Java实现自己的类加载器的示例代码

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
 
public class MyClassLoader extends ClassLoader {
 
    private String classPath;
 
    public MyClassLoader(String classPath) {
        this.classPath = classPath;
    }
 
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        try {
            // 将类名转换为文件路径
            String fileName = classPath + name.replace('.', '/') + ".class";
            // 读取类文件的字节码
            byte[] classBytes = Files.readAllBytes(Paths.get(fileName));
            // 将字节码转换为 Class 对象
            return defineClass(name, classBytes, 0, classBytes.length);
        } catch (IOException e) {
            throw new ClassNotFoundException(name, e);
        }
    }
}

public class CustomClassLoaderDemo {
    public static void main(String[] args) {
        try {
            // 创建自定义类加载器，指定类文件所在路径
            MyClassLoader classLoader = new MyClassLoader("/path/to/classes/");
            // 加载类
            Class<?> clazz = classLoader.loadClass("com.example.MyClass");
            // 创建类的实例
            Object instance = clazz.getDeclaredConstructor().newInstance();
            // 调用方法
            clazz.getMethod("myMethod").invoke(instance);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.net.URL;
 
public class NetworkClassLoader extends ClassLoader {
 
    private String baseUrl;
 
    public NetworkClassLoader(String baseUrl) {
        this.baseUrl = baseUrl;
    }
 
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        try {
            // 将类名转换为 URL
            String url = baseUrl + name.replace('.', '/') + ".class";
            // 从网络读取类文件的字节码
            InputStream inputStream = new URL(url).openStream();
            ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
            int nextValue = 0;
            while ((nextValue = inputStream.read()) != -1) {
                byteStream.write(nextValue);
            }
            byte[] classBytes = byteStream.toByteArray();
            // 将字节码转换为 Class 对象
            return defineClass(name, classBytes, 0, classBytes.length);
        } catch (IOException e) {
            throw new ClassNotFoundException(name, e);
        }
    }
}

public class NetworkClassLoaderDemo {
    public static void main(String[] args) {
        try {
            // 创建自定义类加载器，指定类文件所在的基 URL
            NetworkClassLoader classLoader = new NetworkClassLoader("http://example.com/classes/");
            // 加载类
            Class<?> clazz = classLoader.loadClass("com.example.MyClass");
            // 创建类的实例
            Object instance = clazz.getDeclaredConstructor().newInstance();
            // 调用方法
            clazz.getMethod("myMethod").invoke(instance);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}