Testing concurrent programs for correctness can be extremely challenging because many of the possible failure modes of concurrent programs are low-probability events that are sensitive to timing, load, and other hard-to-reproduce conditions. Further, the testing infrastructure can introduce additional synchronization or timing constraints that can mask concurrency problems in the code being tested. Testing concurrent programs for performance can be equally challenging; Java programs are more difficult to test than programs written in statically compiled languages like C, because timing measurements can be affected by dynamic compilation, garbage collection, and adaptive optimization.
To have the best chance of finding latent bugs before they occur in production, combine traditional testing techniques (being careful to avoid the pitfalls discussed here) with code reviews and automated analysis tools. Each of these techniques finds problems that the others are likely to miss.