstrap ClassLoader: " + bootstrapLoader);
    }
}

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import java.io.*;
import java.nio.file.*;

public class PluginClassLoader extends ClassLoader {

    private final Path pluginDir;

    public PluginClassLoader(Path pluginDir, ClassLoader parent) {
        super(parent);
        this.pluginDir = pluginDir;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        // com.example.MyPlugin → com/example/MyPlugin.class
        String fileName = name.replace('.', File.separatorChar) + ".class";
        Path classFile = pluginDir.resolve(fileName);

        if (!Files.exists(classFile)) {
            throw new ClassNotFoundException("클래스를 찾을 수 없습니다: " + name);
        }

        try {
            byte[] bytes = Files.readAllBytes(classFile);
            return defineClass(name, bytes, 0, bytes.length);
        } catch (IOException e) {
            throw new ClassNotFoundException("클래스 로드 실패: " + name, e);
        }
    }

    public static void main(String[] args) throws Exception {
        Path pluginPath = Paths.get("/opt/app/plugins");
        PluginClassLoader loader = new PluginClassLoader(pluginPath,
            ClassLoader.getSystemClassLoader());

        Class<?> pluginClass = loader.loadClass("com.example.MyPlugin");
        Object plugin = pluginClass.getDeclaredConstructor().newInstance();
        System.out.println("플러그인 로드 성공: " + plugin.getClass().getName());
        System.out.println("사용된 ClassLoader: " + plugin.getClass().getClassLoader());
    }
}