The vecmath jar was originally shipped as part of the java extensions for Java 3D. It is a pretty good library - that we use for matrix calculations in the gt-referencing module.
We are using the older version here on the 1.3.2 tag:
Unfortunately not technically open source. As shown by the above three links it has a Java Research License (JRL) for academics wanting to experiment or Java Distribution License (JDL) for commercial binary distribution.
With upgrading no longer an option there is a wide range of options:
- https://code.google.com/p/gwt-vecmath/ <-- perhaps a drop in replacement
- http://mikiobraun.github.io/jblas/ <-- seems to be fast
- Apache commons math
There is some stack exchange discussion on performance of java matrix libraries which was inconclusive.