Custom JDK Dynamic Proxy for MyBatis Automatic Mapper Implementation

During a casual conversation the author wondered why MyBatis can return an instance of a mapper interface without any concrete implementation class, which led to an exploration of JDK dynamic proxies and their "投鞭断流" (whip‑cut‑stream) technique for automatic mapper creation.

Dynamic proxy functionality : It intercepts method calls via an InvocationHandler , allowing enhancement of the target method and, in this special mode, completely discarding the original target object.

Custom JDK Dynamic Proxy for Automatic Mapper Creation

First, a simple POJO User is defined:

public class User {
    private Integer id;
    private String name;
    private int age;

    public User(Integer id, String name, int age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }
    // getter setter
}

Then the mapper interface UserMapper :

public interface UserMapper {
    public User getUserById(Integer id);
}

A custom InvocationHandler named MapperProxy is created to generate proxy instances and to handle method invocations. When the invoked method belongs to Object , it simply forwards the call; otherwise it returns a new User object, effectively implementing the "投鞭断流" logic.

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class MapperProxy implements InvocationHandler {
    @SuppressWarnings("unchecked")
    public
T newInstance(Class
clz) {
        return (T) Proxy.newProxyInstance(clz.getClassLoader(), new Class[] { clz }, this);
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if (Object.class.equals(method.getDeclaringClass())) {
            try {
                // hashCode(), toString(), equals() etc.
                return method.invoke(this, args);
            } catch (Throwable t) { }
        }
        // 投鞭断流 – create a dummy User
        return new User((Integer) args[0], "zhangsan", 18);
    }
}

A test program demonstrates the proxy in action:

public static void main(String[] args) {
    MapperProxy proxy = new MapperProxy();
    UserMapper mapper = proxy.newInstance(UserMapper.class);
    User user = mapper.getUserById(1001);
    System.out.println("ID:" + user.getId());
    System.out.println("Name:" + user.getName());
    System.out.println("Age:" + user.getAge());
    System.out.println(mapper.toString());
}

Running the program produces:

ID:1001
Name:zhangsan
Age:18
x.y.MapperProxy@6bc7c054

This output reveals the underlying mechanism of MyBatis automatic mapper generation.

The article also notes that mapper methods cannot be overloaded because MyBatis uses the fully‑qualified method name as a key to locate the corresponding SQL statement; overloading would cause ambiguity.

Finally, a brief source‑code analysis of MyBatis's own MapperProxy and MapperProxyFactory classes is provided to show how the framework implements the same proxy pattern.