What is Jython and is it useful at all?

When will I need Jython?

When you want to program in Python but need (or want) to have the result run on a Java virtual machine, or use existing Java components.

What are the drawbacks.

Jython may not be 100% compatible with Python, though any incompatibility would be considered a bug. If you later want/need to run on CPython, any code that uses Java components will have to be rewritten.

I assume it is slow?

That depends, as always, on your specific usecase. It may actually be faster than CPython in some cases; and of course it depends on the specific JVM you run under - these get better all the time.

If you know Python and has bought into the "pythonic" way of doing things, then Jython allows you to bring that philosophy to the JVM stack. If you do this, it is much more than just adding scripting capability.

In our latest projects, all the custom and business logic is built in Jython, at the same time we can still leverage some of the great tried and tested Java libraries like Solr, Jasperreports, Quartz, Jetty, Velocity to name a few.

It does get compiled to bytecode, however, an extra layer is being added, but is no different to using an ORM instead of straight JDBC for example.

What you gain in productivity far out weighs the minuscule lost in performance.

On the server side, Jython is rarely the bottleneck. For mini desktop apps, there may be issues, but very much dependent on what you are trying to do.

The latest JDK, together with containers like Jetty or Tomcat are very mature and stable, adding Python on top, in many cases, gives the best of both worlds.

Quoting Wikipedia:

Jython, successor of JPython, is an implementation of the Python programming language written in Java.

As for what uses it may have:

Jython programs can seamlessly import and use any Java class. Except for some standard modules, Jython programs use Java classes instead of Python modules.

Performance measurements of Jython, IronPython and CPython show that both Jython and IronPython outperform CPython for some cases with large datasets.