MyBatis实现原理：动态代理的运用

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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。
  • 动态代理还有”投鞭断流“的霸权，连目标target都不要的使用方式。

自动映射器Mapper自己实现

定义pojo

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package com.justxzm.proxy;

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
}

定义接口

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package com.justxzm.proxy;

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

定义代理类

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package com.justxzm.proxy;

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);
    }
}

测试代码

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package com.justxzm.proxy;

public class MyTest {

	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());
	}
}
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ID:1001
Name:zhangsan
Age:18
com.justxzm.proxy.MapperProxy@66788a7b

这便是Mybatis自动映射器Mapper的底层实现原理。

自动映射器Mapper源码分析

编写一个测试类

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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接口

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

MapperProxy.java

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

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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);
  }
  // ...

MapperProxyFactory.java

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

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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。

问题（重要）

接口Mapper内的方法能重载（overLoad）吗？ 类似下面：

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

Answer：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）。

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