jtreg is the test harness for regression and unit testing used by the JDK test framework. For many OpenJDK distributions, the jtreg tests are run in addition to the TCK tests. jtreg complements the Technology Compatibility Kit (TCK) but does not replace it. Both together allow production of a good quality OpenJDK distribution. The tests implemented with jtreg are open source functional tests provided by the contributors of features (as opposed to the TCK/JCK, which is not open source, but Oracle proprietary).
The TCK is used for official compatibility and performance tests. jtreg is used for all other tests, for example, for functional tests. As jtreg is open source, it can be used as a regression test by developers contributing to or customizing OpenJDK.
jtreg was developed by Sun starting in 1997 before Junit, ANT, and other common tools were available. jtreg is specific to the OpenJDK project and not commonly used in other projects.
Number of tests in jtreg: 24.758 for Java 8, and 43.731 for Java 11