Unit testing is a method by which the smallest testable part of an application is validated. Unit tests must follow the FIRST Rules; these are Fast, Isolated, Repeatable, Self-Validated and Timely.
It is strange to think about a JEE application without persistence layer (typical Relational databases or new NoSQL databases) so should be interesting to write unit tests of persistence layer too. When we are writing unit tests of persistence layer we should focus on to not break two main concepts of FIRST rules, the fast and the isolated ones.
Our tests will be fast if they don't access network nor filesystem, and in case of persistence systems network and filesystem are the most used resources. In case ofRDBMS ( SQL ), many Java in-memory databases exist like Apache Derby , H2 orHSQLDB . These databases, as their name suggests are embedded into your program and data are stored in memory, so your tests are still fast. The problem is withNoSQL systems, because of their heterogeneity. Some systems work using Document approach (like MongoDb ), other ones Column (like Hbase ), or Graph (like Neo4J ). For this reason the in-memory mode should be provided by the vendor, there is no a generic solution.
Our tests must be isolated from themselves. It is not acceptable that one test method modifies the result of another test method. In case of persistence tests this scenario occurs when previous test method insert an entry to database and next test method execution finds the change. So before execution of each test, database should be found in a known state. Note that if your test found database in a known state, test will be repeatable, if test assertion depends on previous test execution, each execution will be unique. For homogeneous systems like RDBMS , DBUnit exists to maintain database in a known state before each execution. But there is no like DBUnitframework for heterogeneous NoSQL systems.
NoSQLUnit resolves this problem by providing a JUnit extension which helps us to manage lifecycle of NoSQL systems and also take care of maintaining databases into known state.