SpringMVC中RequestContextHolder获取请求信息的方法

RequestContextHolder的作用是：

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 ThreadLocal<RequestAttributes> requestAttributesHolder = new NamedThreadLocal("Request attributes");
  private static final ThreadLocal<RequestAttributes> 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 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;
  }

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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