SpringMVC中的HTTP跳轉
項目開發中經常會碰到需要進行HTTP跳轉的場景，比如用戶請求一個需要登錄之後纔可以看到的頁面，而此時需要跳轉到登錄頁面，待登錄成功之後在跳轉會現在的頁面。那麼SpringMVC是怎樣實現這樣的跳轉的呢？今天就讓我們仔細的研究一下。
一，Servlet中forward與redirect
在原生的Servlet技術中有兩種跳轉方式，分別爲forward和redirect。它們有着各自的特點和用途
1，forward重定向是在容器內部實現的同一個Web應用程序的重定向，所以forward方法只能重定向到同一個Web應用程序中的一個資源，redirect方法可以重定向到任何URL。
2，forward重定向後瀏覽器地址欄URL不變，redirect重定向後瀏覽器地址欄URL改變。
3，forward可以將請求中包含的數據傳遞到跳轉後的地址。redirect則不能，如果想傳遞參數，只能放在請求行中。
3，forward可以將請求中包含的數據傳遞到跳轉後的地址。redirect則不能，如果想傳遞參數，只能放在請求行中。
4，對於客戶端來說，forward重定向的過程不可見。而redirect則需要客戶端配合，再次請求跳轉後的地址。
二，301與302跳轉
二，301與302跳轉
做服務端的不能不知道301和302跳轉，這裏的301、302指的是HTTP的響應的狀態碼。301的意思是原地址永久性的替換爲新地址，302指當前的地址只是暫時的替換爲新地址。它們實現的結果都是一樣的，瀏覽器根據返回的新地址再次發起請求。但是，對於搜索引擎來說則意義大不相同，懂得SEO的人都會慎用301與302跳轉。最後，這裏說的跳轉所指的都是上文說的redirect。
三，spring中redirect的實現
在spring中，當我們想進行redirect跳轉的時候可以讓Controller的方法返回“redirect：”開頭的字符串，或者直接返回RedirectView。
若Controller方法返回的是“redirect：”開頭的字符串（只要返回類型是String，都是同一個結果處理器），則對應的結果處理器是ViewNameMethodReturnValueHandler，來看看它是如何發現並處理這個跳轉結果的：
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());
}
} public void handleReturnValue(
Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws Exception {
if (returnValue == null) {
return;
}
else if (returnValue instanceof View){
View view = (View) returnValue;
mavContainer.setView(view);
if (view instanceof SmartView) {
if (((SmartView) view).isRedirectView()) {
mavContainer.setRedirectModelScenario(true);
}
}
}
else {
// should not happen
throw new UnsupportedOperationException("Unexpected return type: " +
returnType.getParameterType().getName() + " in method: " + returnType.getMethod());
}
} protected void renderMergedOutputModel(
Map<String, Object> model, HttpServletRequest request, HttpServletResponse response)
throws IOException {
String targetUrl = createTargetUrl(model, request);
targetUrl = updateTargetUrl(targetUrl, model, request, response);
FlashMap flashMap = RequestContextUtils.getOutputFlashMap(request);
if (!CollectionUtils.isEmpty(flashMap)) {
UriComponents uriComponents = UriComponentsBuilder.fromUriString(targetUrl).build();
flashMap.setTargetRequestPath(uriComponents.getPath());
flashMap.addTargetRequestParams(uriComponents.getQueryParams());
}
FlashMapManager flashMapManager = RequestContextUtils.getFlashMapManager(request);
flashMapManager.saveOutputFlashMap(flashMap, request, response);
sendRedirect(request, response, targetUrl.toString(), this.http10Compatible);
} renderMergedOutputModel最後悔調用sendRedirect來設置response。
protected void sendRedirect(
HttpServletRequest request, HttpServletResponse response, String targetUrl, boolean http10Compatible)
throws IOException {
String encodedRedirectURL = response.encodeRedirectURL(targetUrl);
if (http10Compatible) {
if (this.statusCode != null) {
response.setStatus(this.statusCode.value());
response.setHeader("Location", encodedRedirectURL);
}
else {
// Send status code 302 by default.
response.sendRedirect(encodedRedirectURL);
}
}
else {
HttpStatus statusCode = getHttp11StatusCode(request, response, targetUrl);
response.setStatus(statusCode.value());
response.setHeader("Location", encodedRedirectURL);
}
}四，spring對forward調整的處理
在spring中，可以通過返回以“forward:”開頭的字符串的方式實現forward調整。在獲取view之前，它與返回redirect字符串的處理過程都是一樣的。而它們對應的View類型不同，上面說到處理redirect的view是RedirectView，而處理forward的view是InternalResourceView。導致它們不同原因是UrlBasedViewResolver創建View時做了區分，如下：
protected View createView(String viewName, Locale locale) throws Exception {
// If this resolver is not supposed to handle the given view,
// return null to pass on to the next resolver in the chain.
if (!canHandle(viewName, locale)) {
return null;
}
// Check for special "redirect:" prefix.
if (viewName.startsWith(REDIRECT_URL_PREFIX)) {
String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());
return applyLifecycleMethods(viewName, view);
}
// Check for special "forward:" prefix.
if (viewName.startsWith(FORWARD_URL_PREFIX)) {
String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
return new InternalResourceView(forwardUrl);
}
// Else fall back to superclass implementation: calling loadView.
return super.createView(viewName, locale);
} 從InternalResourceView的名字可以直觀的看出，forward是要獲取內部資源。來看看它的renderMergedOutputModel方法：
protected void renderMergedOutputModel(
Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
// Determine which request handle to expose to the RequestDispatcher.
HttpServletRequest requestToExpose = getRequestToExpose(request);
// Expose the model object as request attributes.
exposeModelAsRequestAttributes(model, requestToExpose);
// Expose helpers as request attributes, if any.
exposeHelpers(requestToExpose);
// Determine the path for the request dispatcher.
String dispatcherPath = prepareForRendering(requestToExpose, response);
// Obtain a RequestDispatcher for the target resource (typically a JSP).
RequestDispatcher rd = getRequestDispatcher(requestToExpose, dispatcherPath);
if (rd == null) {
throw new ServletException("Could not get RequestDispatcher for [" + getUrl() +
"]: Check that the corresponding file exists within your web application archive!");
}
// If already included or response already committed, perform include, else forward.
if (useInclude(requestToExpose, response)) {
response.setContentType(getContentType());
if (logger.isDebugEnabled()) {
logger.debug("Including resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
}
rd.include(requestToExpose, response);
}
else {
// Note: The forwarded resource is supposed to determine the content type itself.
exposeForwardRequestAttributes(requestToExpose);
if (logger.isDebugEnabled()) {
logger.debug("Forwarding to resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
}
rd.forward(requestToExpose, response);
}
}上面的方法先獲取一個RequestDispatcher，然後通過它將請求轉發到了新的路徑上。這些都是利用Servlet的標準，具體內部的實現可以在tomcat的源碼中查找。
注：本文所述都是基於spring3.1.2的默認環境
