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:

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

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.

到此这篇关于SpringMVC中RequestContextHolder获取请求信息的方法的文章就介绍到这了,更多相关SpringMVC RequestContextHolder请求信息内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！

时间：2020-04-03