AssertJ

Language: Java

Testing

AssertJ was created to improve the readability and expressiveness of assertions in Java tests compared to standard JUnit assertions. Its fluent API allows for clear, chainable, and descriptive assertions, making tests easier to write and maintain.

AssertJ is a fluent and rich assertion library for Java that allows developers to write readable and expressive unit test assertions. It provides a wide variety of assertions for core Java types, collections, exceptions, and more.

Installation

maven: Add org.assertj:assertj-core dependency in pom.xml

gradle: Add testImplementation 'org.assertj:assertj-core:3.26.2' in build.gradle

Usage

AssertJ provides a fluent API for assertions, allowing chaining of methods, clear error messages, and a wide range of assertions for objects, collections, maps, exceptions, and more. It integrates seamlessly with JUnit or TestNG.

Simple assertions

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

int result = 5;
assertThat(result).isEqualTo(5).isPositive();

Asserts that `result` is equal to 5 and positive using fluent chaining.

String assertions

String text = "Hello World";
assertThat(text).startsWith("Hello").endsWith("World").contains("lo Wo");

Performs multiple string checks in a readable fluent style.

Collection assertions

List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
assertThat(numbers).hasSize(4).contains(2, 3).doesNotContain(5);

Performs assertions on collections including size, contents, and absence of elements.

Exception assertions

assertThatThrownBy(() -> { Integer.parseInt("abc"); }).isInstanceOf(NumberFormatException.class).hasMessageContaining("For input string");

Checks that a block of code throws a specific exception with a message containing a given string.

Extracting and asserting on properties

class User { String name; int age; }
List<User> users = List.of(new User("Alice", 25), new User("Bob", 30));
assertThat(users).extracting(User::getName).containsExactly("Alice", "Bob");

Extracts properties from objects in a collection and asserts on their values.

Using soft assertions

SoftAssertions softly = new SoftAssertions();
softly.assertThat(1).isEqualTo(2);
softly.assertThat("abc").startsWith("a");
softly.assertAll();

Allows multiple assertions to be evaluated and reported together without stopping at the first failure.

Error Handling

AssertionError: Occurs when the actual value does not meet the expected condition. Check the test logic and input values.

ClassNotFoundException for Assertions: Ensure AssertJ core dependency is included in your project classpath.

Best Practices

Prefer AssertJ for readable and fluent assertions over standard JUnit assertions.

Use chaining to combine multiple assertions on the same object for clarity.

Leverage collection and property extraction methods for complex data structures.

Use soft assertions when you want to evaluate multiple assertions and report all failures.

Integrate with JUnit 5 or TestNG for seamless test execution.

Official Docs Github