[java] Difference between setUp() and setUpBeforeClass()

When unit testing with JUnit, there are two similar methods, setUp() and setUpBeforeClass(). What is the difference between these methods? Also, what is the difference between tearDown() and tearDownAfterClass()?

Here are the signatures:

@BeforeClass
public static void setUpBeforeClass() throws Exception {
}

@AfterClass
public static void tearDownAfterClass() throws Exception {
}

@Before
public void setUp() throws Exception {
}

@After
public void tearDown() throws Exception {
}

Think of "BeforeClass" as a static initializer for your test case - use it for initializing static data - things that do not change across your test cases. You definitely want to be careful about static resources that are not thread safe.

Finally, use the "AfterClass" annotated method to clean up any setup you did in the "BeforeClass" annotated method (unless their self destruction is good enough).

"Before" & "After" are for unit test specific initialization. I typically use these methods to initialize / re-initialize the mocks of my dependencies. Obviously, this initialization is not specific to a unit test, but general to all unit tests.

From the Javadoc:

Sometimes several tests need to share computationally expensive setup (like logging into a database). While this can compromise the independence of tests, sometimes it is a necessary optimization. Annotating a public static void no-arg method with @BeforeClass causes it to be run once before any of the test methods in the class. The @BeforeClass methods of superclasses will be run before those the current class.

setUpBeforeClass is run before any method execution right after the constructor (run only once)

setUp is run before each method execution

tearDown is run after each method execution

tearDownAfterClass is run after all other method executions, is the last method to be executed. (run only once deconstructor)