Using thread-bound HttpServletRequest exposed by Spring to bring DI on view tier

Im saying about how to obtain request object in a web application at arbitrary point of your web tier code. Sometimes it can be very useful, especially when this object hidden in abstract layers.

Spring provides a couple of very useful classes for doing this. But at first let me explain the problem.

See, sometimes you can have legacy application that wasnt designed for dependency injection, testability, being POJO and etc. For example, it means that somewhere in view tier code, in struts' actions you have:

...
MyActionForm execute(Form inForm, ActionMapping mapping, Session s) {
    ...
    AccountManager accountManager
             = locator.getSessionEjb(AccountManagerHome.JNDI_NAME, s.getEjbProviderUrl());
    ...
}
...

In other words, you have a ubiquitous EJB lookup code which works using http session 'cause it holds some critical info, e.g. providerUrl like in example above. And this code makes your actions:

• not unit-testable;
• being hard to switch from one biz impl. to another, e.g. from EJB to POJO;
• contains RemoteException handling code (even if you have defined RE handling somewhere in superclass you actually need to call this superclass' handling method)

So, here's solution' primary points:

• lookup code moves to spring context xml in the from of FactoryBean implementations;
• add RequestContextListener as listener to web.xm; this class binds request to thread when request is being initialized and unbinds it when it's being destroyed; Or add RequestContextFilter which is faster solution 'cause filter allows to map urls.
• Lookup code needs http session; it'll get it from thread bound request using: RequestContextHolder.getRequestAttributes(), than casts result to ServletRequestAttributes, where method of interest - getRequest();

Implementation of lookup bean:

public class AccountManagerFactoryBean implements FactoryBean {
    // METHODS        

    public Object getObject() throws Exception {
        return Proxy.newProxyInstance(getClass().getClassLoader(),
                new Class[]{AccountManager.class},
                new BaseInvocationHandler() {
                    protected Class getEJBHomeClass() {
                        return AccountManagerHome.class;
                    }
                });
    }

    public Class getObjectType() {
        return AccountManager.class;
    }

    public boolean isSingleton() {
        return true;
    }
}

Where invocation handler that does an actual job:

public abstract class BaseInvocationHandler implements InvocationHandler {
    // METHODS

    /**
     * Method interceptor which guarantees invocation on EJB object obtained
     * through {@link SessionInfo#ejbProviderUrl}
     *
     * @param proxy  proxy object; not used;
     * @param method method; invocation will be comitted on this method;
     * @throws Throwable either RemoteException or application logic exception;
     * here we dont care about it; application exceptions will be propagated
     * to calling code, RemoteException handling will be defined in spring
     * context xml;
     */
    public Object invoke(Object proxy, Method method, Object[] args)
 throws Throwable {
        String interfaceMethodName = method.getName();
        Class[] parameterTypes = method.getParameterTypes();
        Object ejb =
           services.useSessionEjb(
               getEJBHomeClass(), getSessionInfo().getEjbProviderUrl());
        Method ejbDeclaredMethod =
           ejb.getClass().getDeclaredMethod(interfaceMethodName, parameterTypes);
        if (ejbDeclaredMethod != null) {
            return ejbDeclaredMethod.invoke(ejb, args);
        }
        else {
            // MUST BE UNREACHEBLE;
            throw new AssertionError("Not all methods implemented from the EJB");
        }
    }

    /**
     * Obtains session info object;
     *
     * @return session info object;
     */
    protected final SessionInfo getSessionInfo() {
        // getting request;
        // since we use RequestContextFilter in web.xml and we're
        // in servlet container it's legal to call RequestContextHolder;
        RequestAttributes ra = RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = ((ServletRequestAttributes) ra).getRequest();
        // getting http session;
        return SessionInfo.getInstance(request);
    }

    /**
     * @return EJB Home class;
     */
    protected abstract Class getEJBHomeClass();
}

No need to say about benefits of object which is defined as bean in spring context(Im talking about lookup code placed in spring config xml in a form of FactoryBean(s)). One significant - ability to attach remote exception handler(on an object being created by ProxyFactoryBean) and there by get rid of hundreds of MyBaseAction.handleRemoteException() calls in web tier code, making it unaware of actual business logic implementation(being it either EJB or POJO) which is great thing!

Thanks for your time!