我是在java项目中使用的这个jbpm工作流引擎
出现了一下的异常不知道是怎么回事，请教高手指点

1. Exception in thread "main" javax.el.PropertyNotFoundException: Cannot resolve identifier 'username'
   
2.         at de.odysseus.el.tree.impl.ast.AstIdentifier.eval(AstIdentifier.java:86)
   
3.         at de.odysseus.el.tree.impl.ast.AstEval.eval(AstEval.java:51)
   
4.         at de.odysseus.el.tree.impl.ast.AstNode.getValue(AstNode.java:28)
   
5.         at de.odysseus.el.TreeValueExpression.getValue(TreeValueExpression.java:122)
   
6.         at org.jbpm.pvm.internal.el.UelValueExpression.evaluateInScope(UelValueExpression.java:52)
   
7.         at org.jbpm.pvm.internal.el.Expression.evaluate(Expression.java:112)
   
8.         at org.jbpm.pvm.internal.model.ExecutionImpl.initializeAssignments(ExecutionImpl.java:778)
   
9.         at org.jbpm.jpdl.internal.activity.TaskActivity.execute(TaskActivity.java:107)
   
10.         at org.jbpm.jpdl.internal.activity.TaskActivity.execute(TaskActivity.java:58)
    
11.         at org.jbpm.pvm.internal.model.op.ExecuteActivity.perform(ExecuteActivity.java:60)
    
12.         at org.jbpm.pvm.internal.model.ExecutionImpl.performAtomicOperationSync(ExecutionImpl.java:672)
    
13.         at org.jbpm.pvm.internal.model.ExecutionImpl.performAtomicOperation(ExecutionImpl.java:632)
    
14.         at org.jbpm.pvm.internal.model.ExecutionImpl.start(ExecutionImpl.java:217)
    
15.         at org.jbpm.pvm.internal.cmd.StartProcessInstanceInLatestCmd.execute(StartProcessInstanceInLatestCmd.java:63)
    
16.         at org.jbpm.pvm.internal.cmd.StartProcessInstanceInLatestCmd.execute(StartProcessInstanceInLatestCmd.java:36)
    
17.         at org.jbpm.pvm.internal.svc.DefaultCommandService.execute(DefaultCommandService.java:42)
    
18.         at org.jbpm.pvm.internal.tx.StandardTransactionInterceptor.execute(StandardTransactionInterceptor.java:50)
    
19.         at org.jbpm.pvm.internal.svc.EnvironmentInterceptor.executeInNewEnvironment(EnvironmentInterceptor.java:53)
    
20.         at org.jbpm.pvm.internal.svc.EnvironmentInterceptor.execute(EnvironmentInterceptor.java:40)
    
21.         at org.jbpm.pvm.internal.svc.RetryInterceptor.execute(RetryInterceptor.java:56)
    
22.         at org.jbpm.pvm.internal.svc.SkipInterceptor.execute(SkipInterceptor.java:43)
    
23.         at org.jbpm.pvm.internal.svc.ExecutionServiceImpl.startProcessInstanceByKey(ExecutionServiceImpl.java:67)
    
24.         at com.heima.jpbm.JPBMinit.main(JPBMinit.java:32)
    

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