Pascal Davoust created WW-5147:
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Summary: OGNL valid expression is not cached and is parsed over
again in some situations
Key: WW-5147
URL: https://issues.apache.org/jira/browse/WW-5147
Project: Struts 2
Issue Type: Bug
Components: Core
Affects Versions: 2.5.20
Reporter: Pascal Davoust
Profiling an enterprise application under high load shows an unlikely number of
invocations to method `ognl.Ognl.parseExpression` (representing a significant
cumulated CPU time), despite having the OGNL expression cache enabled.
Knowing that there is no dynamic expression in this application, this is
puzzling: once each expression have been encountered at least once, its
parsed/compiled representation should be cached, avoiding the parsing phase
entirely to only keep the execution phase.
After investigation, this is due to the caching logic in
method`com.opensymphony.xwork2.ognl.OgnlUtil.compileAndExecute(String,
Map<String, Object>, OgnlTask<T>)` (same applies to
`com.opensymphony.xwork2.ognl.OgnlUtil.compileAndExecuteMethod(String,
Map<String, Object>, OgnlTask<T>)`) :
{code:java}
private <T> Object compileAndExecute(String expression, Map<String, Object>
context, OgnlTask<T> task) throws OgnlException {
Object tree;
if (enableExpressionCache) {
tree = expressions.get(expression);
if (tree == null) {
tree = Ognl.parseExpression(expression);
checkEnableEvalExpression(tree, context);
}
} else {
tree = Ognl.parseExpression(expression);
checkEnableEvalExpression(tree, context);
} final T exec = task.execute(tree);
// if cache is enabled and it's a valid expression, puts it in
if (enableExpressionCache) {
expressions.putIfAbsent(expression, tree);
}
return exec;
} {code}
As shown above, on cache miss, the expression is parsed, then executed, and
added to the cache {+}only after execution{+}.
Problem is that the execution can fail (especially that Ognl makes use of
exceptions quite extensively): in this case, the parsed AST is lost and not
added to the cache.
As a result, the same expression will go through the cache miss code path next
time.
Unless I'm mistaken, the AST of the parsed expression could be added to the
cache right after parsing and validation, and before execution fires: an AST is
obviously independant from any execution context so it can be reused over and
over again.
As a proof of concept, I patched our enterprise application with the changes
above and ran the benchmark again: we experienced a very significant
improvement in response times and CPU usage, between 10% to 30% decreased
response times (knowing that these pages also perform time consuming tasks such
as database updates and search engine lookups).
I'm happy to provide a PR for you guys to have a look if you deem it worthy.
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