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();

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");

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;
    }

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);
        }
    }

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.