A while ago as I was helping a client re-write a computationally heavy backend system that had a Spring Web Flow wizard interface. The system would retrieve all orders of gas shipments for that day and then run calculations on what orders would be able to ship that day based on a number of various factors. Because the retrieval and calculations could potentially take several minutes, the system used Ajax calls to kick off asynchronous processes for retrieval and calculations, along with polling to deliver progress messages to the user in a modal window. During testing we noticed that our calls to kick off the processes were not always picking up changes as jQuery defaults cache to true for Ajax.

Once we set cache: false in the AJAX call.

$.ajax({  
    url : ...,
    cache: false,
    success : function(data) {
        ...
    }
});

We saw the following helpful stack trace:

Caused by: org.springframework.webflow.execution.FlowExecutionException: Exception thrown in state ‘wizard’ of flow ‘wizardFlow'
at org.springframework.webflow.engine.impl.FlowExecutionImpl.wrap(FlowExecutionImpl.java:571)
at org.springframework.webflow.engine.impl.FlowExecutionImpl.resume(FlowExecutionImpl.java:265)
at org.springframework.webflow.executor.FlowExecutorImpl.resumeExecution(FlowExecutorImpl.java:169)
at org.springframework.webflow.mvc.servlet.FlowHandlerAdapter.handle(FlowHandlerAdapter.java:183)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:900)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:827)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
... 52 more
<strong>Caused by: java.lang.IllegalArgumentException: The expression string to parse is required and must not be empty
at org.springframework.util.Assert.hasText(Assert.java:162)</strong>
at org.springframework.binding.expression.spel.SpringELExpressionParser.parseExpression(SpringELExpressionParser.java:72)
at org.springframework.webflow.mvc.view.AbstractMvcView.addEmptyValueMapping(AbstractMvcView.java:472)
at org.springframework.webflow.mvc.view.AbstractMvcView.addDefaultMappings(AbstractMvcView.java:454)
at org.springframework.webflow.mvc.view.AbstractMvcView.bind(AbstractMvcView.java:373)
at org.springframework.webflow.mvc.view.AbstractMvcView.processUserEvent(AbstractMvcView.java:216)
at org.springframework.webflow.engine.ViewState.handleEvent(ViewState.java:226)
at org.springframework.webflow.engine.ViewState.resume(ViewState.java:196)
at org.springframework.webflow.engine.Flow.resume(Flow.java:545)
at org.springframework.webflow.engine.impl.FlowExecutionImpl.resume(FlowExecutionImpl.java:261)

This was caused by jQuery’s default solution to preventing browsers from caching AJAX requests which is enabled by setting cache: false in the settings of the ajax request, which adds the request param “_” with the request timestamp to the request url.

...app…/wizardFlow.html?execution=e1s1&_eventId=retrieve&gasDay=11/25/2012&_=1365712827950

Spring Web Flow didn’t know what to do with the “_” request parameter when was parsing the request parameters to call the method in our action that handled that sequence in the flow:

public void retrieve(RequestContext requestContext, WizardStateHolder stateHolder, ProcessingStateHolder processingStateHolder, MessageContext messageContext, String gasDay)

In order to resolve this, we registered our own beforeSend method that would replace the “_” request parameter name that jQuery defaulted to with “jqueryCache” as the parameter name.

$.ajaxSetup({
    beforeSend : function(xhr, setting) {
        var url = setting.url;
        url = url.replace("&_=", "&jqueryCache=");
        setting.url = url;
    }
});

This solution allowed jQuery to bypass the browser caching of requests and Web Flow to ignore the request parameter. By registering it as part of ajaxSetup, we were also able to avoid repeating the code wherever we needed to set cache: false, so that developers only had to set cache: false if the use case required it while not worrying about Web Flow.