java - Difference between @Before, @BeforeClass, @BeforeEach and @BeforeAll

Question

What is the main difference between

• @Before and @BeforeClass
  • and in JUnit 5 @BeforeEach and @BeforeAll
• @After and @AfterClass

According to the JUnit Api @Before is used in the following case:

When writing tests, it is common to find that several tests need similar objects created before they can run.

Whereas @BeforeClass can be used to establish a database connection. But couldn't @Before do the same?

Answer 1

The code marked @Before is executed before each test, while @BeforeClass runs once before the entire test fixture. If your test class has ten tests, @Before code will be executed ten times, but @BeforeClass will be executed only once.

In general, you use @BeforeClass when multiple tests need to share the same computationally expensive setup code. Establishing a database connection falls into this category. You can move code from @BeforeClass into @Before, but your test run may take longer. Note that the code marked @BeforeClass is run as static initializer, therefore it will run before the class instance of your test fixture is created.

In JUnit 5, the tags @BeforeEach and @BeforeAll are the equivalents of @Before and @BeforeClass in JUnit 4. Their names are a bit more indicative of when they run, loosely interpreted: 'before each tests' and 'once before all tests'.