Using a HandlerInterceptor to add model attributes with Spring Web Flow

Question

I have a HandlerInterceptor to add some "global" model variables. It works.

Now, I try to reuse it in Spring Web Flow, for the same reason.

But HandlerInterceptors have the ModelAndView parameter set to NULL under Spring Web Flow (couldn't figure why, but it's a fact).

I have referenced my interceptor in the FlowHandlerMapping bean :

<bean class="org.springframework.webflow.mvc.servlet.FlowHandlerMapping"> 
    <property name="order" value="0" /> 
    <property name="flowRegistry" ref="flowRegistry" />
    <property name="interceptors">
        <list>
            <ref bean="myInterceptor" />
        </list>
    </property>
</bean>

How can I add variables to the model ?

Is there a workaround, with the request parameter for example ?

Answer 1

Starting with Spring Webflow 2, the ModelAndView object is not generated anymore (see this post (and thread) at the SpringSource forum).

---

The FlowHandlerAdapter handle() function does not generate a ModedAndView anymore (it just returns null) even if this function is :

public ModelAndView handle(HttpServletRequest request, 
        HttpServletResponse response, Object handler)

So overriding this function is pointless, but this function creates a ServletExternalContext object, which holds all the flow variable, by calling its method :

protected ServletExternalContext createServletExternalContext(
    HttpServletRequest request, HttpServletResponse response)

By overriding this function you can pretty much do what you want with this flow variables.

---

To do this, just create a class that extends FlowHandlerAdapter, register it instead of FlowHandlerAdapter and override the createServletExternalContext function.

Basically you use ServletExternalContext.getSessionMap() to access a SharedAttributeMap and register your properties.

As you have access to the HttpServletRequest and HttpServletResponse objects, this method can act petty much like a HandlerInterceptorAdapter.postHandle function.

See an example below.

I left out how to use generic way to reuse the same code for a HandlerInterceptor for the MVC and this object but it's easy to code, by implementing HandlerInterceptor.

---

MyFlowHandlerAdapter :

package my.package;
public class MyFlowHandlerAdapter extends FlowHandlerAdapter {

    @Override
    protected ServletExternalContext createServletExternalContext(
            HttpServletRequest request,
            HttpServletResponse response) {

        ServletExternalContext context = 
            super.createServletExternalContext(request,response);

        context.getSessionMap().put("myproperty", "myvalue");

        return context;
    }
}

You have the FlowHandlerAdapter object defined in you webflow-context.xml file like that :

<bean class="org.springframework.webflow.mvc.servlet.FlowHandlerAdapter">
    <property name="flowExecutor" ref="flowExecutor"/>
</bean>

Just replace it with :

<bean class="my.package.MyFlowHandlerAdapter">
    <property name="flowExecutor" ref="flowExecutor"/>
</bean>

Answer 2

ModelAndView can be null when the interceptor is processing an Ajax request.

Just check if ModelAndView is null. If not, that's because the interceptor is processing a view-model, so you can add your variables at this time.