RequestContextHolder分析

在spring mvc中，为了随时都能取到当前请求的request对象，可以通过RequestContextHolder的静态方法getRequestAttributes()获取Request相关的变量，如request, response等

1.RequestContextHolder的使用

RequestContextHolder顾名思义,持有上下文的Request容器.使用是很简单的,具体使用如下:

//两个方法在没有使用JSF的项目中是没有区别的
RequestAttributes requestAttributes = RequestContextHolder.currentRequestAttributes();
//                                            RequestContextHolder.getRequestAttributes();
//从session里面获取对应的值
String str = (String) requestAttributes.getAttribute("name",RequestAttributes.SCOPE_SESSION);

HttpServletRequest request = ((ServletRequestAttributes)requestAttributes).getRequest();
HttpServletResponse response = ((ServletRequestAttributes)requestAttributes).getResponse();

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看到这一般都会想到几个问题: 

1. request和response怎么和当前请求挂钩? 

2. request和response等是什么时候设置进去的?

2.解决疑问

2.1 request和response怎么和当前请求挂钩?

首先分析RequestContextHolder这个类,里面有两个ThreadLocal保存当前线程下的request,关于ThreadLocal可以参考我的另一篇博文Java学习记录–ThreadLocal使用案例

//得到存储进去的request
private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
new NamedThreadLocal<RequestAttributes>("Request attributes");
//可被子线程继承的request
private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
new NamedInheritableThreadLocal<RequestAttributes>("Request context");

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再看

getRequestAttributes()

方法,相当于直接获取ThreadLocal里面的值,这样就保证了每一次获取到的Request是该请求的request.

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

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2.2request和response等是什么时候设置进去的?

找这个的话需要对springMVC结构的

DispatcherServlet

的结构有一定了解才能准确的定位该去哪里找相关代码. 

在IDEA中会显示如下的继承关系.



左边1这里是Servlet的接口和实现类. 

右边2这里是使得SpringMVC具有Spring的一些环境变量和Spring容器.类似的XXXAware接口就是对该类提供Spring感知,简单来说就是如果想使用Spring的XXXX就要实现XXXAware,spring会把需要的东西传送过来.

那么剩下要分析的的就是三个类,简单看下源码 

1. HttpServletBean 进行初始化工作 

2. FrameworkServlet 初始化 WebApplicationContext,并提供service方法预处理请求 

3. DispatcherServlet 具体分发处理.

那么就可以在FrameworkServlet查看到该类重写了service(),doGet(),doPost()…等方法,这些实现里面都有一个预处理方法

processRequest(request,
response);

,所以定位到了我们要找的位置

查看

processRequest(request, response);

的实现,具体可以分为三步: 

1. 获取上一个请求的参数 

2. 重新建立新的参数 

3. 设置到XXContextHolder 

4. 父类的service()处理请求 

5. 恢复request 

6. 发布事件

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

long startTime = System.currentTimeMillis();
Throwable failureCause = null;
//获取上一个请求保存的LocaleContext
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
//建立新的LocaleContext
LocaleContext localeContext = buildLocaleContext(request);
//获取上一个请求保存的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());
//具体设置的方法
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 {
//恢复
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");
}
}
}
//发布事件
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}

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再看

initContextHolders(request, localeContext, requestAttributes)

方法,把新的RequestAttributes设置进LocalThread,实际上保存的类型为

ServletRequestAttributes

,这也是为什么在使用的时候可以把RequestAttributes强转为

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);
}
}

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因此RequestContextHolder里面最终保存的为

ServletRequestAttributes

,这个类相比

RequestAttributes

方法是多了很多.