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Modification to readLock example in chapter 12 of JPA Pro

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Hi, for everyone's reference, I modified readLock example of chapter 12 to demonstrate the fact that an optimistic lock can prevent an entity from being updated when it is being read from DB:

When the generateDepartmentSalaryReport method is called, I got this expected error:

Warning: A system exception occurred during an invocation on EJB EmployeeService, method: public examples.model.SalaryReport examples.stateless.EmployeeService.generateDepartmentsSalaryReport(java.util.List)
2017-02-17T21:00:43.818-0500|Warning: javax.ejb.EJBException  ...
Caused by: java.util.ConcurrentModificationException
at java.util.Vector$Itr.checkForComodification(Vector.java:1184)

It is sorta covered in the JavaRanch Style Guide.
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