Spring MVC: How controller been called

When Spring MVC calls user defined controller, following works also performs
1. passes data from request as parameters to method of controller
2. binds data from request to business object
3. converts controller method return value to response

#Strategy interface

Classes with interface *org.springframework.web.method.support.HandlerMethodArgumentResolver* for achieving work 1 and 2.

```java
public interface HandlerMethodArgumentResolver {

	boolean supportsParameter(MethodParameter parameter);

	@Nullable
	Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception;

}
```

For example class *org.springframework.web.servlet.mvc.method.annotation.PathVariableMethodArgumentResolver* responds for process method parameters annotated with *org.springframework.web.bind.annotation.PathVariable*. And class *org.springframework.web.method.annotation.ModelAttributeMethodProcessor* responds for process method parameters annotated with *org.springframework.web.bind.annotation.ModelAttribute*.

Classes with interface *org.springframework.web.method.support.HandlerMethodReturnValueHandler* for achieving work 3.

```java
public interface HandlerMethodReturnValueHandler {

	boolean supportsReturnType(MethodParameter returnType);

	void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;

}
```

For example class *org.springframework.web.servlet.mvc.method.annotation.ViewMethodReturnValueHandler* responds for process method return of class with *org.springframework.web.servlet.View* interface.

#Data binding and parameter selection

Method **org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(NativeWebRequest, ModelAndViewContainer, Object...)** performs actual work.

```java
	protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {
	// ** providedArgs always null when being called **

		MethodParameter[] parameters = getMethodParameters();
		if (ObjectUtils.isEmpty(parameters)) {
			return EMPTY_ARGS;
		}

		Object[] args = new Object[parameters.length];
		for (int i = 0; i < parameters.length; i++) {
			MethodParameter parameter = parameters[i];
			parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
			args[i] = findProvidedArgument(parameter, providedArgs);
			if (args[i] != null) {
				continue;
			}
			if (!this.resolvers.supportsParameter(parameter)) {
				throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
			}
			try {
				// ** performs look up at here **
				args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
			}
			catch (Exception ex) {
				// Leave stack trace for later, exception may actually be resolved and handled...
				if (logger.isDebugEnabled()) {
					String exMsg = ex.getMessage();
					if (exMsg != null && !exMsg.contains(parameter.getExecutable().toGenericString())) {
						logger.debug(formatArgumentError(parameter, exMsg));
					}
				}
				throw ex;
			}
		}
		return args;
	}
```

Method *org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(HttpServletRequest, HttpServletResponse, HandlerMethod)* instantiating *org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod* class (which is the son of *org.springframework.web.method.support.InvocableHandlerMethod*), setting resolver and processor instances and invokes it.

```java
	/**
	 * Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView}
	 * if view resolution is required.
	 * @since 4.2
	 * @see #createInvocableHandlerMethod(HandlerMethod)
	 */
	@Nullable
	protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

		ServletWebRequest webRequest = new ServletWebRequest(request, response);
		try {
			WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
			ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

			ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
			if (this.argumentResolvers != null) {
				invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
			}
			if (this.returnValueHandlers != null) {
				invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
			}
			invocableMethod.setDataBinderFactory(binderFactory);
			invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

			ModelAndViewContainer mavContainer = new ModelAndViewContainer();
			mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
			modelFactory.initModel(webRequest, mavContainer, invocableMethod);
			mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

			AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
			asyncWebRequest.setTimeout(this.asyncRequestTimeout);

			WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
			asyncManager.setTaskExecutor(this.taskExecutor);
			asyncManager.setAsyncWebRequest(asyncWebRequest);
			asyncManager.registerCallableInterceptors(this.callableInterceptors);
			asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

			if (asyncManager.hasConcurrentResult()) {
				Object result = asyncManager.getConcurrentResult();
				mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
				asyncManager.clearConcurrentResult();
				LogFormatUtils.traceDebug(logger, traceOn -> {
					String formatted = LogFormatUtils.formatValue(result, !traceOn);
					return "Resume with async result [" + formatted + "]";
				});
				invocableMethod = invocableMethod.wrapConcurrentResult(result);
			}

			// ** invokes **
			invocableMethod.invokeAndHandle(webRequest, mavContainer);
			if (asyncManager.isConcurrentHandlingStarted()) {
				return null;
			}

			return getModelAndView(mavContainer, modelFactory, webRequest);
		}
		finally {
			webRequest.requestCompleted();
		}
	}
```

# Method return

Method return is processed on *org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletWebRequest, ModelAndViewContainer, Object...)*.

```java
	/**
	 * Invoke the method and handle the return value through one of the
	 * configured {@link HandlerMethodReturnValueHandler HandlerMethodReturnValueHandlers}.
	 * @param webRequest the current request
	 * @param mavContainer the ModelAndViewContainer for this request
	 * @param providedArgs "given" arguments matched by type (not resolved)
	 */
	public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {

		Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
		setResponseStatus(webRequest);

		if (returnValue == null) {
			if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
				disableContentCachingIfNecessary(webRequest);
				mavContainer.setRequestHandled(true);
				return;
			}
		}
		else if (StringUtils.hasText(getResponseStatusReason())) {
			mavContainer.setRequestHandled(true);
			return;
		}

		mavContainer.setRequestHandled(false);
		Assert.state(this.returnValueHandlers != null, "No return value handlers");
		try {
			// ** actual handling location **
			this.returnValueHandlers.handleReturnValue(
					returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
		}
		catch (Exception ex) {
			if (logger.isTraceEnabled()) {
				logger.trace(formatErrorForReturnValue(returnValue), ex);
			}
			throw ex;
		}
	}
```