SpringMVC @SessionAttributes 使用详解以及源码分析

超、凢脫俗 2022-10-02 00:35 128阅读 0赞

@sessionattributes

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@Target({ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Inherited
    @Documented
    public @interface SessionAttributes {
        String[] value() default {};
        Class[] types() default {};
    }

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@sessionattributes注解应用到Controller上面，可以将Model中的属性同步到session当中。

先看一个最基本的方法

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@Controller
    @RequestMapping("/Demo.do")
    @SessionAttributes(value={"attr1","attr2"})
    public class Demo {
        
        @RequestMapping(params="method=index")
        public ModelAndView index() {
            ModelAndView mav = new ModelAndView("index.jsp");
            mav.addObject("attr1", "attr1Value");
            mav.addObject("attr2", "attr2Value");
            return mav;
        }
        
        @RequestMapping(params="method=index2")
        public ModelAndView index2(@ModelAttribute("attr1")String attr1, @ModelAttribute("attr2")String attr2) {
            ModelAndView mav = new ModelAndView("success.jsp");
            return mav;
        }
    }

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index方法返回一个ModelAndView 其中包括视图index.jsp 和 两个键值放入model当中，在没有加入@sessionattributes注解的时候，放入model当中的键值是request级别的。

现在因为在Controller上面标记了@SessionAttributes(value=\{"attr1","attr2"\}) 那么model中的attr1,attr2会同步到session中，这样当你访问index 然后在去访问index2的时候也会获取这俩个属性的值。

当需要清除session当中的值得时候，我们只需要在controller的方法中传入一个SessionStatus的类型对象 通过调用setComplete方法就可以清除了。

@RequestMapping(params="method=index3")
    　　public ModelAndView index4(SessionStatus status) {
    　　ModelAndView mav = new ModelAndView("success.jsp");
    　　status.setComplete();
    　　return mav;
    }

下面就直接分析代码来看Spring是如可封装的。

首先我们需要看2个类 DefaultSessionAttributeStore和SessionAttributesHandler

DefaultSessionAttributeStore这个类是用来往WebRequest存取数据的工具类，WebRequest是Spring包装的HttpServletRequest，大家理解为普通的HttpServletRequest就行了。

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public class DefaultSessionAttributeStore implements SessionAttributeStore {
    
        private String attributeNamePrefix = "";
    
    
        public void setAttributeNamePrefix(String attributeNamePrefix) {
            this.attributeNamePrefix = (attributeNamePrefix != null ? attributeNamePrefix : "");
        }
    
    
        public void storeAttribute(WebRequest request, String attributeName, Object attributeValue) {
            Assert.notNull(request, "WebRequest must not be null");
            Assert.notNull(attributeName, "Attribute name must not be null");
            Assert.notNull(attributeValue, "Attribute value must not be null");
            String storeAttributeName = getAttributeNameInSession(request, attributeName);
            request.setAttribute(storeAttributeName, attributeValue, WebRequest.SCOPE_SESSION);
        }
    
        public Object retrieveAttribute(WebRequest request, String attributeName) {
            Assert.notNull(request, "WebRequest must not be null");
            Assert.notNull(attributeName, "Attribute name must not be null");
            String storeAttributeName = getAttributeNameInSession(request, attributeName);
            return request.getAttribute(storeAttributeName, WebRequest.SCOPE_SESSION);
        }
    
        public void cleanupAttribute(WebRequest request, String attributeName) {
            Assert.notNull(request, "WebRequest must not be null");
            Assert.notNull(attributeName, "Attribute name must not be null");
            String storeAttributeName = getAttributeNameInSession(request, attributeName);
            request.removeAttribute(storeAttributeName, WebRequest.SCOPE_SESSION);
        }
    
    
        protected String getAttributeNameInSession(WebRequest request, String attributeName) {
            return this.attributeNamePrefix + attributeName;
        }
    
    }

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Spring会为每一个Controller初始化一个SessionAttributesHandler实例,用来记录@SessionAttributes(value=\{"attr1","attr2"\})里面的属性信息，当需要同步model的值时，会先判断是否在SessionAttributes当中定义。

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public class SessionAttributesHandler {
    
        private final Set<String> attributeNames = new HashSet<String>();
    
        private final Set<Class<?>> attributeTypes = new HashSet<Class<?>>();
    
        private final Set<String> knownAttributeNames = Collections.synchronizedSet(new HashSet<String>(4));
    
        private final SessionAttributeStore sessionAttributeStore;
    
        public SessionAttributesHandler(Class<?> handlerType, SessionAttributeStore sessionAttributeStore) {
            Assert.notNull(sessionAttributeStore, "SessionAttributeStore may not be null.");
            this.sessionAttributeStore = sessionAttributeStore;
    
            SessionAttributes annotation = AnnotationUtils.findAnnotation(handlerType, SessionAttributes.class);
            if (annotation != null) {
                this.attributeNames.addAll(Arrays.asList(annotation.value()));
                this.attributeTypes.addAll(Arrays.<Class<?>>asList(annotation.types()));
            }
    
            this.knownAttributeNames.addAll(this.attributeNames);
        }
    
    
        public boolean hasSessionAttributes() {
            return ((this.attributeNames.size() > 0) || (this.attributeTypes.size() > 0));
        }
    
    
        public boolean isHandlerSessionAttribute(String attributeName, Class<?> attributeType) {
            Assert.notNull(attributeName, "Attribute name must not be null");
            if (this.attributeNames.contains(attributeName) || this.attributeTypes.contains(attributeType)) {
                this.knownAttributeNames.add(attributeName);
                return true;
            }
            else {
                return false;
            }
        }
    
    
        public void storeAttributes(WebRequest request, Map<String, ?> attributes) {
            for (String name : attributes.keySet()) {
                Object value = attributes.get(name);
                Class<?> attrType = (value != null) ? value.getClass() : null;
    
                if (isHandlerSessionAttribute(name, attrType)) {
                    this.sessionAttributeStore.storeAttribute(request, name, value);
                }
            }
        }
    
        public Map<String, Object> retrieveAttributes(WebRequest request) {
            Map<String, Object> attributes = new HashMap<String, Object>();
            for (String name : this.knownAttributeNames) {
                Object value = this.sessionAttributeStore.retrieveAttribute(request, name);
                if (value != null) {
                    attributes.put(name, value);
                }
            }
            return attributes;
        }
    
    
        public void cleanupAttributes(WebRequest request) {
            for (String attributeName : this.knownAttributeNames) {
                this.sessionAttributeStore.cleanupAttribute(request, attributeName);
            }
        }
    
    
        Object retrieveAttribute(WebRequest request, String attributeName) {
            return this.sessionAttributeStore.retrieveAttribute(request, attributeName);
        }
    
    }

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当我们访问controller中的一个方法时，会调用RequestMappingHandlerAdapter类当中的invokeHandleMethod的方法。

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private ModelAndView invokeHandleMethod(HttpServletRequest request,
                HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
    
            ServletWebRequest webRequest = new ServletWebRequest(request, response);
    
            WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
            ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
            ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);
    
            ModelAndViewContainer mavContainer = new ModelAndViewContainer();
            mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
            modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);
            mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);
    
            AsyncWebRequest asyncWebRequest = AsyncWebUtils.createAsyncWebRequest(request, response);
            asyncWebRequest.setTimeout(this.asyncRequestTimeout);
    
            final WebAsyncManager asyncManager = AsyncWebUtils.getAsyncManager(request);
            asyncManager.setTaskExecutor(this.taskExecutor);
            asyncManager.setAsyncWebRequest(asyncWebRequest);
    
            if (asyncManager.hasConcurrentResult()) {
                Object result = asyncManager.getConcurrentResult();
                mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
                asyncManager.clearConcurrentResult();
    
                if (logger.isDebugEnabled()) {
                    logger.debug("Found concurrent result value [" + result + "]");
                }
                requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
            }
    
            requestMappingMethod.invokeAndHandle(webRequest, mavContainer);
    
            if (asyncManager.isConcurrentHandlingStarted()) {
                return null;
            }
    
            return getModelAndView(mavContainer, modelFactory, webRequest);
        }

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我们看这行代码

modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);

跟进去看源码

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public void initModel(NativeWebRequest request, ModelAndViewContainer mavContainer, HandlerMethod handlerMethod)
                throws Exception {
    
            Map<String, ?> attributesInSession = this.sessionAttributesHandler.retrieveAttributes(request);
            mavContainer.mergeAttributes(attributesInSession);
    
            invokeModelAttributeMethods(request, mavContainer);
    
            for (String name : findSessionAttributeArguments(handlerMethod)) {
                if (!mavContainer.containsAttribute(name)) {
                    Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
                    if (value == null) {
                        throw new HttpSessionRequiredException("Expected session attribute '" + name + "'");
                    }
                    mavContainer.addAttribute(name, value);
                }
            }
        }

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其中这两句是关键的地方,把session的值取出来全部放入到ModelAndViewContainer当中去，这样我就可以再controller当中的方法中获取了，当然直接从session中拿也是可以的。

Map<String, ?> attributesInSession = this.sessionAttributesHandler.retrieveAttributes(request);  
mavContainer.mergeAttributes(attributesInSession);

在controller中获取sessionAttributes的只有两种方式。

一、public ModelAndView index2(@ModelAttribute("attr1")String attr1, @ModelAttribute("attr2")String attr2)

二、public ModelAndView index2(ModelMap map)

这俩种方式的区别是，如果使用@ModelAttribute属性获取值，并且@SessionAttributes注解当中还设置了该属性，当属性为null时会跑出异常，因为这几行代码。

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for (String name : findSessionAttributeArguments(handlerMethod)) {
                if (!mavContainer.containsAttribute(name)) {
                    Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
                    if (value == null) {
                        throw new HttpSessionRequiredException("Expected session attribute '" + name + "'");
                    }
                    mavContainer.addAttribute(name, value);
                }
    }

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以上分析的都是取值的过程，那么Spring是如何将Model中的数据同步到session当中的呢。

我们看上面invokeHandleMethod方法中最后一句话 return getModelAndView(mavContainer, modelFactory, webRequest); 我们跟进去看源码。

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private ModelAndView getModelAndView(ModelAndViewContainer mavContainer,
                ModelFactory modelFactory, NativeWebRequest webRequest) throws Exception {
    
            modelFactory.updateModel(webRequest, mavContainer);
    
            if (mavContainer.isRequestHandled()) {
                return null;
            }
            ModelMap model = mavContainer.getModel();
            ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model);
            if (!mavContainer.isViewReference()) {
                mav.setView((View) mavContainer.getView());
            }
            if (model instanceof RedirectAttributes) {
                Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
                HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);
                RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
            }
            return mav;
        }

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有一行代码是modelFactory.updateModel(webRequest, mavContainer); 我们在跟进去看源码。

我们看到首先判断一下ModelAndViewContainer中的SessionStatus是否是完成状态，如果是TRUE就清除session中的值，反之将ModelAndViewContainer中的model值设置进去。

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public void updateModel(NativeWebRequest request, ModelAndViewContainer mavContainer) throws Exception {
            
            if (mavContainer.getSessionStatus().isComplete()){
                this.sessionAttributesHandler.cleanupAttributes(request);
            }
            else {
                this.sessionAttributesHandler.storeAttributes(request, mavContainer.getModel());
            }
            
            if (!mavContainer.isRequestHandled()) {
                updateBindingResult(request, mavContainer.getModel());
            } 
        }

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看到这里可能还有一个疑问，就是我们的数据时设置到ModelAndView里面的,那么这些数据时如何跑到ModelAndViewContainer当中去的呢，其实是在方法执行完成之后Spring帮我们做的，

具体细节查看 ModelAndViewMethodReturnValueHandler方法中的最后一行代码mavContainer.addAllAttributes(mav.getModel());

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public void handleReturnValue(
                Object returnValue, MethodParameter returnType,
                ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
                throws Exception {
    
            if (returnValue == null) {
                mavContainer.setRequestHandled(true);
                return;
            }
    
            ModelAndView mav = (ModelAndView) returnValue;
            if (mav.isReference()) {
                String viewName = mav.getViewName();
                mavContainer.setViewName(viewName);
                if (viewName != null && viewName.startsWith("redirect:")) {
                    mavContainer.setRedirectModelScenario(true);
                }
            }
            else {
                View view = mav.getView();
                mavContainer.setView(view);
                if (view instanceof SmartView) {
                    if (((SmartView) view).isRedirectView()) {
                        mavContainer.setRedirectModelScenario(true);
                    }
                }
            }
            mavContainer.addAllAttributes(mav.getModel());
        }

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原文链接：[https://www.cnblogs.com/daxin/p/SessionAttributes.html][https_www.cnblogs.com_daxin_p_SessionAttributes.html]

[copycode.gif]: /images/20220113/78fc99ab4a9e4570ae28f7ff3f4e1dae.png
[ExpandedBlockStart.gif]: /images/20220113/e2aa252b412d46418d5fa85488a6f8a5.png
[https_www.cnblogs.com_daxin_p_SessionAttributes.html]: https://www.cnblogs.com/daxin/p/SessionAttributes.html