I always like to start with the field.sometimes you can learn it's purpose from it's naming.even if it's hard to understand,you can just remember it.then you just go on.
no more word,just watch this picture.

field and constructor.png

ok,this class only hava one field.the configuration saves all the data which passed from the xml file.and just as you can see,there is only one contructor and it's very simple.just make the field configuration point to the parameter configuration.
so next ,we analyse the following method.

openSession.png

there are so many overloaded function.i just capture some.Generally speaking they invoke the two main function called openSessionFromDataSource and openSessionFromConnection.

openSessionFromConnection.png

openSessionFromDataSource.png

ok,here is the function.list the things it will do:
first,get environment from configuration.(when we parse the xml file,we had already save this info into configuration,so just take it out)
second,get transactionFactory from environment.(because in the xml file,we have transaction node under the environment node)
third,use transactionFactory to create a new transaction.(according to the different type,choose different implemation to achive the transactional goal)
forth,use transaction and execType to get a new executor,which is the most impotant thing in execute the sql.
fifth,return a new defaultSqlSession.
that's all.
end.