JFluid is a profiler that uses bytecode instrumentation and, unlike
most of the other tools, can perform it on-the-fly, while the profiled
application is running. This is enabled by the code hotswapping
technology in the experimental version of Sun's HotSpot JVM that
JFluid uses. JFluid allows you to instrument and de-instrument limited
portions of a running application, and/or turn on and off different
kinds of profiling (CPU, memory) at run time.

The main added features in this version is the object liveness
analysis and initial support for memory leak debugging. These features
are supposed to scale better than in other existing profilers, since
heap snapshots are not used. There have also been some usability
improvements and bug fixes since the previous version. For more
information and free downloads, see
http://research.sun.com/projects/jfluid.