OGNL thread safety

Question

I'm going to reuse OGNL library out of Struts2 scope. I have rather large set of formulas, that is why I would like to precompile all of them:

Ognl.parseExpression(expressionString);

But I'm not sure if precompiled expression can be used in multi-thread environment. Does anybody knows if it can be used?

Answer 1

I think your best option is to contact original developers, directly or through mailing list:
http://www.opensymphony.com/ognl/members.action
https://ognl.dev.java.net/servlets/ProjectMailingListList

The project seems to be abandoned for some time, so there is hardly anybody else who knows :/

Answer 2

This PropertyUtils code from OGNL is written to be thread-safe, and so I would guess that compiled expressions are intended to be thread safe.

Further evidence is that most of the accessor API provide the mutable state as a context parameter (e.g. see PropertyAccessor), so the classes themselves have little mutable state. Immutable classes are intrinsicly thread-safe. The developer guide urges extensions to be thread-safe, and finally looking through the code, where there is mutable state, it is guarded in a synchronized block, for example see EvaluationPool.

In summary, it seems OGNL has been designed to be thread-safe. Whether it actually is or not is another question! You could write a quick test to see for sure, using for example Concutest. Alternatively, if the number of threads is reasonable, storing all the expressions in a ThreadLocal sidesteps the issue altogether, at the cost of a little extra memory (or possibly not, as OGNL does expression caching.)