Mybatis - parameter mapping in QueryBuilder class

Question

I am fairly comfortable at using mybatis as an ORM tool. But i am not able to understand how parameter mapping works in mybatis.

say i have a mybatis mapper interface defined that has a method to fetch the user details.

And I have my Querybuilder class defined, which has the select query .

public interface UserMapper {

    @SelectProvider(type = UserQueryBuilder.class, method = "getUserId")
    Long getUserId(@Param("first") String firstName, @Param("last") String lastName, @Param("location") String location);

}


public class UserQueryBuilder(){

    public String getUserId(String firstName, String lastName, String location) {
        return new SQL() {{
            SELECT("USER_TABLE.USER_ID");
            FROM("USER_TABLE");
            WHERE("USER_TABLE.FIRST_NAME" + " = #{first}");
            WHERE("USER_TABLE.LAST_NAME" + " = #{last}");
            WHERE("USER_TABLE.LOCATION" + " = #{location}");

        }}.toString();
    }

}

In the above mentioned QueryBuilder , how the SQL query parameters were able to get mapped to "first" and "last" param values, defined in the 'userMapper' interface .

Answer 1

The important thing is that UserQueryBuilder does not get parameters defines in UserMapper interface.

There are several components participating in query execution:

1. the client of the mapper
2. mapper interface
3. query builder
4. spring-mybatis library
5. mybatis library

The mapper interface purpose is:

1. to specify the query that would be used
2. to specify parameters that query require
3. to define actual java interface

Mapper may define the query directly using annotation like @Select , implicitly using the query defined in xml mapping or using the query builder via @SelectProvider . The purpose of the builder is the same as of @Select or xml mapping, namely to provide the query text to mapper.

The actual implementation of the mapper is provided by the spring-mybatis . That implementation is created dynamically based on the mapper class. The implementation works like this: when a method on the mapper is invoked it uses reflection to invoke appropriate methods (like selectOne or selectList based on the return type defined in the mapper method) on mybatis SqlSession . In order to do that it needs query text and parameters. Query text is taken from builder/annotation/xml. Parameters are available as parameters to mapper method invocation. Then SqlSessoin itself uses the query and parameters to execute the query using JDBC API.

Answer 2

MyBatis builds an dynamic proxy for mapper like Spring AOP Proxy , for MyBatis is using MapperProxyFactory to create the proxy instance MapperProxy with the proxy interface ( UserMapper ).

So when invoke the getUserId , MapperProxy will catch the target method and parameters by:

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable

this will invoke the correspond MapperMethod to execute the sql with converted args to parameters by:

Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);

Since method.convertArgsToSqlCommandParam actually will generated named parameters by @Param annotations.

and also need to replace placeholder ( #{first} ) to ask and with correspond parameters, after this it will create BoundSql that own the raw sql and parameters , these will hand to jdbc dirver to execute like we use the jdbcTemplate directly.

• sqlSourceParser.parse(replacePlaceholder(sql), parameterType, new HashMap());