spring mvc redirect

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter implements BeanFactoryAware,
		InitializingBean {

.....
protected final ModelAndView handleInternal(HttpServletRequest request,
												HttpServletResponse response,
												HandlerMethod handlerMethod) throws Exception {

		if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
			// Always prevent caching in case of session attribute management.
			checkAndPrepare(request, response, this.cacheSecondsForSessionAttributeHandlers, true);
		}
		else {
			// Uses configured default cacheSeconds setting.
			checkAndPrepare(request, response, true);
		}
		
		// Execute invokeHandlerMethod in synchronized block if required.
		if (this.synchronizeOnSession) {
			HttpSession session = request.getSession(false);
			if (session != null) {
				Object mutex = WebUtils.getSessionMutex(session);
				synchronized (mutex) {
					return invokeHandlerMethod(request, response, handlerMethod);
				}
			}
		}
		
		return invokeHandlerMethod(request, response, handlerMethod);
	}


private ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response,
			HandlerMethod handlerMethod) throws Exception {
		
		ServletWebRequest webRequest = new ServletWebRequest(request, response);

		WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
		ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
		ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);

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

		requestMappingMethod.invokeAndHandle(webRequest, mavContainer);
		modelFactory.updateModel(webRequest, mavContainer);

		if (mavContainer.isRequestHandled()) {
			return null;
		}
		else {
			ModelMap model = mavContainer.getModel();
			ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model);
			if (!mavContainer.isViewReference()) {
				mav.setView((View) mavContainer.getView());
			}
			if (model instanceof RedirectAttributes) {
				Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
				RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
			}
			return mav;
		}
	}


.....

}

public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {

.......

public final void invokeAndHandle(
			NativeWebRequest request, ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {

		Object returnValue = invokeForRequest(request, mavContainer, providedArgs);

		setResponseStatus((ServletWebRequest) request);

		if (returnValue == null) {
			if (isRequestNotModified(request) || hasResponseStatus() || mavContainer.isRequestHandled()) {
				mavContainer.setRequestHandled(true);
				return;
			}
		}

		mavContainer.setRequestHandled(false);

		try {
			returnValueHandlers.handleReturnValue(returnValue, getReturnType(), mavContainer, request);
		} catch (Exception ex) {
			if (logger.isTraceEnabled()) {
				logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
			}
			throw ex;
		}
	}


.......

}

public class HandlerMethodReturnValueHandlerComposite implements HandlerMethodReturnValueHandler {

...

public void handleReturnValue(
			Object returnValue, MethodParameter returnType,
			ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
			throws Exception {

		HandlerMethodReturnValueHandler handler = getReturnValueHandler(returnType);
		Assert.notNull(handler, "Unknown return value type [" + returnType.getParameterType().getName() + "]");
		handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
	}

...

}

public class ViewNameMethodReturnValueHandler implements HandlerMethodReturnValueHandler {

...

public void handleReturnValue(
			Object returnValue, MethodParameter returnType,
			ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
			throws Exception {

		if (returnValue == null) {
			return;
		}
		else if (returnValue instanceof String) {
			String viewName = (String) returnValue;
			mavContainer.setViewName(viewName);
			if (isRedirectViewName(viewName)) {
				mavContainer.setRedirectModelScenario(true);
			}
		}
		else {
			// should not happen
			throw new UnsupportedOperationException("Unexpected return type: " +
					returnType.getParameterType().getName() + " in method: " + returnType.getMethod());
		}
	}
	
	/**
	 * Whether the given view name is a redirect view reference.
	 * @param viewName the view name to check, never {@code null}
	 * @return "true" if the given view name is recognized as a redirect view
	 * reference; "false" otherwise.
	 */
	protected boolean isRedirectViewName(String viewName) {
		return viewName.startsWith("redirect:");
	}
...
}