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SpringMVC之RequestContextHolder詳細(xì)解析

更新時(shí)間：2023年11月28日 10:35:34 作者：ZhaoJuFei

這篇文章主要介紹了SpringMVC之RequestContextHolder詳細(xì)解析,正常來(lái)說(shuō)在service層是沒(méi)有request的,然而直接從controlller傳過(guò)來(lái)的話(huà)解決方法太粗暴,后來(lái)發(fā)現(xiàn)了SpringMVC提供的RequestContextHolder,需要的朋友可以參考下

前言

最近遇到的問(wèn)題是在service獲取request和response,正常來(lái)說(shuō)在service層是沒(méi)有request的,然而直接從controlller傳過(guò)來(lái)的話(huà)解決方法太粗暴,后來(lái)發(fā)現(xiàn)了SpringMVC提供的RequestContextHolder遂去分析一番,并借此對(duì)SpringMVC的結(jié)構(gòu)深入了解一下,后面會(huì)再發(fā)文章詳細(xì)分析源碼。

ps:正常情況下，service應(yīng)該是無(wú)狀態(tài)的，這個(gè)RequestContextHolder可以在一些工具類(lèi)里使用。

1.RequestContextHolder的使用

RequestContextHolder顧名思義,持有上下文的Request容器.使用是很簡(jiǎn)單的,具體使用如下:

//兩個(gè)方法在沒(méi)有使用JSF的項(xiàng)目中是沒(méi)有區(qū)別的
RequestAttributes requestAttributes = RequestContextHolder.currentRequestAttributes();
//RequestContextHolder.getRequestAttributes();
//從session里面獲取對(duì)應(yīng)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(huì)想到幾個(gè)問(wèn)題:

request和response怎么和當(dāng)前請(qǐng)求掛鉤?
request和response等是什么時(shí)候設(shè)置進(jìn)去的?

2.解決疑問(wèn)

2.1 request和response怎么和當(dāng)前請(qǐng)求掛鉤?

首先分析RequestContextHolder這個(gè)類(lèi),里面有兩個(gè)ThreadLocal保存當(dāng)前線(xiàn)程下的request。

// 得到存儲(chǔ)進(jìn)去的request
private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
new NamedThreadLocal<RequestAttributes>("Request attributes");
// 可被子線(xiàn)程繼承的request
private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
new NamedInheritableThreadLocal<RequestAttributes>("Request context");

再看`getRequestAttributes()`方法,相當(dāng)于直接獲取ThreadLocal里面的值,這樣就保證了每一次獲取到的Request是該請(qǐng)求的request.

public static RequestAttributes getRequestAttributes() {
        RequestAttributes attributes = requestAttributesHolder.get();
        if (attributes == null) {
            attributes = inheritableRequestAttributesHolder.get();
        }
        return attributes;
    }

2.2request和response等是什么時(shí)候設(shè)置進(jìn)去的?

找這個(gè)的話(huà)需要對(duì)springMVC結(jié)構(gòu)的`DispatcherServlet`的結(jié)構(gòu)有一定了解才能準(zhǔn)確的定位該去哪里找相關(guān)代碼.

在IDEA中會(huì)顯示如下的繼承關(guān)系.

左邊1這里是Servlet的接口和實(shí)現(xiàn)類(lèi).

右邊2這里是使得SpringMVC具有Spring的一些環(huán)境變量和Spring容器.類(lèi)似的XXXAware接口就是對(duì)該類(lèi)提供Spring感知,簡(jiǎn)單來(lái)說(shuō)就是如果想使用Spring的XXXX就要實(shí)現(xiàn)XXXAware,spring會(huì)把需要的東西傳送過(guò)來(lái).

那么剩下要分析的的就是三個(gè)類(lèi),簡(jiǎn)單看下源碼

1. HttpServletBean 進(jìn)行初始化工作

2. FrameworkServlet 初始化 WebApplicationContext,并提供service方法預(yù)處理請(qǐng)

3. DispatcherServlet 具體分發(fā)處理.

那么就可以在FrameworkServlet查看到該類(lèi)重寫(xiě)了service(),doGet(),doPost()...等方法,這些實(shí)現(xiàn)里面都有一個(gè)預(yù)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(shí)現(xiàn),具體可以分為三步:

獲取上一個(gè)請(qǐng)求的參數(shù)
重新建立新的參數(shù)
設(shè)置到XXContextHolder
父類(lèi)的service()處理請(qǐng)求
恢復(fù)request
發(fā)布事

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
//獲取上一個(gè)請(qǐng)求保存的LocaleContext
    LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
//建立新的LocaleContext
    LocaleContext localeContext = buildLocaleContext(request);
//獲取上一個(gè)請(qǐng)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(shè)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(fù)
        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(fā)布事件
        publishRequestHandledEvent(request, response, startTime, failureCause);
    }
}

再看initContextHolders(request, localeContext, requestAttributes)方法,把新的RequestAttributes設(shè)置進(jìn)LocalThread,實(shí)際上保存的類(lèi)型為ServletRequestAttributes,這也是為什么在使用的時(shí)候可以把RequestAttributes強(qiáng)轉(zhuǎn)為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里面最終保存的為ServletRequestAttributes,這個(gè)類(lèi)相比`RequestAttributes`方法是多了很多

到此這篇關(guān)于SpringMVC之RequestContextHolder詳細(xì)解析的文章就介紹到這了,更多相關(guān)RequestContextHolder詳細(xì)解析內(nèi)容請(qǐng)搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！

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