最近遇到的問題是在service獲取request和response,正常來說在service層是沒有request的,然而直接從controlller傳過來的話解決方法太粗暴,後來發現了SpringMVC提供的RequestContextHolder遂去分析一番,並藉此對SpringMVC的結構深入瞭解一下,後面會再發文章詳細分析源碼
1.RequestContextHolder的使用
RequestContextHolder顧名思義,持有上下文的Request容器.使用是很簡單的,具體使用如下:
//兩個方法在沒有使用JSF的項目中是沒有區別的
RequestAttributes requestAttributes = RequestContextHolder.currentRequestAttributes();
// RequestContextHolder.getRequestAttributes();
//從session裏面獲取對應的值
String str = (String) requestAttributes.getAttribute("name",RequestAttributes.SCOPE_SESSION);
HttpServletRequest request = ((ServletRequestAttributes)requestAttributes).getRequest();
HttpServletResponse response = ((ServletRequestAttributes)requestAttributes).getResponse();
看到這一般都會想到幾個問題:
- request和response怎麼和當前請求掛鉤?
- request和response等是什麼時候設置進去的?
2.解決疑問
2.1 request和response怎麼和當前請求掛鉤?
首先分析RequestContextHolder這個類,裏面有兩個ThreadLocal保存當前線程下的request,關於ThreadLocal可以參考我的另一篇博文[Java學習記錄--ThreadLocal使用案例]
//得到存儲進去的request
private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
new NamedThreadLocal<RequestAttributes>("Request attributes");
//可被子線程繼承的request
private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
new NamedInheritableThreadLocal<RequestAttributes>("Request context");
再看`getRequestAttributes()`方法,相當於直接獲取ThreadLocal裏面的值,這樣就保證了每一次獲取到的Request是該請求的request.
public static RequestAttributes getRequestAttributes() {
RequestAttributes attributes = requestAttributesHolder.get();
if (attributes == null) {
attributes = inheritableRequestAttributesHolder.get();
}
return attributes;
}
2.2request和response等是什麼時候設置進去的?
找這個的話需要對springMVC結構的`DispatcherServlet`的結構有一定了解才能準確的定位該去哪裏找相關代碼.
在IDEA中會顯示如下的繼承關係.
左邊1這裏是Servlet的接口和實現類.
右邊2這裏是使得SpringMVC具有Spring的一些環境變量和Spring容器.類似的XXXAware接口就是對該類提供Spring感知,簡單來說就是如果想使用Spring的XXXX就要實現XXXAware,spring會把需要的東西傳送過來.
那麼剩下要分析的的就是三個類,簡單看下源碼
1. HttpServletBean 進行初始化工作
2. FrameworkServlet 初始化 WebApplicationContext,並提供service方法預處理請
3. DispatcherServlet 具體分發處理.
那麼就可以在FrameworkServlet查看到該類重寫了service(),doGet(),doPost()...等方法,這些實現裏面都有一個預處理方法`processRequest(request, response);`,所以定位到了我們要找的位置
查看`processRequest(request, response);`的實現,具體可以分爲三步:
- 獲取上一個請求的參數
- 重新建立新的參數
- 設置到XXContextHolder
- 父類的service()處理請求
- 恢復request
- 發佈事件
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
//獲取上一個請求保存的LocaleContext
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
//建立新的LocaleContext
LocaleContext localeContext = buildLocaleContext(request);
//獲取上一個請求保存的RequestAttributes
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
//建立新的RequestAttributes
ServletRequestAttributes requestAttributes = buildRequestAttributes(request,
response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(),
new RequestBindingInterceptor());
//具體設置的方法
initContextHolders(request, localeContext, requestAttributes);
try {
doService(request, response);
}
catch (ServletException ex) {
failureCause = ex;
throw ex;
}
catch (IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
//恢復
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
if (logger.isDebugEnabled()) {
if (failureCause != null) {
this.logger.debug("Could not complete request", failureCause);
}
else {
if (asyncManager.isConcurrentHandlingStarted()) {
logger.debug("Leaving response open for concurrent processing");
}
else {
this.logger.debug("Successfully completed request");
}
}
}
//發佈事件
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
再看initContextHolders(request, localeContext, requestAttributes)方法,把新的RequestAttributes設置進LocalThread,實際上保存的類型爲ServletRequestAttributes,這也是爲什麼在使用的時候可以把RequestAttributes強轉爲ServletRequestAttributes.
private void initContextHolders(HttpServletRequest request,
LocaleContext localeContext,
RequestAttributes requestAttributes) {
if (localeContext != null) {
LocaleContextHolder.setLocaleContext(localeContext,
this.threadContextInheritable);
}
if (requestAttributes != null) {
RequestContextHolder.setRequestAttributes(requestAttributes,
this.threadContextInheritable);
}
if (logger.isTraceEnabled()) {
logger.trace("Bound request context to thread: " + request);
}
}
因此RequestContextHolder裏面最終保存的爲ServletRequestAttributes,這個類相比`RequestAttributes`方法是多了很多.