Jar's can be found here: https://bintray.com/robstoll/tutteli-jars/atrium/0.4.0
API Maturity: Almost Stable
Implementation Maturity: Development

I provide more or less a stable API for users of Atrium. Only the ReporterBuilder, which you use only to build your own assertion verb might slightly change its API (so migrating will be just a few lines in one file). There won't be any changes in the API of the assertion functions/builders.
However, I do not provide yet a stable API for the core of Atrium -- this might affect you if you write your own assertion functions as well as when you provide your own implementation of the core of Atrium.

Included in this release:

Functions to create single assertions and assertion groups.

assertion functions and builders

• assertion for nullable types
• assertions about thrown exceptions
• feature assertions for properties and return values of methods
• assertion builders for Iterable<T>
• and other assertion functions and assertion builders

Components

• Translator including a PropertiesBasedTranslationSupplier
• OnlyFailureReporting
• TextSameLineAssertionFormatter
• DetailedObjectFormatter
• ArgumentFormatter

Improvements compared to 0.3.0

• splitting of API and implementation; which enables to define multiple APIs
  • atrium-cc-en_UK-robstoll
  • atrium-cc-de_CH-robstoll
• feature assertion builders for collection/iterable
• introduction of explanatory assertion groups
• fixes for IAssertionPlantCheckingLazily -> simplified, introduce ReportingPlant as a replacement
• improved README, better explained examples

Not included in this release yet

• overload for Iterable<T> contains assertions which support identification lambdas for nullable types.