面试：Mybatis接口Mapper内的方法为啥不能重载

动态代理的功能

通过拦截器方法回调，对目标target方法进行增强。言外之意就是为了增强目标target方法。上面这句话没错，但也不要认为它就是真理，殊不知，动态代理还有投鞭断流的霸权，连目标target都不要的科幻模式。
注：本文默认认为，对动态代理的原理是理解的，如果不明白target的含义，难以看懂本篇文章，建议先理解动态代理。

1. 自定义JDK动态代理之投鞭断流实现自动映射器Mapper

首先定义一个实体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
}

再定义一个接口UserMapper

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

接下来我们看看如何使用动态代理之投鞭断流，实现实例化接口并调用接口方法返回数据的

自定义一个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 {
    // 诸如hashCode()、toString()、equals()等方法，将target指向当前对象this
    return method.invoke(this, args);
   } catch (Throwable t) {
   }
  }
  // 投鞭断流
  return new User((Integer) args[0], "zhangsan", 18);
 }
}

代码中的target，在执行Object.java内的方法时，target被指向了this，target已经变成了傀儡、象征、占位符。在投鞭断流式的拦截时，已经没有了target

写一个测试代码：

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());
}

输出output

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

这便是Mybatis自动映射器Mapper的底层实现原理

2. Mybatis自动映射器Mapper的源码分析

编写一个测试类

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();
}
}

Mapper

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

org.apache.ibatis.binding.MapperProxy.java部分源码

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;

  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

  @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) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    // 投鞭断流
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

org.apache.ibatis.binding.MapperProxyFactory.java部分源码

public class MapperProxyFactory<T> {

  private final Class<T> mapperInterface;

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

这便是Mybatis使用动态代理之投鞭断流

3. 接口Mapper内的方法能重载（overLoad）吗？（重要）

举例

public User getUserById(Integer id);
public User getUserById(Integer id, String name);

答案

不能。
原因：在投鞭断流时，Mybatis使用package+Mapper+method全限名作为key，去xml内寻找唯一sql来执行的。类似：key=x.y.UserMapper.getUserById，那么，重载方法时将导致矛盾。对于Mapper接口，Mybatis禁止方法重载（overLoad）。