how to do DAO(db) layer unit test?

since the dao layer methods will be dependent on data in the database, in complex systems some operations will depend on lots of tables, in开发者_高级运维 this way unit test are not repeatable and independent,

i'm wondering how good TDD layers do this? thx.

IMHO you are on to something... If you are communicating with your Sql Server, then you are not doing unit test but integration tests. If you do TDD, then you realise this and start to put server communication into wrappers, so you can stub and mock any test data, instead of using a framework like DbUnit to control database state. I think that Your business logic should not be directly in touch with databases -- or webservices or other external resources. If it is, odds are that you will never write anything but integration tests.

A testing framework like DbUnit is exactly what you want. From their site:

DbUnit is a JUnit extension (also usable with Ant) targeted at database-driven projects that, among other things, puts your database into a known state between test runs. This is an excellent way to avoid the myriad of problems that can occur when one test case corrupts the database and causes subsequent tests to fail or exacerbate the damage.

DbUnit also supports a variety of RDBMS's but I might recommend something like HSQLDB, which can be embedded into your project/tests so your unit tests aren't dependent on being able to connect to a database somewhere in your company's basement. :) Although, on the other hand, you will be testing using a different RDBMS than you would be using in a production environment...