assertj

Overview

AssertJ is "a Java library that provides a rich set of assertions and truly helpful error messages, improves test code readability, and is designed to be super easy to use within your favorite IDE" (assertj.github.io/doc). It ships modules for JDK core types, Guava, Joda-Time, and databases; this skill covers assertj-core (JDK types).

Works alongside junit5-tests, testng-tests, or spock-tests. AssertJ handles the assertion layer; those skills handle the test runner.

This skill is a reference - defines the matcher catalog and patterns; does not run tests. Advanced per-feature variants live in references/matcher-catalog.md; provenance links are consolidated under References.

When to use

• JVM project (Java 8+ or Kotlin 1.9+) using any test framework.
• Need richer assertion failure messages than built-in assertEquals / assertTrue.
• Deep-equality checking between object graphs without overriding equals.
• Verifying collections, exception details, or running multiple assertions without failing on the first one.

Step 1 - Install

Maven:

<dependency>
  <groupId>org.assertj</groupId>
  <artifactId>assertj-core</artifactId>
  <version>3.27.7</version>
  <scope>test</scope>
</dependency>

Gradle:

testImplementation("org.assertj:assertj-core:3.27.7")

The 3.27.7 version is current at time of writing; check Maven Central for the latest assertj-core release before copying.

Static import the entry class once per test file:

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

Step 2 - assertThat entry point

assertThat(actual) returns a type-specific assertion object. All assertions chain fluently from it.

assertThat(frodo.getName()).isEqualTo("Frodo");

assertThat("text").isNotNull()
                  .startsWith("te")
                  .contains("ex");

Common predicates available on every assertion:

assertThat(value).isEqualTo(expected);
assertThat(value).isNotEqualTo(other);
assertThat(value).isNull();
assertThat(value).isNotNull();
assertThat(value).isSameAs(ref);            // reference equality
assertThat(value).isInstanceOf(MyClass.class);
assertThat(flag).isTrue();
assertThat(flag).isFalse();

Use .as("description") to label an assertion in failure output:

assertThat(user.getAge()).as("user age").isGreaterThan(0);

Step 3 - Collection assertions

assertThat(list).hasSize(9);
assertThat(list).contains(frodo, sam);                // any order, subset
assertThat(list).containsExactly(frodo, sam, pippin); // exact order, exact set

Element-level verification:

assertThat(hobbits).allSatisfy(c -> {
    assertThat(c.getRace()).isEqualTo(HOBBIT);
    assertThat(c.getName()).isNotEqualTo("Sauron");
});

Extraction:

// Extract a property then assert on extracted values
assertThat(fellowship).extracting("name")
                      .contains("Boromir", "Gandalf", "Frodo");

containsOnly / doesNotContain / isEmpty, multi-property tuple extraction, filteredOn, and anySatisfy are in references/matcher-catalog.md.

Step 4 - Exception assertions

Primary form - assertThatThrownBy:

assertThatThrownBy(() -> parser.parse(null))
    .isInstanceOf(IllegalArgumentException.class)
    .hasMessageContaining("null input");

Type-first (assertThatExceptionOfType), BDD capture (catchThrowable / catchThrowableOfType), cause-chain inspection, and no-exception (assertThatCode) forms are in references/matcher-catalog.md.

Step 5 - SoftAssertions

SoftAssertions collect failures instead of stopping at the first one. All violations are reported together in a single error.

Instance form:

SoftAssertions softly = new SoftAssertions();
softly.assertThat(actual.getName()).isEqualTo("Frodo");
softly.assertThat(actual.getAge()).isEqualTo(33);
softly.assertThat(actual.getRace()).isEqualTo(HOBBIT);
softly.assertAll(); // throws one error listing all failures

Static helper - assertSoftly: manages lifecycle automatically; assertAll() is called on exit:

assertSoftly(softly -> {
    softly.assertThat(frodo.getName()).isEqualTo("Frodo");
    softly.assertThat(frodo.getAge()).isEqualTo(33);
    softly.assertThat(frodo.getRace()).isEqualTo(HOBBIT);
});

Use SoftAssertions when a test covers multiple independent properties of the same subject, so a single run reveals all mismatches rather than stopping at the first.

Step 6 - Recursive comparison

usingRecursiveComparison() compares object graphs field-by-field without requiring equals overrides.

assertThat(sherlock).usingRecursiveComparison()
                    .isEqualTo(sherlockClone);

Exclude fields:

assertThat(actual).usingRecursiveComparison()
                  .ignoringFields("id", "home.address.street")
                  .isEqualTo(expected);

Regex field exclusion, ignoringActualNullFields, per-type comparators, and withStrictTypeChecking are in references/matcher-catalog.md.

Step 7 - Custom assertions

Extend AbstractAssert with type parameters <SELF, ACTUAL> (SELF is the concrete assertion class, for chaining), then expose a static factory mirroring assertThat:

public class PersonAssert extends AbstractAssert<PersonAssert, Person> {
    public PersonAssert(Person actual) { super(actual, PersonAssert.class); }

    public PersonAssert hasName(String name) {
        isNotNull();
        if (!actual.getName().equals(name)) {
            failWithMessage("Expected name <%s> but was <%s>", name, actual.getName());
        }
        return this;
    }
}

public static PersonAssert assertThat(Person actual) {
    return new PersonAssert(actual);
}

Usage then reads like built-in assertions: assertThat(person).hasName("Alice"). A multi-method example is in references/matcher-catalog.md.

Example - full test method

@Test
void order_ships_to_correct_address() {
    Order order = orderService.place(items, address);

    assertThat(order).isNotNull();
    assertThat(order.getItems()).hasSize(2)
                                .extracting("sku")
                                .containsExactly("ITEM-001", "ITEM-002");
    assertThat(order).usingRecursiveComparison()
                     .ignoringFields("id", "createdAt")
                     .isEqualTo(expectedOrder);
}

@Test
void invalid_quantity_throws() {
    assertThatThrownBy(() -> orderService.place(emptyItems, address))
        .isInstanceOf(IllegalArgumentException.class)
        .hasMessageContaining("at least one item");
}

@Test
void order_summary_fields_all_valid() {
    OrderSummary summary = orderService.summarize(orderId);

    assertSoftly(softly -> {
        softly.assertThat(summary.getTotal()).isGreaterThan(BigDecimal.ZERO);
        softly.assertThat(summary.getItemCount()).isEqualTo(2);
        softly.assertThat(summary.getStatus()).isEqualTo(OrderStatus.CONFIRMED);
    });
}

Anti-patterns

Limitations

• assertj-core targets Java 8+; Kotlin works but the API is more ergonomic in Java (Kotlin users may prefer AssertK).
• usingRecursiveComparison on cyclic object graphs requires .withCyclicSafeComparison(); defaults to withIgnoreAllOverriddenEquals() which skips equals overrides.
• SoftAssertions does not propagate assertion context automatically to nested lambdas; each lambda needs its own softly.assertThat(...) call.
• The extracting overload using string property names uses reflection; use the Function-based overload for compile-time safety.

References

• AssertJ docs - landing (install, overview)
• Basic assertions - isEqualTo, isNull, isTrue, chaining
• Collection assertions - iterable matchers, extracting, filtering
• Exception assertions - assertThatThrownBy, catchThrowable
• Soft assertions - SoftAssertions, assertSoftly
• Recursive comparison - usingRecursiveComparison variants
• Custom assertions - AbstractAssert
• references/matcher-catalog.md - advanced per-feature variants
• github.com/assertj/assertj - repository and issue tracker
• github.com/assertj/assertj-examples - worked examples including soft assertions
• junit5-tests, testng-tests, spock-tests - sister skills (test runners)