There are numerous frameworks available for unit testing within Java. The most popular option by far is JUnit. It is documented under the following:
JUnit4 - Proposed tag for JUnit4 features; not yet implemented.
Other unit test frameworks do exist, and have documentation available:
There are several other tools used for unit testing:
Mockito - Mocking framework; allows objects to be mimicked. Useful for mimicking the expected behavior of an external unit within a given unit's test, as to not link the external unit's behavior to the given unit's tests.
JBehave - BDD Framework. Allows tests to be linked to user behaviors (allowing requirement/scenario validation). No documents tag available at time of writing; here is an external link.