[JUG CH - LU] What's wrong with AssertJ?!

https://www.jug.ch/html/events/2026/assertj-lu.html

Transcript

1. Stefano Cordio J u l y 1 s t , 2 0 2 6 What’s Wrong with AssertJ?! How a decade of technical debt is addressed in version 4
3. $ whoami Lead Software Engineer Technical Lead @ Large Swiss Bank
4. $ whoami
5. What is AssertJ?
6. Java Assertion Library Fluent API Type-specific Assertions
7. Supported Types • boolean, boolean[], boolean[][] • byte, byte[], byte[][] • char, char[], char[][] • short, short[], short[][] • int, int[], int[][] • long, long[], long[][] • float, float[], float[][] • double, double[], double[][] Primitives • CompletableFuture • CompletionStage • Future java.util.concurrent • DoublePredicate • IntPredicate • LongPredicate • Predicate java.util.function • Boolean • Byte • Character • CharSequence • Class • Comparable • Double • Float • Integer • Iterable • Long • Object[] • Short • String • StringBuffer • StringBuilder • Throwable java.lang • Duration • Instant • LocalDate • LocalDateTime • LocalTime • OffsetDateTime • OffsetTime • Period • YearMonth • ZonedDateTime java.time • Collection • Date • HashSet • Iterator • List • Map • Optional • OptionalDouble • OptionalInt • OptionalLong • Spliterator java.util • DoubleStream • IntStream • LongStream • Stream java.util.stream • File • InputStream java.io • URI • URL java.net • BigDecimal • BigInteger java.math • Path java.nio.file • SQLException java.sql • Matcher java.util.regex • AtomicBoolean • AtomicInteger • AtomicIntegerArray • AtomicIntegerFieldUpdater • AtomicLong • AtomicLongArray • AtomicLongFieldUpdater • AtomicMarkableReference • AtomicReference • AtomicReferenceArray • AtomicReferenceFieldUpdater • AtomicStampedReference • LongAdder java.util.concurrent.atomic
8. Designed around IDE auto-completion
9. Helpful Error Messages
10. Javadoc with Examples
11. Coordinates assertj/assertj <dependency> <groupId>org.assertj</groupId> <artifactId>assertj-core</artifactId> <version>3.27.7</version> <scope>test</scope> </dependency> testImplementation("org.assertj:assert-core:3.27.7")
12. Most Popular Assertion Library
13. Trusted by Popular Open Source Projects JUnit 5 Mockito Spring Framework Spring Boot LangChain4j
14. AssertJ Core AssertJ Guava AssertJ Generator joel-costigliola Joel Costigliola scordio Stefano Cordio AssertJ Vavr Grzegorz Piwowarek pivovarit AssertJ DB VanRoy Julien Roy AssertJ Neo4j Florent Biville fbiville AssertJ Swing Christian Rösch croesch Matt Bertolini mattbertolini AssertJ Eclipse Collections Riccardo Cardin rcardin AssertJ Arrow Core AssertJ Protobuf The Team filiphr Filip Hrisafov Nava2 Kevin Brightwell
15. So, What’s Wrong with AssertJ?!
16. The AssertJ History FEST Assert 1.x 2.x 3.x 4.x 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2027 2026 1.0 1.4 1.0 1.7.1 2.0 2.9.1 3.0 3.27.7 Fork! 3.28 2.3 2.6 2.7 2.9 3.3 3.6 3.8 3.10 3.13 3.17 3.20 3.22 3.24 3.26 3.27 1.6 Java 8 Java 17 Java 7 4.0-M1 4.0 4.0-M2
17. Entry Points
18. Entry Points
19. Entry Points Ambiguities • Multiple unrelated interface overloads • CharBuffer matching assertThat(CharSequence) and assertThat(Comparable) • Bounded type as <T extends X> with X contributing to overload competition • Date subtype matching assertThat(Date) and assertThat(Comparable)
20. Entry Points Target • Single set of assertThat methods in Assertions • Explicit assertThat<Type> helpers only for ambiguity cases • Deprecation in 3.28, removal in 4.0
21. Inconsistent Return Types
22. Inconsistent Return Types
23. Inconsistent Return Types Abstract class → Contract Remove “Abstract” prefix Concrete class → Implementation details Never public, controlled instantiation via factory methods
24. Soft Assertions
25. Soft Assertions Performance penalty due to proxying at runtime with Byte Buddy Alternatives: • Byte Buddy proxying at compile time • Source code generation (JavaPoet, Java Template Engine) • Explicit wrapping logic ✓ Recently contributed by filiphr Filip Hrisafov
26. Kotlin Interoperability
27. Groovy Interoperability
28. What Else to Expect?
29. 3.28 → 4.0 Migration with OpenRewrite Assertions for New JDK Classes Multi-release JAR to support JDK types > 17 Diff-like Error Messages for Strings and Collections MCP Server for LLM Integration? AssertJ Incubator?
30. User Guide with Antora Javadoc Markdown and Executable Snippets Integration Testing with Popular Projects (Spring, JUnit, Mockito, etc.)
31. How Do We Build AssertJ?
32. For Java 17 Or Java 8 (3.x) With Java 25 The latest and greatest build-time goodies from the JDK Java Module System and OSGi Native
33. Cross-Platform Testing
34. Cross-Version Testing
35. Binary Compatibility
36. Code Quality Mutation Testing TestLens
37. Static Analysis In Evaluation Kover Qodana SonarQube
38. Recognition and Sponsorship
39. GitHub Secure Open Source Fund (2025) Spotify FOSS Fund (2023) “This Java library helps engineers write clean tests by making assertions and errors much more readable than with plain JUnit or other libraries. Used in many of our critical tests, it’s found in 764 of our repositories containing production components.” “Investing in security for fast-growing dependencies that support larger projects can mitigate risks and enhance OSS security, especially in the age of AI. Providing funding directly to maintainers enables them to focus on security while giving them expert guidance and emergency support.”
40. GitHub Secure Open Source Fund
41. GitHub Secure Open Source Fund Top Learnings • Limited attack surface • Resilience to Supply Chain Attacks • Dependencies and GitHub Actions • Mindset shift Upcoming Milestones • Incident Response Plan • Threat Modeling • Fuzzing Community • Direct contact with GitHub Security experts • Collaboration with major Open Source projects • Fast-lane to GitHub Product Teams • Issues, Actions, Dependabot, CodeQL • Feeling supported and not left alone in Nebraska
42. We Want to Hear from You!
43. Get in touch assertj/assertj Use Cases • Missing Features • Friction Try Out Milestone Versions Snapshots, too, if you are brave!
44. Open Source in Switzerland November 2026 Hochschule Luzern Rotkreuz switzerland2026.hack-commit-pu.sh Monthly evenings Zürich / Luzern / Basel www.hackergarten.net
45. Thank you!