Unlocking MyBatis Mapper Magic: Custom JDK Dynamic Proxy Explained

Custom JDK Dynamic Proxy for Automatic Mapper

Dynamic proxies intercept method calls to enhance the target method, providing automatic mapping capabilities.

1. Define POJO

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
}

2. Define Mapper Interface

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

3. Implement InvocationHandler

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

public class MapperProxy implements InvocationHandler {
  @SuppressWarnings("unchecked")
  public <T> T newInstance(Class<T> 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 {
        return method.invoke(this, args);
      } catch (Throwable t) {}
    }
    // 投鞭断流
    return new User((Integer) args[0], "zhangsan", 18);
  }
}

Test code and output demonstrate 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());
}

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

This demonstrates the underlying principle of MyBatis automatic mapper.

2. MyBatis Mapper Source Analysis

Example test class using SqlSession to obtain a StudentMapper and list students.

public static void main(String[] args) {
  SqlSession sqlSession = MybatisSqlSessionFactory.openSession();
  try {
    StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
    List<Student> students = studentMapper.findAllStudents();
    for (Student student : students) {
      System.out.println(student);
    }
  } finally {
    sqlSession.close();
  }
}

StudentMapper interface definition.

public interface StudentMapper {
  List<Student> findAllStudents();
  Student findStudentById(Integer id);
  void insertStudent(Student student);
}

Key excerpts from org.apache.ibatis.binding.MapperProxy and MapperProxyFactory show how MyBatis employs dynamic proxies and the “投鞭断流” technique.

3. Can Mapper Methods Be Overloaded?

Answer: No. MyBatis uses the fully qualified method name as a key to locate a unique SQL statement; overloaded methods would cause key collisions, so overload is prohibited.