[jboss-jira] [JBoss JIRA] (DROOLS-483) Repeated class loading of absent services

[Frank Pavageau (JIRA)]

Frank Pavageau created DROOLS-483:
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             Summary: Repeated class loading of absent services
                 Key: DROOLS-483
                 URL: https://issues.jboss.org/browse/DROOLS-483
             Project: Drools
          Issue Type: Enhancement
      Security Level: Public (Everyone can see)
    Affects Versions: 6.0.1.Final, 5.6.0.Final, 5.5.0.Final, 6.1.0.Final
            Reporter: Frank Pavageau
            Assignee: Mark Proctor


h3. Context

The {{ServiceRegistry}} is used to get the providers for a number of services, several of which don't have an implementation in a Drools artifact but in jBPM, for example.

Those providers include:
- {{ProcessBuilderFactoryService}} (implemented by {{org.jbpm.process.builder.ProcessBuilderFactoryServiceImpl}}) called in the constructor of {{KnowledgeBuilderImpl}} through {{ProcessBuilderFactory}}
- {{ProcessRuntimeFactoryService}} (implemented by {{org.jbpm.process.instance.ProcessRuntimeFactoryServiceImpl}}) called in the constructor of {{StatefulKnowledgeSessionImpl}} through {{ProcessRuntimeFactory}}, so this one especially can be called frequently

Both {{ProcessBuilderFactory}} and {{ProcessRuntimeFactory}} use the fact that the static field containing the service is not null to know they've been initialized, which means they'll continuously try to load the class if it's not actually in the classpath.

h3. Enhancement

If this behavior is not needed for the OSGi support, I'd like to see the initialization test use a boolean field to only try loading the class once. And if it is needed for OSGi, maybe the behavior could be configured.

h3. Rationale

Attempting to load a class is a costly operation, especially in the context of a web application running on Tomcat. I've pushed a [JMH|http://openjdk.java.net/projects/code-tools/jmh/] benchmark on [Github|https://github.com/fpavageau/bench-for-name] to illustrate the point (see the [README.md|https://github.com/fpavageau/bench-for-name#results]): a failed call to Class.forName() takes ~22 microseconds using the context classloader of the benchmark (i.e. probably the system classloader), ~75 microseconds using a Tomcat classloader configured ({{WebappClassLoader}}) with 100 empty jars (that need to be scanned for the missing class). Combined with the fact that {{Class.forName()}} calls {{ClassLoader.loadClass()}} which in the case of {{WebappClassLoader}} is a synchronized method, it becomes a nice bottleneck in a high-traffic application creating {{StatefulKnowledgeSessions}} extensively, because of the lock contention.

Another side-effect of this flow is that {{ServiceRegistryImpl}} continuously gets a {{ClassNotFoundException}}, then throws an {{IllegalArgumentException}}, which means 2 stacktraces to fill for each call, and that is also an [expensive operation|http://shipilev.net/blog/2014/exceptional-performance/] when throwing exceptions is not exceptional and becomes the regular control flow.

I've been able to grab stacktraces from Drools to {{WebappClassLoader.loadClass()}} when doing thread dumps on an application with sufficient load, so it's frequent enough that it can be observed by sampling and not just a rhetorical question.

I could create a pull request, but I'd like to get some feedback first.

Note: most calls to {{ServiceRegistry.get()}} use the same pattern, but the 2 I have picked out are the ones which happen the most frequently for my use case.



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