Strategies for Using Mock Objects when Unit Testing DAOs

Question

I am curious what strategies folks have found for unit testing a data access class that does not involve loading (and presumably unloading) a real database for each test method? Are you using mock objects to represent the database connection? If so, are you required to pass the mock object into every method-under-test, and thus forcing the API to require a real db connection as a parameter to every method? Or, are you passing a mock object into the constructor at setup()?

I have a class that is implementing what I believe is a Data Mapper (or maybe gateway) pattern. It is the class responsible for encapsulating SQL and returning (or saving) "business objects". The rest of the code can interact with this mapper layer and the business objects, with total disregard for the persistence model. This code needs to have/maintain, or just know about, a live db connection in the real system. Emulating this under test is tricky.

The problem is how to unit test one of these mapper classes. The practice for creating a unit test under xUnit that I have seen most often is using the setup() method of the test to instantiate the SUT (system under test), usually your object that you're testing, and store it in a local variable in the test class. Then each of your test methods, interact with a unique instance of that SUT.

The assumption though is that whatever you're doing in the setup() method will presumably be replicated somewhere in your real code. So, you have to think about the setup process as "is this something I will want to repeatedly reproduce every time I need to use this object in the real world." If I am passing a db connection into the mapper's constructor in the setup that's fine, but doesn't that mean I'll have to pass a live db connection into the mapper object's constructor every time I want to really use one? Imagine that you'll have all kinds of places where you need to retrieve or store a business object and that to use a data mapper object, you need to pass in the db connection every time?

In my case, I am trying to establish tests for these data mapper objects that achieve the following:

1. Do not require the database connection object to be instantiated and passed into every method of the mapper class.
2. Do not require that the test case either connect to a real db or create a real, but "test", db on the fly for each test method.

I have basically seen two suggestions, pass the connection object as a parameter (which I have already addressed) or extend the SUT class just for the test and override whatever db connection setup process you have in the real world to use a mock system instead.

I am curious if anyone else is facing these issues, with any language, and what you have done to solve them? Maybe there is something obvious that I am missing?

Answer 1

In my experience, the responsibility for connection to a database is a sore point in data access. I solved this by letting the DAO take care of that based on the configuration file (app.config, etc). This way I don't need to worry about that when I write my tests. The DAL keeps one or more database connection profiles and connects/disconnects on every data access because in the end the connection pool will take care of physically connecting/disconnecting.

Another thing that helped me was using dbUnit to load baseline data before running the tests. I found it easier to go straight to the database instead of using mock objects. Also by connecting to a real database I can (to a certain point) test concurrency by issuing commands in different threads - mock objects wouldn't give me the real behavior.

Answer 2

You can use DbUnit to test SQL

Answer 3

It depends on what you're really trying to test. If you want to test that your SQL does what you expect, that's really heading into Integration Test territory. Assuming you're using Java, there are several pure-java RDBMS solutions (Apache Derby, HSQLDB, H2) you can use for that.

If on the other hand you're really just testing your Java <-> JDBC code (i.e. reading from ResultSets), then you can mock out pretty much all the relevant parts of JDBC since they're mostly interfaces. JMock is great for this. Simply add a setConnection() method to your Class Under Test, and pass in the mocked java.sql.Connection that will do your bidding. This works really well for keeping tests short and sweet.