类加载器家族：启动、扩展、应用类加载器

java.lang.*      // 核心类（String、Object、System等）
java.util.*      // 工具类（List、Map等）
java.io.*        // IO类
java.nio.*       // NIO类
java.net.*       // 网络类
java.security.*  // 安全类
...

public class BootstrapDemo {
    public static void main(String[] args) {
        // 获取String类的类加载器
        ClassLoader loader = String.class.getClassLoader();
        System.out.println("String类的类加载器: " + loader);

        // 获取Object类的类加载器
        ClassLoader loader2 = Object.class.getClassLoader();
        System.out.println("Object类的类加载器: " + loader2);
    }
}

public class BootstrapPathDemo {
    public static void main(String[] args) {
        // 获取启动类加载器的搜索路径
        String paths = System.getProperty("sun.boot.class.path");
        String[] pathArray = paths.split(":");

        System.out.println("启动类加载器的搜索路径：");
        for (String path : pathArray) {
            System.out.println(path);
        }
    }
}

javax.*          // Java扩展包
sun.*            // Sun专有实现
com.sun.*        // Sun公司的工具类
...

public class ExtensionDemo {
    public static void main(String[] args) {
        // 获取扩展类加载器
        ClassLoader extLoader = ClassLoader.getSystemClassLoader().getParent();
        System.out.println("扩展类加载器: " + extLoader);

        // 查看扩展类加载器的搜索路径
        String paths = System.getProperty("java.ext.dirs");
        System.out.println("扩展类加载器搜索路径:");
        if (paths != null) {
            for (String path : paths.split(":")) {
                System.out.println(path);
            }
        } else {
            System.out.println("Java 9+ 已移除 java.ext.dirs");
        }
    }
}

public class AppDemo {
    public static void main(String[] args) {
        // 获取应用类加载器
        ClassLoader appLoader = ClassLoader.getSystemClassLoader();
        System.out.println("应用类加载器: " + appLoader);

        // 获取当前类的类加载器
        ClassLoader myLoader = AppDemo.class.getClassLoader();
        System.out.println("AppDemo的类加载器: " + myLoader);

        // 验证是否相同
        System.out.println("是否相同: " + (appLoader == myLoader));

        // 查看classpath
        String classpath = System.getProperty("java.class.path");
        System.out.println("\nClasspath:");
        for (String path : classpath.split(":")) {
            System.out.println(path);
        }
    }
}

public abstract class ClassLoader {
    private final ClassLoader parent;  // 父加载器引用

    protected ClassLoader(ClassLoader parent) {
        this.parent = parent;
    }

    public final ClassLoader getParent() {
        return parent;
    }
}

public class ParentDemo {
    public static void main(String[] args) {
        ClassLoader appLoader = ClassLoader.getSystemClassLoader();
        ClassLoader extLoader = appLoader.getParent();
        ClassLoader bootLoader = extLoader.getParent();

        System.out.println("应用类加载器: " + appLoader);
        System.out.println("扩展类加载器: " + extLoader);
        System.out.println("启动类加载器: " + bootLoader);

        // 验证层次关系
        System.out.println("\n层次关系：");
        ClassLoader current = appLoader;
        int level = 1;
        while (current != null) {
            System.out.println("Level " + level + ": " + current);
            current = current.getParent();
            level++;
        }
        System.out.println("Level " + level + ": Bootstrap ClassLoader (C++)");
    }
}

public class LoaderUtil {
    public static void printClassLoader(Class<?> clazz) {
        System.out.println("=== " + clazz.getName() + " ===");

        ClassLoader loader = clazz.getClassLoader();
        if (loader == null) {
            System.out.println("加载器: Bootstrap ClassLoader (C++)");
        } else {
            System.out.println("加载器: " + loader.getClass().getName());

            // 打印父加载器链
            System.out.println("父加载器链:");
            int level = 1;
            ClassLoader parent = loader;
            while (parent != null) {
                System.out.println("  Level " + level + ": " + parent.getClass().getName());
                parent = parent.getParent();
                level++;
            }
            System.out.println("  Level " + level + ": Bootstrap ClassLoader (C++)");
        }
        System.out.println();
    }
}

public class LoaderTest {
    public static void main(String[] args) {
        // 1. JDK核心类
        LoaderUtil.printClassLoader(String.class);
        LoaderUtil.printClassLoader(Object.class);

        // 2. JDK扩展类
        LoaderUtil.printClassLoader(sun.security.ec.SunEC.class);

        // 3. 应用程序类
        LoaderUtil.printClassLoader(LoaderTest.class);

        // 4. 第三方库类（如果有）
        // LoaderUtil.printClassLoader(org.springframework.context.ApplicationContext.class);
    }
}

// 设置线程上下文类加载器
Thread.currentThread().setContextClassLoader(classLoader);

// 获取线程上下文类加载器
ClassLoader loader = Thread.currentThread().getContextClassLoader();

// DriverManager（JDK核心类）
public class DriverManager {
    static {
        // 使用线程上下文类加载器加载驱动
        loadInitialDrivers();
    }

    private static void loadInitialDrivers() {
        // 获取线程上下文类加载器
        ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(
            Driver.class,
            Thread.currentThread().getContextClassLoader()  // ← 使用上下文加载器
        );

        Iterator<Driver> driversIterator = loadedDrivers.iterator();
        while(driversIterator.hasNext()) {
            driversIterator.next();
        }
    }
}