DWR has a concept called handlers for error handling and exception handling on the browser side. Four different handlers have been specified by DWR:
errorHandler is used when DWR knows for certain that something is broken, for example, when the application server has been stopped.
warningHandler is used by DWR when something is wrong, but DWR cannot automatically define the severity of that "something". This is useful during development while debugging an application.
exceptionHandler is used to catch exceptions that are thrown by the server side code. Since we are likely to want to know about exceptions, errorHandler is used if no exceptionHandler is defined.