SpringMVC之RequestContextHolder分析_requestcontextholder.currentrequestattributes()

1. web应用环境相关的Bean作用域

如果用户使用spring的webApplicationContext,则可以使用另外3种Bean的作用域:request,session和globalSession.不过在使用这些作用域之前,首先必须在web容器中进行一些额外的配置,在高版本的web容器中,则可以利用HTTP请求监听器进行配置:

<web-app>
...
<listener>
<listener-class>
org.springframework.web.context.request.RequestContextListener
</listener-class>
</listener>
...
</web-app>
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细心的朋友可能有一个疑问:在介绍webApplicationContext初始化时,我们已经通过ContextLoaderListener将web容器与spring容器整合,为什么这里又要引入一个额外的RequestContextListener以支持Bean的另外3个作用域呢?

在整合spring容器时使用ContextLoaderListener,它实现了ServletContextListener监听器接口,ServletContextListener只负责监听web容器启动和关闭的事件.而RequestContextListener实现ServletRequestListener监听器接口,该监听器监听HTTP请求事件,web服务器接收的每一次请求都会通知该监听器.spring容器启动和关闭操作由web容器的启动和关闭事件触发,但如果spring容器中的Bean需要request,session,globalsession作用域的支持,spring容器本身就必须获得web容器的HTTP请求事件,以HTTP请求的事件"驱动"Bean作用域的控制逻辑。

2. 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: request和response怎么和当前请求挂钩? request和response等是什么时候设置进去的?

3. 解决疑问

• 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
  ThreadLocal 参考链接https://www.cnblogs.com/shuilangyizu/p/8621733.html

//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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• request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.
  idea Ctrl + H 查看类的层次结构 eclipse F4 或者 Ctrl + T 查看类的层次结构
  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
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:
   获取上一个请求的参数
   重新建立新的参数
   设置到XXContextHolder
   父类的service()处理请求
   恢复request
   发布事

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.


ServletRequestAttributes继承AbstractRequestAttributes，AbstractRequestAttributes实现了RequestAttributes