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

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'.

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Difference between each annotation are :

+-------------------------------------------------------------------------------------------------------+
¦                                       Feature                            ¦   Junit 4    ¦   Junit 5   ¦
¦--------------------------------------------------------------------------+--------------+-------------¦
¦ Execute before all test methods of the class are executed.               ¦ @BeforeClass ¦ @BeforeAll  ¦
¦ Used with static method.                                                 ¦              ¦             ¦
¦ For example, This method could contain some initialization code          ¦              ¦             ¦
¦-------------------------------------------------------------------------------------------------------¦
¦ Execute after all test methods in the current class.                     ¦ @AfterClass  ¦ @AfterAll   ¦
¦ Used with static method.                                                 ¦              ¦             ¦
¦ For example, This method could contain some cleanup code.                ¦              ¦             ¦
¦-------------------------------------------------------------------------------------------------------¦
¦ Execute before each test method.                                         ¦ @Before      ¦ @BeforeEach ¦
¦ Used with non-static method.                                             ¦              ¦             ¦
¦ For example, to reinitialize some class attributes used by the methods.  ¦              ¦             ¦
¦-------------------------------------------------------------------------------------------------------¦
¦ Execute after each test method.                                          ¦ @After       ¦ @AfterEach  ¦
¦ Used with non-static method.                                             ¦              ¦             ¦
¦ For example, to roll back database modifications.                        ¦              ¦             ¦
+-------------------------------------------------------------------------------------------------------+

Most of annotations in both versions are same, but few differs.

Reference

Order of Execution.

Dashed box -> optional annotation.

https://i.stack.imgur.com/HKspz.png

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Before and BeforeClass in JUnit

The function @Before annotation will be executed before each of test function in the class having @Test annotation but the function with @BeforeClass will be execute only one time before all the test functions in the class.

Similarly function with @After annotation will be executed after each of test function in the class having @Test annotation but the function with @AfterClass will be execute only one time after all the test functions in the class.

SampleClass

public class SampleClass {
    public String initializeData(){
        return "Initialize";
    }

    public String processDate(){
        return "Process";
    }
 }

SampleTest

public class SampleTest {

    private SampleClass sampleClass;

    @BeforeClass
    public static void beforeClassFunction(){
        System.out.println("Before Class");
    }

    @Before
    public void beforeFunction(){
        sampleClass=new SampleClass();
        System.out.println("Before Function");
    }

    @After
    public void afterFunction(){
        System.out.println("After Function");
    }

    @AfterClass
    public static void afterClassFunction(){
        System.out.println("After Class");
    }

    @Test
    public void initializeTest(){
        Assert.assertEquals("Initailization check", "Initialize", sampleClass.initializeData() );
    }

    @Test
    public void processTest(){
        Assert.assertEquals("Process check", "Process", sampleClass.processDate() );
    }

}

Output

Before Class
Before Function
After Function
Before Function
After Function
After Class

In Junit 5

@Before = @BeforeEach
@BeforeClass = @BeforeAll
@After = @AfterEach
@AfterClass = @AfterAll

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@Before(JUnit4) -> @BeforeEach(JUnit5) - method is called before every test

@After(JUnit4) -> @AfterEach(JUnit5) - method is called after every test

@BeforeClass(JUnit4) -> @BeforeAll(JUnit5) - static method is called before executing all tests in this class. It can be a large task as starting server, read file, making db connection...

@AfterClass(JUnit4) -> @AfterAll(JUnit5) - static method is called after executing all tests in this class.

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import org.junit.Assert
import org.junit.Before
import org.junit.BeforeClass
import org.junit.Test

class FeatureTest {
    companion object {
        private lateinit var heavyFeature: HeavyFeature
        @BeforeClass
        @JvmStatic
        fun beforeHeavy() {
            heavyFeature = HeavyFeature()
        }
    }

    private lateinit var feature: Feature

    @Before
    fun before() {
        feature = Feature()
    }

    @Test
    fun testCool() {
        Assert.assertTrue(heavyFeature.cool())
        Assert.assertTrue(feature.cool())
    }

    @Test
    fun testWow() {
        Assert.assertTrue(heavyFeature.wow())
        Assert.assertTrue(feature.wow())
    }
}

Same as

import org.junit.Assert
import org.junit.Test

 class FeatureTest {
    companion object {
        private val heavyFeature = HeavyFeature()
    }

    private val feature = Feature()

    @Test
    fun testCool() {
        Assert.assertTrue(heavyFeature.cool())
        Assert.assertTrue(feature.cool())
    }

    @Test
    fun testWow() {
        Assert.assertTrue(heavyFeature.wow())
        Assert.assertTrue(feature.wow())
    }
}

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The basic difference between all these annotations is as follows -

@BeforeEach - Use to run a common code before( eg setUp) each test method execution. analogous to JUnit 4’s @Before. @AfterEach - Use to run a common code after( eg tearDown) each test method execution. analogous to JUnit 4’s @After. @BeforeAll - Use to run once per class before any test execution. analogous to JUnit 4’s @BeforeClass. @AfterAll - Use to run once per class after all test are executed. analogous to JUnit 4’s @AfterClass.

All these annotations along with the usage is defined on Codingeek - Junit5 Test Lifecycle

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@BeforeClass will be run before the entire test suits whereas @Before will be run 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.

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