SpringMVC学习记录(九)–RequestContextHolder分析

标签（空格分隔）： javaWEB

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最近遇到的问题是在service获取request和response,正常来说在service层是没有request的,然而直接从controlller传过来的话解决方法太粗暴,后来发现了SpringMVC提供的RequestContextHolder遂去分析一番,并借此对SpringMVC的结构深入了解一下,后面会再发文章详细分析源码

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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.

项目示例可以参考:
SSM框架整合: https://github.com/nl101531/JavaWEB