在上一篇文章Mybatis：Mapper接口编程原理分析（二）中，已经知道 mapper 接口是怎么注册的了，那么现在就是需要获取 mapper 接口的代理了。
在使用 Mybatis 时，我们都是通过如下代码去获取 mapper 接口的代理的：

sqlSession.getMapper(UserMapper.class)

代码只有一行，就获取了 mapper 接口的代理对象了，那么在内部是怎么一回事呢？

• DefaultSqlSession

public <T> T getMapper(Class<T> type) {
        // 从 Configuration 获取 mapper 接口代理
        return this.configuration.getMapper(type, this);
}

• Configuration

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
         // 从 MapperRegistry 获取 mapper 接口代理
        return this.mapperRegistry.getMapper(type, sqlSession);
}

• MapperRegistry

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        // 获取 mapper 接口的代理工厂
        MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
        if (mapperProxyFactory == null) {
            throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
        } else {
            try {
                // 生成 mapper 代理
                return mapperProxyFactory.newInstance(sqlSession);
            } catch (Exception var5) {
                throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
            }
        }
}

• MapperProxyFactory

public T newInstance(SqlSession sqlSession) {
        // MapperProxy 实现了 InvocationHandler
        MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
        // 获取 mapper 代理
        return this.newInstance(mapperProxy);
 }

// 生成 mapper 代理对象
protected T newInstance(MapperProxy<T> mapperProxy) {
        return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
}

序列图如下：

获取 mapper 代理