SpringMVC中RequestContextHolder获取请求信息的方法

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 requestAttributesHolder = 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 ThreadLocal requestAttributesHolder =

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.threadContexhttp://tInheritable);

}

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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