Let us assume a multithreaded application, that tries to initialise & start several processes using a StatefulKnowledgeSession in order to use in these processes rule flow. The flow in all the processes is the same but every process instance must have an own StatefulKnowledgeSession instance because of stateful session.

Actually the problem:

At the initializing phase occurs an org.mvel2.CompileException exception from time to time at KnowledgeBuilder.add(...):
Exception in thread "Thread-60" [Error: incomplete statement: (possible use of reserved keyword as identifier: )]
[Near :

{... globals != empt ....}

]
^
[Line: 0, Column: 0]
at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:153)
at org.mvel2.MVELInterpretedRuntime.parse(MVELInterpretedRuntime.java:44)
at org.mvel2.MVEL.eval(MVEL.java:514)
at org.mvel2.templates.res.IfNode.eval(IfNode.java:61)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.res.TerminalNode.eval(TerminalNode.java:39)
at org.mvel2.templates.res.ForEachNode.eval(ForEachNode.java:116)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.res.TerminalNode.eval(TerminalNode.java:39)
at org.mvel2.templates.res.IfNode.eval(IfNode.java:64)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:195)
at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:190)
at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:180)
at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:169)
at org.drools.rule.builder.dialect.java.AbstractJavaRuleBuilder.generatTemplates(AbstractJavaRuleBuilder.java:126)
at org.drools.rule.builder.dialect.java.JavaConsequenceBuilder.build(JavaConsequenceBuilder.java:128)
at org.drools.rule.builder.RuleBuilder.build(RuleBuilder.java:86)
at org.drools.compiler.PackageBuilder.addRule(PackageBuilder.java:1159)
at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:649)
at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:290)
at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:488)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:25)
at com.myfirm.RuleFlowProcess.init(RuleFlowProcess.java:25)
at com.myfirm.RuleFlowProcess.clone(RuleFlowProcess.java:53)
at com.myfirm.ThreadTest$CloneThread.run(ThreadTest.java:35)
Caused by: java.lang.NullPointerException
at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:113)
... 27 more
Exception in thread "Thread-72" [Error: incomplete statement: (possible use of reserved keyword as identifier: )]
[Near :

{... declarations != empt ....}

]
^
[Line: 0, Column: 0]
at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:153)
at org.mvel2.MVELInterpretedRuntime.parse(MVELInterpretedRuntime.java:44)
at org.mvel2.MVEL.eval(MVEL.java:514)
at org.mvel2.templates.res.IfNode.eval(IfNode.java:61)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.res.TerminalNode.eval(TerminalNode.java:39)
at org.mvel2.templates.res.ForEachNode.eval(ForEachNode.java:116)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.res.TerminalNode.eval(TerminalNode.java:39)
at org.mvel2.templates.res.ForEachNode.eval(ForEachNode.java:116)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:195)
at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:190)
at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:180)
at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:169)
at org.drools.rule.builder.dialect.java.AbstractJavaRuleBuilder.generatTemplates(AbstractJavaRuleBuilder.java:135)
at org.drools.rule.builder.dialect.java.JavaConsequenceBuilder.build(JavaConsequenceBuilder.java:128)
at org.drools.rule.builder.RuleBuilder.build(RuleBuilder.java:86)
at org.drools.compiler.PackageBuilder.addRule(PackageBuilder.java:1159)
at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:649)
at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:290)
at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:488)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:25)
at com.myfirm.RuleFlowProcess.init(RuleFlowProcess.java:25)
at com.myfirm.RuleFlowProcess.clone(RuleFlowProcess.java:53)
at com.myfirm.ThreadTest$CloneThread.run(ThreadTest.java:35)
Caused by: java.lang.NullPointerException
at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:113)
... 33 more

To demonstrate this problem I have created a simple test maven project that reproduces this error (s. attach):
I have a class that creates & inits stateful knowledge session in a init method (RuleFlowProcess):

/**

• inits the rule flow

*/

public class RuleFlowProcess {
private StatefulKnowledgeSession ksession;

public void init() {
KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
kbuilder.add(ResourceFactory.newClassPathResource("threadSafeTest.rf"), ResourceType.DRF);
kbuilder.add(ResourceFactory.newClassPathResource("testRules1.drl"), ResourceType.DRL);
kbuilder.add(ResourceFactory.newClassPathResource("testRules2.drl"), ResourceType.DRL);
kbuilder.add(ResourceFactory.newClassPathResource("testRules3.drl"), ResourceType.DRL);
kbuilder.add(ResourceFactory.newClassPathResource("testRules4.drl"), ResourceType.DRL);

KnowledgeBuilderErrors errors = kbuilder.getErrors();
if (errors.size() > 0) {
StringBuilder errorString = new StringBuilder();
for (KnowledgeBuilderError error : errors)

{ errorString.append(error); }

throw new RuntimeException(errorString.toString());
}
KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();
kbase.addKnowledgePackages(kbuilder.getKnowledgePackages());
ksession = kbase.newStatefulKnowledgeSession();
}
...
}
I create & init concurrently in several (200) threads this RuleFlowProcess and some threads crash with this exceptions while adding the resources.