public class ClassLoaderDemo {
    public static void main(String[] args) {
        // 1. 核心類：由Bootstrap加載，輸出null
        ClassLoader bootstrapLoader = String.class.getClassLoader();
        System.out.println("String類加載器: " + bootstrapLoader);

        // 2. 系統(tǒng)類加載器（應(yīng)用類加載器）
        ClassLoader appLoader = ClassLoaderDemo.class.getClassLoader();
        System.out.println("自定義類加載器: " + appLoader);

        // 3. 應(yīng)用類加載器的父加載器：擴展類加載器
        ClassLoader extLoader = appLoader.getParent();
        System.out.println("應(yīng)用類加載器的父加載器: " + extLoader);

        // 4. 擴展類加載器的父加載器：Bootstrap，輸出null
        System.out.println("擴展類加載器的父加載器: " + extLoader.getParent());
    }
}

protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
    synchronized (getClassLoadingLock(name)) {
        // 1. 檢查類是否已經(jīng)被當前加載器加載
        Class<?> c = findLoadedClass(name);
        if (c == null) {
            try {
                // 2. 遞歸委托父加載器加載
                if (parent != null) {
                    c = parent.loadClass(name, false);
                } else {
                    // 3. 無父加載器，直接使用啟動類加載器
                    c = findBootstrapClassOrNull(name);
                }
            } catch (ClassNotFoundException e) {
                // 父加載器無法加載，捕獲異常
            }
            // 4. 父加載器加載失敗，當前加載器自行加載
            if (c == null) {
                c = findClass(name);
            }
        }
        // 解析類
        if (resolve) {
            resolveClass(c);
        }
        return c;
    }
}

import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.IOException;

/**
 * 打破雙親委派的自定義類加載器
 */
public class CustomClassLoader extends ClassLoader {
    // 自定義類加載路徑
    private final String classPath;

    public CustomClassLoader(String classPath) {
        // 關(guān)鍵：不指定父加載器（默認父加載器為系統(tǒng)類加載器，也可手動設(shè)置）
        this.classPath = classPath;
    }

    /**
     * 重寫loadClass，核心：跳過雙親委派的向上委托邏輯
     */
    @Override
    public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
        synchronized (getClassLoadingLock(name)) {
            // 1. 檢查是否已加載
            Class<?> clazz = findLoadedClass(name);
            if (clazz != null) {
                return clazz;
            }

            // 2. 打破規(guī)則：核心類仍交給Bootstrap加載，避免JDK核心類加載異常
            if (name.startsWith("java.")) {
                return getSystemClassLoader().loadClass(name);
            }

            // 3. 不委托父加載器，直接自己加載
            try {
                clazz = findClass(name);
            } catch (Exception e) {
                // 加載失敗，降級為系統(tǒng)類加載器加載
                return super.loadClass(name, resolve);
            }

            if (resolve) {
                resolveClass(clazz);
            }
            return clazz;
        }
    }

    /**
     * 重寫findClass：從自定義路徑讀取字節(jié)碼
     */
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classData = getClassBytes(name);
        if (classData == null) {
            throw new ClassNotFoundException("無法找到類：" + name);
        }
        // 將字節(jié)碼轉(zhuǎn)換為Class對象
        return defineClass(name, classData, 0, classData.length);
    }

    /**
     * 從文件系統(tǒng)讀取.class字節(jié)碼
     */
    private byte[] getClassBytes(String className) {
        String path = classPath + "/" + className.replace(".", "/") + ".class";
        try (FileInputStream fis = new FileInputStream(path);
             ByteArrayOutputStream bos = new ByteArrayOutputStream()) {

            byte[] buffer = new byte[1024];
            int len;
            while ((len = fis.read(buffer)) != -1) {
                bos.write(buffer, 0, len);
            }
            return bos.toByteArray();
        } catch (IOException e) {
            return null;
        }
    }
}

public class TestCustomLoader {
    public static void main(String[] args) throws Exception {
        // 自定義加載路徑
        CustomClassLoader loader = new CustomClassLoader("D:/test/classes");
        // 加載自定義類
        Class<?> clazz = loader.loadClass("com.test.DemoClass");
        // 打印加載器，驗證為自定義加載器
        System.out.println("類加載器: " + clazz.getClassLoader());
    }
}