mybatis mapper生成流程

mybatis提供了一种非常简便的方式去访问数据库，定义接口和sql之后，就能自动帮你完成jdbc操作。这得益于它的mapper机制，本篇文章在于分析mapper的流程。

DataSource dataSource = BlogDataSourceFactory.getBlogDataSource();
TransactionFactory transactionFactory = new JdbcTransactionFactory();
Environment environment = new Environment("development", transactionFactory, dataSource);
Configuration configuration = new Configuration(environment);
configuration.addMapper(BlogMapper.class);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);

从mybatis的官方示例代码可以看到，mapper接口的加载是通过Configuration类的addMapper方法去实现的。

public <T> void addMapper(Class<T> type) {
    this.knownMappers.put(type, new MapperProxyFactory(type));
    MapperAnnotationBuilder parser = new MapperAnnotationBuilder(this.config, type);
    parser.parse();
}

可以看到，这里创建了一个MapperProxyFactory对象，当用户调用getMapper方法时，则会调用该factory生成一个基于jdk的代理

public T newInstance(SqlSession sqlSession) {
  final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
  return newInstance(mapperProxy);
}
protected T newInstance(MapperProxy<T> mapperProxy) {
  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

最终调用方法时，则是通过MapperProxy的invoke方法完成逻辑调用。

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  final MapperMethod mapperMethod = cachedMapperMethod(method);
  return mapperMethod.execute(sqlSession, args);
}

MapperProxy内部由MapperMethod的execute完成方法调用，该方法调用sqlSession完成操作，即完成了mapper的封装。

if (SqlCommandType.INSERT == command.getType()) {
  Object param = method.convertArgsToSqlCommandParam(args);
  result = rowCountResult(sqlSession.insert(command.getName(), param));
...