好像新的mybatis要求mapper中对于允许为空的字段，必须注明jdbcType，例如：
#{projectName,jdbcType=VARCHAR} ,
否则在增加、修改数据时会出现错误：
### Error updating database.? Cause: org.apache.ibatis.type.TypeException: Error setting null parameter.? Most JDBC drivers require that the JdbcType must be specified for all nullable parameters. Cause: java.sql.SQLException: 无效的列类型: 1111
### The error may involve defaultParameterMap
### The error occurred while setting parameters
### Cause: org.apache.ibatis.type.TypeException: Error setting null parameter.? Most JDBC drivers require that the JdbcType must be specified for all nullable parameters. Cause: java.sql.SQLException: 无效的列类型: 1111