RequestContextHolder详解

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:

1. //两个方法在没有使用JSF的项目中是没有区别的

2. RequestAttributes requestAttributes = RequestContextHolder.currentRequestAttributes();

3. //RequestContextHolder.getRequestAttributes();

4. //从session里面获取对应的值

5. String str = (String) requestAttributes.getAttribute("name",RequestAttributes.SCOPE_SESSION);

7. HttpServletRequest request = ((ServletRequestAttributes)requestAttributes).getRequest();

8. 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]

1. //得到存储进去的request

2. private static final ThreadLocal requestAttributesHolder =

3. new NamedThreadLocal("Request attributes");

4. //可被子线程继承的request

5. private static final ThreadLocal inheritableRequestAttributesHolder =

6. 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.

1. public static RequestAttributes getRequestAttributes() {

2. RequestAttributes attributes = requestAttributesHolder.get();

3. if (attributes == null) {

4. attributes = inheritableRequestAttributesHolder.get();

5. }

6. return attributes;

7. }

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.

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

1. protected final void processRequest(HttpServletRequest request, HttpServletResponse response)

2. throws ServletException, IOException {

3. long startTime = System.currentTimeMillis();

4. Throwable failureCause = null;

5. //获取上一个请求保存的LocaleContext

6. LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();

7. //建立新的LocaleContext

8. LocaleContext localeContext = buildLocaleContext(request);

9. //获取上一个请求保存的RequestAttributes

10. RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();

11. //建立新的RequestAttributes

12. ServletRequestAttributes requestAttributes = buildRequestAttributes(request,

13. response, previousAttributes);

14. WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

15. asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(),

16. new RequestBindingInterceptor());

17. //具体设置的方法

18. initContextHolders(request, localeContext, requestAttributes);

19. try {

20. doService(request, response);

21. }

22. catch (ServletException ex) {

23. failureCause = ex;

24. throw ex;

25. }

26. catch (IOException ex) {

27. failureCause = ex;

28. throw ex;

29. }

30. catch (Throwable ex) {

31. failureCause = ex;

32. throw new NestedServletException("Request processing failed", ex);

33. }

34. finally {

35. //恢复

36. resetContextHolders(request, previousLocaleContext, previousAttributes);

37. if (requestAttributes != null) {

38. requestAttributes.requestCompleted();

39. }

40. if (logger.isDebugEnabled()) {

41. if (failureCause != null) {

42. this.logger.debug("Could not complete request", failureCause);

43. }

44. else {

45. if (asyncManager.isConcurrentHandlingStarted()) {

46. logger.debug("Leaving response open for concurrent processing");

47. }

48. else {

49. this.logger.debug("Successfully completed request");

50. }

51. }

52. }

53. //发布事件

54. publishRequestHandledEvent(request, response, startTime, failureCause);

55. }

56. }

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.

1. private void initContextHolders(HttpServletRequest request,

2. LocaleContext localeContext,

3. RequestAttributes requestAttributes) {

4. if (localeContext != null) {

5. LocaleContextHolder.setLocaleContext(localeContext,

6. this.threadContextInheritable);

7. }

8. if (requestAttributes != null) {

9. RequestContextHolder.setRequestAttributes(requestAttributes,

10. this.threadContextInheritable);

11. }

12. if (logger.isTraceEnabled()) {

13. logger.trace("Bound request context to thread: " + request);

14. }

15. }

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.