springmvc--11--ViewResolver和View

短命女 2022-11-07 11:35 163阅读 0赞

# springmvc–`ViewResolver`和`View` #

### 文章目录 ###

*  springmvc--\`ViewResolver\`和\`View\`
 *  1 前言
 *  2 视图解析和响应原理
 *  3 视图解析器
 *  4 视图

# 1 前言 #

虽然现在前后端分离是主流，但是我们还是需要了解一下传统项目。当我们在处理器方法中返回一个`String`字符串并且没有使用`@ResponseBody`注解的时候，`springmvc`就会使用视图解析器解析这个`String`字符串得到视图对象，最终调用视图对象的`render()`方法完成视图渲染，响应给客户端。

# 2 视图解析和响应原理 #

经过前面的学习，我们知道`DispatcherServlet`在它的`doDispatch(request, response)`方法中完成请求的处理和响应，下面是该方法的源码

/** * Process the actual dispatching to the handler. * <p>The handler will be obtained by applying the servlet's HandlerMappings in order. * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters * to find the first that supports the handler class. * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers * themselves to decide which methods are acceptable. * @param request current HTTP request * @param response current HTTP response * @throws Exception in case of any kind of processing failure */
    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { 
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;
    
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    
        try { 
            ModelAndView mv = null;
            Exception dispatchException = null;
    
            try { 
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);
    
                // Determine handler for the current request.
                //从处理器映射器中得到处理器执行链
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) { 
                    noHandlerFound(processedRequest, response);
                    return;
                }
    
                // Determine handler adapter for the current request.
                //获取支持该处理器的处理器适配器
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
    
                // Process last-modified header, if supported by the handler.
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) { 
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { 
                        return;
                    }
                }
    
                //执行拦截器applyPreHandle()方法
                if (!mappedHandler.applyPreHandle(processedRequest, response)) { 
                    return;
                }
    
                // Actually invoke the handler.
                //处理器适配器调用处理器方法， 并返回一个ModelAndView对象
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
    
                if (asyncManager.isConcurrentHandlingStarted()) { 
                    return;
                }
    
                //没有指定视图名，则使用默认的视图名
                applyDefaultViewName(processedRequest, mv);
                //执行拦截器applyPostHandle()方法
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) { 
                dispatchException = ex;
            }
            catch (Throwable err) { 
                // As of 4.3, we're processing Errors thrown from handler methods as well,
                // making them available for @ExceptionHandler methods and other scenarios.
                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            //处理异常和ModelAndView对象，使用视图解析器解析视图，响应视图
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) { 
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Throwable err) { 
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                                   new NestedServletException("Handler processing failed", err));
        }
        finally { 
            if (asyncManager.isConcurrentHandlingStarted()) { 
                // Instead of postHandle and afterCompletion
                if (mappedHandler != null) { 
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else { 
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) { 
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

> 我们重点看一下`processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);`方法，该方法调用视图解析器解析视图并响应给客户端

/** * Handle the result of handler selection and handler invocation, which is * either a ModelAndView or an Exception to be resolved to a ModelAndView. */
    private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
                                       @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
                                       @Nullable Exception exception) throws Exception { 
    
        boolean errorView = false;
    
        if (exception != null) { 
            if (exception instanceof ModelAndViewDefiningException) { 
                logger.debug("ModelAndViewDefiningException encountered", exception);
                mv = ((ModelAndViewDefiningException) exception).getModelAndView();
            }
            else { 
                //解析异常，生成一个ModelAndView对象
                Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
                mv = processHandlerException(request, response, handler, exception);
                errorView = (mv != null);
            }
        }
    
        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) { 
            //解析ModelAndView对象，生成视图对象，渲染视图，响应给客户端浏览器
            render(mv, request, response);
            if (errorView) { 
                WebUtils.clearErrorRequestAttributes(request);
            }
        }
        else { 
            if (logger.isTraceEnabled()) { 
                logger.trace("No view rendering, null ModelAndView returned.");
            }
        }
    
        if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { 
            // Concurrent handling started during a forward
            return;
        }
    
        if (mappedHandler != null) { 
            // Exception (if any) is already handled..
            mappedHandler.triggerAfterCompletion(request, response, null);
        }
    }

> 重点看这个`render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response)`方法

/** * Render the given ModelAndView. * <p>This is the last stage in handling a request. It may involve resolving the view by name. * @param mv the ModelAndView to render * @param request current HTTP servlet request * @param response current HTTP servlet response * @throws ServletException if view is missing or cannot be resolved * @throws Exception if there's a problem rendering the view */
    protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception { 
        // Determine locale for request and apply it to the response.
        Locale locale =
            (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
        response.setLocale(locale);
    
        View view;
        //视图名
        String viewName = mv.getViewName();
        if (viewName != null) { 
            // We need to resolve the view name.
            //解析视图名得到视图对象，方法内部遍历它拥有的所有的视图解析器，获取第一个解析成功的视图对象
            view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
            if (view == null) { 
                throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
                                           "' in servlet with name '" + getServletName() + "'");
            }
        }
        else { 
            // No need to lookup: the ModelAndView object contains the actual View object.
            //获取视图对象
            view = mv.getView();
            if (view == null) { 
                throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
                                           "View object in servlet with name '" + getServletName() + "'");
            }
        }
    
        // Delegate to the View object for rendering.
        if (logger.isTraceEnabled()) { 
            logger.trace("Rendering view [" + view + "] ");
        }
        try { 
            if (mv.getStatus() != null) { 
                response.setStatus(mv.getStatus().value());
            }
            //调用视图对象的render()方法完成视图渲染，响应给客户端浏览器
            view.render(mv.getModelInternal(), request, response);
        }
        catch (Exception ex) { 
            if (logger.isDebugEnabled()) { 
                logger.debug("Error rendering view [" + view + "]", ex);
            }
            throw ex;
        }
    }

> 其实视图解析原理很简单：
> 
> 处理器方法返回一个`String`字符串，这个字符串被视图解析器解析，生成视图对象，最后调用视图对象的`render()`方法渲染视图，响应给浏览器。

# 3 视图解析器 #

先来看一下`springmvc`定义的视图解析器接口

public interface ViewResolver { 
    
        /** * 解析视图名，得到视图对象 */
        @Nullable
        View resolveViewName(String viewName, Locale locale) throws Exception;
    
    }

> 这个接口只有一个方法，就是根据视图名得到视图对象。我们再看一下`springmvc`提供的默认的视图解析器

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMDEyOTgx_size_16_color_FFFFFF_t_70_pic_center]

上面这些就是`springmvc`提供的默认的视图解析器

<table> 
 <thead> 
  <tr> 
   <th align="left">视图解析器</th> 
   <th>作用</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"><code>UrlBasedViewResolver</code></td> 
   <td>解析得到转发和重定向视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>InternalResourceViewResolver</code></td> 
   <td>解析得到<code>jsp</code>视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>ContentNegotiatingViewResolver</code></td> 
   <td>根据客户端可接收的内容类型调用其他的视图解析器创建视图对象</td> 
  </tr> 
  <tr> 
   <td align="left"><code>FreeMarkerViewResolver</code></td> 
   <td>解析得到<code>FreeMaker</code>模板视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>GroovyMarkupViewResolver</code></td> 
   <td>解析得到<code>Groovy XML/XHTML</code>模板视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>BeanNameViewResolver</code></td> 
   <td>根据视图名从<code>spring</code>容器中获取对应的视图对象</td> 
  </tr> 
  <tr> 
   <td align="left"><code>ResourceBundleViewResolver</code></td> 
   <td>和<code>BeanNameViewResolver</code>很相似，只是该视图解析器只会默认获取<code>views包</code>定义的视图<code>bean</code>对象</td> 
  </tr> 
  <tr> 
   <td align="left"><code>XmlViewResolver</code></td> 
   <td>和<code>BeanNameViewResolver</code>很相似，只是该视图解析器只会默认获取<code>/WEB-INF/views.xml文件</code>定义的视图<code>bean</code>对象</td> 
  </tr> 
 </tbody> 
</table>

> 视图解析器只负责将视图名解析为对应的视图对象，视图渲染过程由视图对象的`render()`方法完成

# 4 视图 #

先来看一下`springmvc`定义的视图接口

public interface View { 
    
        /** * 响应状态码 * org.springframework.web.servlet.View.responseStatus */
        String RESPONSE_STATUS_ATTRIBUTE = View.class.getName() + ".responseStatus";
    
        /** * 模板变量 * org.springframework.web.servlet.View.pathVariables */
        String PATH_VARIABLES = View.class.getName() + ".pathVariables";
    
        /** * 选中的视图类型 * org.springframework.web.servlet.View.selectedContentType */
        String SELECTED_CONTENT_TYPE = View.class.getName() + ".selectedContentType";
    
    
        //获取响应的内容类型
        @Nullable
        default String getContentType() { 
            return null;
        }
    
        //渲染视图
        void render(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response)
            throws Exception;
    
    }

> 这个接口的核心方法就是`render()`方法，它负责将视图内容写入到响应体中。另外还定义了`3`个常量
> 
>  *  `RESPONSE_STATUS_ATTRIBUTE`
>  *  `PATH_VARIABLES`
>  *  `SELECTED_CONTENT_TYPE`

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMDEyOTgx_size_16_color_FFFFFF_t_70_pic_center 1]

上面这些就是`springmvc`提供的默认的视图

<table> 
 <thead> 
  <tr> 
   <th align="left">视图解析器</th> 
   <th>作用</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"><code>InternalResourceView</code></td> 
   <td><code>jsp</code>视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>JstlView</code></td> 
   <td>可解析<code>jstl</code>标签的<code>jsp</code>视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>RedirectView</code></td> 
   <td>重定向视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>MappingJackson2JsonView</code></td> 
   <td>响应<code>json</code>数据的视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>MappingJackson2XmlView</code></td> 
   <td>响应<code>xml</code>数据的视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>FreeMarkerView</code></td> 
   <td>可解析<code>FreeMarkerView</code>标签的视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>GroovyMarkupView</code></td> 
   <td>可解析<code>Groovy</code>标签的视图</td> 
  </tr> 
  <tr> 
   <td align="left"><code>AbstractXlsxView</code></td> 
   <td><code>excel</code>表格下载的视图，需要用户自己实现具体逻辑</td> 
  </tr> 
 </tbody> 
</table>

> `springmvc`提供很多默认的视图类型，用户可以根据需要选择合适的视图响应

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMDEyOTgx_size_16_color_FFFFFF_t_70_pic_center 1]: /images/20221023/55b4a67702f749c484e419df7ebf8315.png