JsonUnit is a mature Java library for comparing JSON documents in unit and integration tests. It provides a unified comparison engine with pluggable JSON library backends (Jackson 2/3, Gson, Moshi, Johnzon, org.json) via SPI, and integrates with major testing frameworks: AssertJ, Hamcrest, Spring MVC (MockMvc, WebTestClient, RestTestClient), and Kotest. The core engine supports sophisticated features like placeholder tokens, regex-based path matchers, custom number tolerances, array order ignoring, and path-specific configuration overrides. Its architecture is centered around a clean Node abstraction over various JSON representations, enabling transparent cross-backend support without requiring users to manage parser-specific APIs. Beyond basic equality checking, JsonUnit offers a rich set of options for tolerance, ignoring extra fields, treating null as absent, and fail-fast behavior. The Configuration object is an immutable builder that can be shared and reused across tests. The library also includes Kotlin DSLs for AssertJ and Spring integrations, providing a fluent and type-safe assertion experience. Its multi-module Maven project separates concerns cleanly, and the extensive abstract test base ensures consistent behavior across all supported JSON libraries and assertion APIs. JsonUnit is widely adopted (over 1000 GitHub stars) and actively maintained, with recent updates supporting Java 21+, Jackson 3, and modern Spring versions.