Java JUnit Testing Cheat Sheet

Covers JUnit 5 test lifecycle annotations, assertions, parameterized tests, and Mockito mocking patterns for writing effective unit tests.

2 PagesIntermediateApr 10, 2026

Basic Test Structure

Core JUnit 5 lifecycle annotations.

java

import org.junit.jupiter.api.*;import static org.junit.jupiter.api.Assertions.*;class CalculatorTest {    private Calculator calc;    @BeforeEach    void setUp() {        calc = new Calculator();       // Runs before each test    }    @Test    void addsTwoNumbers() {        assertEquals(5, calc.add(2, 3));    }    @AfterEach    void tearDown() {        calc = null;                   // Runs after each test    }}

Common Assertions

Assertion methods from org.junit.jupiter.api.Assertions.

java

assertEquals(expected, actual);assertEquals(expected, actual, "custom failure message");assertNotEquals(a, b);assertTrue(condition);assertFalse(condition);assertNull(obj);assertNotNull(obj);assertThrows(IllegalArgumentException.class, () -> calc.divide(1, 0));assertAll(    () -> assertEquals(4, calc.add(2, 2)),    () -> assertEquals(0, calc.add(0, 0)));assertArrayEquals(new int[]{1, 2}, result);assertTimeout(Duration.ofMillis(100), () -> calc.slowOp());

Parameterized Tests

Run the same test logic with multiple inputs.

java

import org.junit.jupiter.params.ParameterizedTest;import org.junit.jupiter.params.provider.ValueSource;import org.junit.jupiter.params.provider.CsvSource;@ParameterizedTest@ValueSource(ints = {2, 4, 6, 8})void isEven(int number) {    assertEquals(0, number % 2);}@ParameterizedTest@CsvSource({"1,2,3", "2,3,5", "10,20,30"})void addsPairs(int a, int b, int expected) {    assertEquals(expected, calc.add(a, b));}

Key Annotations

Lifecycle and configuration annotations in JUnit 5.

@Test- Marks a method as a test case.
@BeforeEach / @AfterEach- Runs before/after every test method in the class.
@BeforeAll / @AfterAll- Runs once before/after all tests in the class; method must be static (unless using @TestInstance(PER_CLASS)).
@Disabled- Skips a test method or class, optionally with a reason string.
@DisplayName- Sets a custom human-readable name for a test or class in reports.
@Nested- Groups related tests in an inner non-static class for hierarchical organization.
@Tag- Labels tests for filtered execution (e.g. mvn test -Dgroups=slow).
@ExtendWith- Registers an extension, e.g. @ExtendWith(MockitoExtension.class).

Mocking with Mockito

Common Mockito patterns used alongside JUnit 5.

java

import static org.mockito.Mockito.*;@ExtendWith(MockitoExtension.class)class OrderServiceTest {    @Mock    private PaymentGateway gateway;    @InjectMocks    private OrderService service;    @Test    void chargesCustomer() {        when(gateway.charge(100)).thenReturn(true);        boolean result = service.checkout(100);        assertTrue(result);        verify(gateway, times(1)).charge(100);    }}

Pro Tip

Prefer assertThrows over try/catch + fail() for exception testing - it returns the exception so you can also assert on its message, and it clearly signals the intended failure path.

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