Mybatis接口编程示例

（1）首先定义接口编程需要的接口及其方法

public interface IUserMapper {  

  public User getById(int id);//接口方法，不需要实现
}

（2）创建mybatis的mapper文件，其中namespace的值为接口的完整类名

<mapper namespace="com.tianjunwei.learn.learn2.IUserMapper">//接口完整类名
   
     <select id="getById" parameterType="int" 
         resultType="com.tianjunwei.learn.learn1.entity.User">
      select * from users where id=#{id}
     </select>
     
 </mapper>

（3）创建接口调用操作

public class Learn2Main {
  
  public static void main(String [] args){
    
    String resource = "learn/mybatis-config.xml";
      InputStream is = Learn1Main.class.getClassLoader().getResourceAsStream(resource);
      SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(is);
      SqlSession session = sessionFactory.openSession();
      IUserMapper userMapper = session.getMapper(IUserMapper.class);//获取接口，已经是代理接口
      userMapper.getById(1);
      RowBounds rowBounds = new RowBounds(2, 4);//接口分页编程
      System.out.println(userMapper.page(rowBounds).size());
  }
}

Mybatis接口编程相关的类主要有MapperProxyFactory、MapperProxy、MapperMethod和MapperRegister四个类：

MapperProxyFactory：通过类名我们可以猜到这是一个MapperProxy的工厂类，用于创建MapperProxy的，通过函数newInstance(SqlSession sqlSession)来创建MapperProxy的代理类。

源码如下：

//这个类赋值创建具体mapper接口代理对象的工厂
public class MapperProxyFactory<T> {

  //具体Mapper接口的class对象
  private final Class<T> mapperInterface;
  //该接口下面的缓存，key是方法对象，value是对接口中方法对象的封装
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

  public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  public Class<T> getMapperInterface() {
    return mapperInterface;
  }

  public Map<Method, MapperMethod> getMethodCache() {
    return methodCache;
  }

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
  //创建一个代理类并返回
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
  //在这里创建MapperProxy对象，这个类实现了JDK的动态代理接口 InvocationHandler
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
  //调用上面的方法，返回一个接口的代理类
    return newInstance(mapperProxy);
  }

}


MapperProxy：通过类名可以猜到这个类为一个代理类，它实现了JDK动态代理接口InvocationHandler，通过查看源码发现它又不像一个真正的代理类，它一般不会真正执行被代理类的函数方法，只是在执行被代理类函数方法时来执行MapperMethod类的execute方法，具体逻辑详看invoke函数

源码如下：

//实现了JDK动态代理的接口，InvocationHandler
//在invoke方法中实现了代理方法调用的细节
public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  //sqlSession
  private final SqlSession sqlSession;
  //接口的类型对象
  private final Class<T> mapperInterface;
  //接口中方法的缓存，由MapperProxyFactory传递过来
  private final Map<Method, MapperMethod> methodCache;

  //构造函数
  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }
  //接口代理对象所有的方法调用都会调用该方法
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  //判断是不是基础方法比如toString、hashCode等，这些方法直接调用不需要处理
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
  //进行缓存
    final MapperMethod mapperMethod = cachedMapperMethod(method);
  //调用mapperMethod.execute 核心的地方就在这个方法里，这个方法对才是真正对SqlSession进行的包装调用
    return mapperMethod.execute(sqlSession, args);
  }
  //缓存处理
  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

}