Java Assertion is a mechanism to indicate that the programmer thinks that something unexpected has happened. It is like a sanity check in a code and should be used to ensure that the code works as expected. Assertion statements are available since Java 1.4 and can be enabled or disabled as per requirement. Assertions should not be used for user input validation, the normal flow of program execution, or any exception handling. Java Assertion Libraries is one of the most popular open-source libraries for assertion in Java. These libraries are useful to check whether the actual result and expected result are equal or not. Let's take a look at some of the most popular and useful assertion open source libraries in 2022 truth - Fluent assertions for Java and Android; assertj-core - library providing easy to use rich typed assertions; ArchUnit - Java architecture test library.