@DisallowConcurrentExecution is an annotation that can be added to the Job class that tells Quartz not to execute multiple instances of a given job definition (that refers to the given job class) concurrently. Notice the wording there, as it was chosen very carefully. In the example from the previous section, if "SalesReportJob" has this annotation, than only one instance of "SalesReportForJoe" can execute at a given time, but it can execute concurrently with an instance of "SalesReportForMike". The constraint is based upon an instance definition (JobDetail), not on instances of the job class. However, it was decided (during the design of Quartz) to have the annotation carried on the class itself, because it does often make a difference to how the class is coded.
If you dont want SalesReportForMike and SalesReportForJoe to run concurrently ,then you can set the scheduler's ThreadPool size to 1. So at any given time only one job will run.
Also take a look at StatefulJob