SpringWeb Cannot expose request attribute 'XXX' because of an existing model object of the same name

发生场景：

导致这个异常只有一种原因，数据模型跟request中的attributes存在同名属性，大致在几种场景下出现： 1. Model和Request对象中有同名的attribute - 比如你在Interceptor中request.setAttribute("key1","niubi")，然后又在Controller中调用model.addAttribute("key1","wocao") 2. PathVariable 和 Model/Request中的attribute同名

解决方法

根据模板引擎添加对应配置即可。

不过本人还是建议能改个属性名就改名吧，不到迫不得已，尽量少一些个性化的配置，日后代码维护或交接的时候会轻松许多。

freemarker

# 指定HttpServletRequest的属性是否可以覆盖controller的model的同名项
spring.freemarker.allow-request-override=true

velocity

# 指定HttpServletRequest的属性是否可以覆盖controller的model的同名项
spring.velocity.allow-request-override=true

groovy

# 指定HttpServletRequest的属性是否可以覆盖controller的model的同名项
spring.groovy.template.allow-request-override=true

追根溯源，揭开神秘面纱

顺着堆栈debug

Java代码：

    @GetMapping("/www.tongtian.icu/{test}")
    public String tongtian_icu(@PathVariable String test,HttpServletRequest request) {
        request.setAttribute("test","123456");
        return "TestTemplate";
    }

堆栈信息：

javax.servlet.ServletException: Cannot expose request attribute 'test' because of an existing model object of the same name
	at org.springframework.web.servlet.view.AbstractTemplateView.renderMergedOutputModel(AbstractTemplateView.java:124) ~[spring-webmvc-5.3.4.jar:5.3.4]
	at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:316) ~[spring-webmvc-5.3.4.jar:5.3.4]
	at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1393) ~[spring-webmvc-5.3.4.jar:5.3.4]
	at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1138) ~[spring-webmvc-5.3.4.jar:5.3.4]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1077) ~[spring-webmvc-5.3.4.jar:5.3.4]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:962) ~[spring-webmvc-5.3.4.jar:5.3.4]
# 到DispatcherServlet就可以了，后续略...

renderMergedOutputModel 渲染合并后的输出模型

这个方法作用是判断是否公开request中的attributes，如果exposeRequestAttributes为true，会将request中的attributes复制到model中。

第十行可以看到，在exposeRequestAttributes为true，且allowRequestOverride不为true的情况下，会抛出异常，这也就是场景1中出现这种情况的原因，所以我们解决方法中的配置就允许覆盖。

	@Override
	protected final void renderMergedOutputModel(
			Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

		if (this.exposeRequestAttributes) {
			Map<String, Object> exposed = null;
			for (Enumeration<String> en = request.getAttributeNames(); en.hasMoreElements();) {
				String attribute = en.nextElement();
				if (model.containsKey(attribute) && !this.allowRequestOverride) {
					throw new ServletException("Cannot expose request attribute '" + attribute +
						"' because of an existing model object of the same name");
				}
				Object attributeValue = request.getAttribute(attribute);
				if (logger.isDebugEnabled()) {
					exposed = exposed != null ? exposed : new LinkedHashMap<>();
					exposed.put(attribute, attributeValue);
				}
				model.put(attribute, attributeValue);
			}
			if (logger.isTraceEnabled() && exposed != null) {
				logger.trace("Exposed request attributes to model: " + exposed);
			}
		}

render

注释的意思是，将静态属性与requestContext属性合并（17行），再委托renderMergedOutputModel进行实际渲染。

	/**
	 * Prepares the view given the specified model, merging it with static
	 * attributes and a RequestContext attribute, if necessary.
	 * Delegates to renderMergedOutputModel for the actual rendering.
	 * @see #renderMergedOutputModel
	 */
	@Override
	public void render(@Nullable Map<String, ?> model, HttpServletRequest request,
			HttpServletResponse response) throws Exception {

		if (logger.isDebugEnabled()) {
			logger.debug("View " + formatViewName() +
					", model " + (model != null ? model : Collections.emptyMap()) +
					(this.staticAttributes.isEmpty() ? "" : ", static attributes " + this.staticAttributes));
		}

		Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);
		prepareResponse(request, response);
		renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
	}

createMergedOutputModel

19行可以看到，我们的pathVariable全部都被加入到了mergedModel中，所以再通过request.setAttribude方法设置同名属性，就会在renderMergedOutputModel实际渲染的时候抛出异常。这也就是场景2中抛出异常的原因，针对场景2，我们可以单独设置exposePathVariables为false解决报错。

	/**
	 * Creates a combined output Map (never {@code null}) that includes dynamic values and static attributes.
	 * Dynamic values take precedence over static attributes.
	 */
	protected Map<String, Object> createMergedOutputModel(@Nullable Map<String, ?> model,
			HttpServletRequest request, HttpServletResponse response) {

		@SuppressWarnings("unchecked")
		Map<String, Object> pathVars = (this.exposePathVariables ?
				(Map<String, Object>) request.getAttribute(View.PATH_VARIABLES) : null);

		// Consolidate static and dynamic model attributes.
		int size = this.staticAttributes.size();
		size += (model != null ? model.size() : 0);
		size += (pathVars != null ? pathVars.size() : 0);

		Map<String, Object> mergedModel = CollectionUtils.newLinkedHashMap(size);
		mergedModel.putAll(this.staticAttributes);
		if (pathVars != null) {
			mergedModel.putAll(pathVars);
		}
		if (model != null) {
			mergedModel.putAll(model);
		}

		// Expose RequestContext?
		if (this.requestContextAttribute != null) {
			mergedModel.put(this.requestContextAttribute, createRequestContext(request, response, mergedModel));
		}

		return mergedModel;
	}