Details
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Improvement
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Status: Closed
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Major
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Resolution: Fixed
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Scripting JSP 2.0.28
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None
Description
We have seen problems with JSP compilation under heavy load. It looks like when multiple threads try to compile the same JSP or tag file the result can be NoClassDefFoundError, ClassNotFoundException or simply compilation failures (saying method return types not match the expected type, etc.)
While I was investigating the issue I have found the following things:
First: multiple wrapper instances can exist at the same time for a given JSP file because the caching of them (creating/storing) is not atomic. I have verified that multiple threads can execute the compilation inside the prepareServlet method at the same time. This was my initial prime suspect.
Synchronizing access for this method didn't solve all the problems so I have looked into the taglib related parts as taglib related exceptions/errors showed up frequently in stack traces.
Finally I've found out that the problem (for us) is around the loadTagFile method of the JspServletWrapper class. This method is invoked from the TagFileProcessor.loadTagFile method which - I believe - again, can have different JspServletWrapper instances for the same taglib at the same time and compiling them concurrently can lead to problems (so it seems). By adding a per-file locking around the body of this method I am no longer able to reproduce the compilation errors, JSP files are compiled correctly.
I will attach a patch with my changes to the 2.0.28 version (I haven't found any noticeable changes around this area since) which solved the problems, for us at least.
I think I may lock for a larger scope than it is absolutely necessary so maybe someone can advise me on this.