JUnit is the preeminent testing frameworK:
Never in the field of software development was so much owed by so many to so few lines of code.--Martin Fowler
JUnit was developed for Java 1.1
Based on reflection
But there are alternatives: TestNG, JTiger, Artima SuiteRunner, etc.
However JUnit has the broadest adoption
And by far the broadest tool support; Eclipse, IntelliJ, NetBeans, Maven, Ant, XDevelop, BlueJ, PMD, etc.
This has limited uptake of other, more modern tools