Testing multi-threaded, concurrent Java is hard since we can only test one thread interleaving at a time. An obvious solution is to repeat the test multiple times, hoping that we execute all possible interleavings.
The problem with this solution is that every field access leads to a potential new interleaving. So the solution only works for small unit tests. This led me to develop the open-source tool VMLens.

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VMLens is an open-source Java testing tool that addresses the challenge of testing multi-threaded code by separating the problem into two parts: executing all possible thread interleavings based on synchronization actions, and detecting data races. The tool operates as a Java agent, transforming bytecode at runtime to trace field accesses and synchronization actions. It focuses on synchronization actions to define thread interleavings and checks for data races using happens-before relations. VMLens treats Java's concurrency utilities as correctly implemented abstractions, reducing the number of necessary interleavings to test. The tool aims to help developers efficiently utilize modern multi-core processors by ensuring correct usage of Java's concurrency features.

A new way to test multi-threaded and concurrent Java

Unit tests are a missing piece to use the cores efficiently