JUnit5 Assertions

Basic Assertions

import static org.junit.jupiter.api.Assertions.*;

@Test
void basicAssertions() {
    // Equality
    assertEquals(4, 2 + 2);
    assertEquals("hello", result.toLowerCase());
    assertEquals(3.14, Math.PI, 0.01);  // delta for doubles

    // Null checks
    assertNull(user.getMiddleName());
    assertNotNull(response.getBody());

    // Boolean
    assertTrue(list.isEmpty());
    assertFalse(user.isBanned());

    // Reference equality
    assertSame(singleton, Singleton.getInstance());
    assertNotSame(obj1, obj2);

    // Array equality (deep)
    assertArrayEquals(new int[]{1, 2, 3}, actual);

    // Iterable equality
    assertIterableEquals(List.of("a", "b"), result);

    // With failure message (last param or supplier)
    assertEquals(200, response.getStatus(), "Expected OK status");
    assertTrue(predicate, () -> "Computed message: " + computeExpected());
}

assertThrows & assertDoesNotThrow

@Test
void exceptionAssertions() {
    // Assert specific exception is thrown
    IllegalArgumentException ex = assertThrows(
        IllegalArgumentException.class,
        () -> service.create(null)
    );
    assertEquals("Name cannot be null", ex.getMessage());

    // Assert exception type and cause
    RuntimeException wrapped = assertThrows(RuntimeException.class,
        () -> service.callExternal());
    assertInstanceOf(IOException.class, wrapped.getCause());

    // Assert NO exception is thrown
    assertDoesNotThrow(() -> service.healthCheck());

    // assertThrowsExactly — no subclasses
    assertThrowsExactly(
        NullPointerException.class,
        () -> parseNull()
    );
}

assertAll — Grouped Assertions

@Test
void userFieldsAreCorrect() {
    User user = service.findById(1L);

    // All assertions execute even if one fails
    assertAll("user properties",
        () -> assertEquals("Alice", user.getName()),
        () -> assertEquals("alice@example.com", user.getEmail()),
        () -> assertTrue(user.isActive()),
        () -> assertNotNull(user.getCreatedAt())
    );
}

// Nested assertAll
@Test
void addressIsValid() {
    Address address = user.getAddress();
    assertAll("full address check",
        () -> assertAll("street",
            () -> assertNotNull(address.getStreet()),
            () -> assertFalse(address.getStreet().isBlank())
        ),
        () -> assertNotNull(address.getCity()),
        () -> assertEquals(5, address.getZipCode().length())
    );
}

assertTimeout & assertTimeoutPreemptively

import java.time.Duration;

@Test
void operationCompletesInTime() {
    // Fails if execution exceeds duration (waits for completion)
    String result = assertTimeout(Duration.ofSeconds(2), () -> {
        return processBigFile();
    });
    assertNotNull(result);
}

@Test
void operationAbortedIfTooSlow() {
    // Interrupts execution if duration exceeded (different thread)
    assertTimeoutPreemptively(Duration.ofMillis(500), () -> {
        Thread.sleep(200);  // This would fail if changed to 600
        return fetchData();
    });
}

// Note: assertTimeout uses same thread (ThreadLocal state works)
// assertTimeoutPreemptively uses new thread (ThreadLocal state lost)

Assumptions — assumeTrue / assumingThat

import static org.junit.jupiter.api.Assumptions.*;

@Test
void onlyRunOnLinux() {
    assumeTrue("Linux".equals(System.getProperty("os.name")));
    // Test only runs on Linux, skipped otherwise (not failed)
}

@Test
void onlyRunWithRealDatabase() {
    assumeTrue(System.getenv("USE_REAL_DB") != null,
        "Skipping: USE_REAL_DB env var not set");
    // ...
}

@Test
void conditionalBehavior() {
    assumingThat(
        Boolean.getBoolean("ci.build"),
        () -> {
            // Only runs in CI
            assertEquals(0, getWarnings().size());
        }
    );
    // This always runs:
    assertNotNull(mainResult);
}

Custom Assertions with AssertJ

// Add AssertJ for fluent, readable assertions
// Maven: org.assertj:assertj-core:3.x

import static org.assertj.core.api.Assertions.*;

@Test
void assertJExamples() {
    // Strings
    assertThat("Hello World")
        .startsWith("Hello")
        .containsIgnoringCase("world")
        .hasSize(11);

    // Collections
    assertThat(list)
        .hasSize(3)
        .contains("apple", "banana")
        .doesNotContain("cherry");

    // Objects
    assertThat(user)
        .extracting("name", "email")
        .containsExactly("Alice", "alice@example.com");

    // Exceptions
    assertThatThrownBy(() -> service.divide(1, 0))
        .isInstanceOf(ArithmeticException.class)
        .hasMessage("/ by zero");

    // Numbers
    assertThat(price).isBetween(9.99, 29.99);
}

JUnit5 Assertions

Basic Assertions

assertThrows & assertDoesNotThrow

assertAll — Grouped Assertions

assertTimeout & assertTimeoutPreemptively

Assumptions — assumeTrue / assumingThat

Custom Assertions with AssertJ

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