SpringMVC的RequestContextHolder详解分析

作者：本科IT人
链接：https://zhuanlan.zhihu.com/p/24293062
来源：知乎
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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

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

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);
    }
}

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: nl101531/JavaWEB