自定义类加载器实现(Java)

方法：

• 重写loadclass方法，不推荐因为会破坏双亲委派模型
• 重写findClass方法，推荐

public class MyClassLoader extends ClassLoader{

    //1
    private String codePath;

    //2.
    public MyClassLoader(ClassLoader parent, String codePath) {
        super(parent);
        this.codePath = codePath;
    }

    public MyClassLoader(String codePath) {
        this.codePath = codePath;
    }


    @Override
    protected Class findClass(String name) throws ClassNotFoundException {
        BufferedInputStream bis = null;
        ByteArrayOutputStream baos = null;

        try {
            //1.字节码路径
            String fileName = codePath + name + ".class";
            //获取输入流
            bis = new BufferedInputStream(new FileInputStream(fileName));
            //获取输出流
            baos = new ByteArrayOutputStream();
            //进行io读写
            int len;
            byte[] data = new byte[1024];
            while ((len = bis.read(data)) != -1){
                baos.write(data,0,len);
            }
            //获取内存中的字节数组
            byte[] byteCode = baos.toByteArray();
            //调用defineClass将字节数组转成Class对象
            Class defineClass = defineClass(null, byteCode, 0, byteCode.length);
            //返回
            return defineClass;
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            //关闭资源
            try {
                bis.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                baos.close();
            } catch (IOException e) {
                e.printStackTrace();
            }

        }
        return null;
    }
}

public class String {
    static {
        System.out.println("自定义的String...");
    }
}

测试：

public class MyClassLoaderTest {
    public static void main(String[] args) {
        MyClassLoader classLoader = new MyClassLoader("d:/");
        try {
            Class clazz = classLoader.loadClass("String");
            System.out.println("由：" + clazz.getClassLoader().getClass().getName() + "加载的");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

结果：

由：com.zzyuan.lagou.util2.MyClassLoader加载的