SpringMVC之RequestContextHolder分析

╰半橙微兮° 2022-05-26 03:29 183阅读 0赞

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\[[Java学习记录--ThreadLocal使用案例][Java_--ThreadLocal]\]

//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.

![v2-e4bbd502427ab89db852b438644793f9_b.jpg][]

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.

![v2-776dae1be28205c2e2e53bb0019941d8_b.jpg][]

项目示例可以参考:

SSM框架整合: [nl101531/JavaWEB][nl101531_JavaWEB]

转自https://zhuanlan.zhihu.com/p/24293062

[Java_--ThreadLocal]: https://link.zhihu.com/?target=http%3A//www.jianshu.com/p/5675690b351e
[v2-e4bbd502427ab89db852b438644793f9_b.jpg]: /images/20220526/ad15a7b23b894b8880bb7dc075a80047.png
[v2-776dae1be28205c2e2e53bb0019941d8_b.jpg]: /images/20220526/a63181a3ef6840a3b84d958900b8323e.png
[nl101531_JavaWEB]: https://link.zhihu.com/?target=https%3A//github.com/nl101531/JavaWEB