Monday, March 29, 2010

My attempt at Spring MVC custom annotations

While working on a simple project using SpringMVC, I stumbled upon the need to access a request or session attribute directly from within a controller method. I needed to bypass the binder and just access a request attribute set by a filter. This is obviously easy enough to do:

public String view(WebRequest request){
request.getAttribute("someAttribute", WebRequest.SCOPE_REQUEST);

Given modern mocking frameworks like Mockito and spring's own test libraries, it's easy enough to mock a WebRequest or even an HttpServletRequest. But I am pedantic, so I decided to see if I could create my own @RequestAttribute annotation instead! Ideally, I figured I'd be able to create a new annotation representing new behaviour without touching any of the spring source code.

An initial cursory look over AnnotationMethodHandlerAdapter did not reveal any clean hooks, so I decided to go with an aspect.

First Approach - Aspects

At first, I tried creating a pointcut around HandlerMethodInvoker#resolveHandlerArguments. At this point I discussed my plan with coworker and friend Martin Snyman, who quickly pointed out that it might be simpler to write the pointcut to intercept @RequestMapping annotated controller classes instead.

Starting out, my original pointcut looked like this:

@Around("execution(@org.springframework.web.bind.annotation.RequestMapping * *(..))")

This worked well enough, but it created unncessary proxies. The pointcut picked up the execution of all methods annotated with @RequestMapping - or worse.

What I really needed was to pick up execution of all methods annotated with @RequestMapping that had at least one parameter annotated with my special annotation - "@RequestAttribute". This turned out to be difficult. I could not for the life of me figure out how to write a pointcut that picked up annotated parameters rather than matching annotations on the actual classes of passed parameters. After a couple of stabs during the weekend, I found a blog post that discussed the issue I was facing in some detail. With that, I was able to write the following pointcut:

@Around("execution(@org.springframework.web.bind.annotation.RequestMapping * *.*(..,@o.c.RequestAttribute (*),..))")

I wrote a supporting interface to allow my aspect to support any number of annotations:

public interface ArgumentModifyingAnnotationBehavior<T extends Annotation> {
public Object getArgumentValue(Object argumentValue, T annotation);

The actual aspect then looked something like this:

protected final Map<Class<Annotation>, ArgumentModifyingAnnotationBehavior<Annotation>> annotationRegistry = new ConcurrentHashMap<Class<Annotation>, ArgumentModifyingAnnotationBehavior<Annotation>>();
@Around("execution(@org.springframework.web.bind.annotation.RequestMapping * *.*(..,@o.c.RequestAttribute (*),..))")
private Object resolveHandlerArguments(ProceedingJoinPoint pjp)
throws Throwable {
MethodSignature sig = (MethodSignature) pjp.getSignature();
Annotation[][] annotations = sig.getMethod().getParameterAnnotations();
Object[] arguments = pjp.getArgs();
if (annotations != null) {
/* iterate arguments */
for (int argumentIndex = 0; argumentIndex < annotations.length; argumentIndex++) {
Annotation[] argumentAnnotations = annotations[argumentIndex];
if (argumentAnnotations != null) {
for (int annoationIndex = 0; annoationIndex < argumentAnnotations.length; annoationIndex++) {
Annotation argumentAnnotation = argumentAnnotations[annoationIndex];
ArgumentModifyingAnnotationBehavior behavior = annotationRegistry
if (behavior != null) {
arguments[argumentIndex] = behavior
return pjp.proceed(arguments);

I was very proud of myself. Then disaster struck...

public String view(@RequestAttribute("stringAttribute") String stringAttribute){...

worked like a champ.

public String view(@RequestAttribute("objectAttribute") java.util.Map objectAttribute){...

however, grenaded. My annotation worked great with concrete types but blew up when it was used to annotate interfaces or abstract classes.

Second Approach - WebArgumentResolvers

Debugging revealed that HandlerMethodInvoker was attempting to resolve "objectAttribute" to a model attribute and actually attempted to instantiate java.util.Map.

Debugging also revealed the curious new WebArgumentResolver class. By this point, I've spent hours of my time writing and debugging my awesome aspect only to find that HandlerMethodInvoker actually *did* have a hook for allowing custom behaviour using a WebArgumentResolvers!

A little bit more digging revealed that one can actually register any number of WebArgumentResolvers with the AnnotationMethodHandlerAdapter, which would pass these on through to the HandlerMethodInvoker

After 8+ hours of screwing around with my Aspect, I found I could actually achieve what I needed to do without the creation of any proxies with this:

public class RequestAttributeCustomArgumentResolver implements WebArgumentResolver {
public static @interface RequestAttribute {
String value() default "";
public Object resolveArgument(MethodParameter methodParameter,
NativeWebRequest webRequest) throws Exception {
RequestAttribute annotation=methodParameter.getParameterAnnotation(RequestAttribute.class);
return webRequest.getAttribute(annotation.value(),WebRequest.SCOPE_REQUEST);

and the following in the applciation context XML:

<bean id="handlerAdapter"
<property name="customArgumentResolvers">
<bean class="org.campuseai.spring.webmvc.annotation.RequestAttributeCustomArgumentResolver"/>

Ouch. Googling "WebArgumentResolver" also resulted in this blog post which does something similar to what I attempted to do.