jdbc:sqlserver://localhost\SQLEXPRESS:1433;database=mydb;user=username;password=psw

jdbc:sqlserver://localhost;instanceName=SQLEXPRESS;database=mydb;user=username;‌​password=psw'

instanceName = SQLEXPRESS;
database = mydb;  -- database mydb should exists
user = userName;‌ -- user should exits and have permission to mydb
​password = userPassword

USE [master]
GO
CREATE LOGIN [userName] WITH PASSWORD=N'userPassword',
             DEFAULT_DATABASE=[YourDB], CHECK_EXPIRATION=OFF,CHECK_POLICY=OFF

GO
USE [YourDB] -- in your case its mydb
GO
CREATE USER [userName] FOR LOGIN [userName] WITH DEFAULT_SCHEMA=[dbo]
GO

jdbc:sqlserver://localhost\SQLEXPRESS;database=mydb;user=username;password=psw

jdbc:sqlserver://localhost\SQLEXPRESS,1433;database=mydb;user=username;password=psw