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You may want to build the openmrs-api and openmrs-web, before building the openmrs-webapp, as it allows to get all the dependencies that are required for OpenMRS into the local maven repository.
After getting all the dependencies, Clean and Build the openmrs-api and the openmrs-web modules.
The openmrs-web application can then be tested to build and run on jetty by defining a Goal to run through the jetty:run command. You may also want to skip tests or build offline etc can be configured through this screen.
On opening the openmrs-webapp maven module, we can run the web application in one of the supplied application servers (Apache Tomcat or Glassfish) by right-clicking on the project and Run or through the 
icon in the toolbar. 
Tips to Skip Tests
If unit tests are taking too long and you want to skip them at times, when you are not running tests specifically.... then you can go to Tools->Options->Miscellaneous->Maven-> Check the "Skip Tests for any build executions not directly related to testing
