SpringMVC中RequestContextHolder获取请求信息的方法
RequestContextHolder的作用是:
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在Service层获取获取request和response信息
代码示例:
ServletRequestAttributes attrs = (ServletRequestAttributes)RequestContextHolder.getRequestAttributes(); HttpServletRequest request = attrs.getRequest();
源码分析:
定义了两个ThreadLocal变量用来存储Request
private static final ThreadLocalrequestAttributesHolder = new NamedThreadLocal("Request attributes"); private static final ThreadLocal inheritableRequestAttributesHolder = new NamedInheritableThreadLocal("Request context");
设置方法
public static void setRequestAttributes(@Nullable RequestAttributes attributes) { setRequestAttributes(attributes, false); } public static void setRequestAttributes(@Nullable RequestAttributes attributes, boolean inheritable) { if (attributes == null) { resetRequestAttributes(); } else if (inheritable) { inheritableRequestAttributesHolder.set(attributes); requestAttributesHolder.remove(); } else { requestAttributesHolder.set(attributes); inheritableRequestAttributesHolder.remove(); } }
是在SpringMVC处理Servlet的类FrameworkServlet的类中,doget/dopost方法,调用processRequest方法进行初始化上下文方法中initContextHolders设置进去的
private void initContextHolders(HttpServletRequest request, @Nullable LocaleContext localeContext, @Nullable RequestAttributes requestAttributes) { if (localeContext != null) { LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable); } if (requestAttributes != null) { RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable); } if (this.logger.isTraceEnabled()) { this.logger.trace("Bound request context to thread: " + request); } }
再看一下请求信息怎么获取
@Nullable public static RequestAttributes getRequestAttributes() { RequestAttributes attributes = (RequestAttributes)requestAttributesHolder.get(); if (attributes == null) { attributes = (RequestAttributes)inheritableRequestAttributesHolder.get(); } return attributes; }
解决疑问
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 ThreadLocalrequestAttributesHolder = new NamedThreadLocal ("Request attributes"); //可被子线程继承的request private static final ThreadLocal inheritableRequestAttributesHolder = new NamedInheritableThreadLocal ("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; }
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:
- 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); } }
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.
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