Tutorial Step 3a Adding Server-side Validation in Java

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32 Tutorial Step 3a Adding Server-side Validation
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How does our call to dojoxhrGet differ from the standard usage for XHR First, and most obviously, we aren t creating a new XHR objectThe new object does get created eventually somewhere deep in the internals of Dojo (actually not that deep but more on that later) But we have a simpler syntax using an existing Dojo function Second, rather that pass the HTTP message type as a parameter, it is built into the name of the functionTo do an HTTP GET, we use dojoxhrGet, while to do a POST we use dojoxhrPost instead Third, and finally, we pass the callback function, userNameValidationHandler, as a property of our argument object, not by setting an XHR propertyThere are some benefits that aren t obvious from viewing this codeWhen using XHR directly, our callback method has to test the state and status properties of the XHR object before it can safely execute the handler codeWhen using dojoxhrGet, Dojo will perform the checks before calling our handler, allowing us to write simple handler codeThe less code we write, the less the potential for errorThat s a good thing, as Martha Stewart might say 3223 Handle the Response from the Server The server receives our request, processes it, and returns the response back to the browserThe browser executes a callback function internal to Dojo Dojo, in turn, calls the function that we specified as the callback, userNameValidationHandler, in the dojoxhrGet function call All we have to do is code that functionWhat must this function do At a minimum, it should display an error message stating that someone else has already taken the user nameThe following code will display an error message
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function userNameValidationHandler(response) { // Clear any error messages that may have been displayed dijitbyId("userName")displayMessage(); if (!responsevalid) { var errorMessage = "User name already taken"; // Display error message as tooltip next to field dijitbyId("userName")displayMessage(errorMessage); } }
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Note that we re making sure to clear the error message firstThis is necessary to get rid of the error message if the user is entering this field a second time after having failed to enter a valid user name the first time There is at least one thorny issue leftWhat if the call to the server to validate user name takes a long time maybe 20 seconds or more The user might already be entering the next fieldWe don t want to interrupt what the person is doing by switching focus back to the user name field But what will they think when an error message suddenly appears next to a field they aren t even working on We could block the user from working while the validation is being done by making the XHR call synchronous, but
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that could also be frustrating for the userWe ll discuss this issue further in Part II when we explore Dojo widgets in more detail Our example rests on the assumption that the server can validate the user nameTo do that we re traveling past the boundaries of DojoWe assume that there is some resource on the server called validateUserNamejsp This resource takes the user name as a parameter and returns a JavaScript Object Notation (JSON) string, defining an object with a property called valid that might either be true or false Dojo doesn t care how you write this resource or what kind of server it is running on, just so long as the server can talk HTTP Following is a simple JSP page that would validate the user name with a hard coded check to see if the value is olduser As long as it isn t, then the user name would be considered validThis will allow our example to work, but obviously the server program should be more sophisticated
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<%@ page contentType="text/plain"%> <% try { Systemoutprintln("UserName : " + requestgetParameter("userName")); if (requestgetParameter("userName")equals("olduser")) { outprintln("{valid: false}"); Systemoutprintln("To Browser : false" ); } else { outprintln("{valid: true}"); Systemoutprintln("To Browser : true" ); } } catch (Exception ex) { outprintln(exgetMessage()); exprintStackTrace(); } %>
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