SpringMVC源码阅读：视图解析器

1.前言

SpringMVC是目前J2EE平台的主流Web框架，不熟悉的园友可以看SpringMVC源码阅读入门,它交代了SpringMVC的基础知识和源码阅读的技巧

本文将通过源码(基于Spring4.3.7)分析，弄清楚SpringMVC如何完成视图解析的

2.源码分析

在SpringMVC源码阅读：拦截器分析过doDispatch的运行过程，这里再分析一遍

回到DispatcherServlet类的doDispatch方法，看看doDispatch如何获取ModelAndView

HandlerMapping根据request获得HandlerExecutionChain

根据HandlerExecutionChain获取HandlerAdapter

HandlerAdapter根据request,response和HandlerExecutionChain调用handle方法返回ModelAndView

然后交由processDispatchResult处理

1023行检测从doDispatch方法运行到此是否有异常

1037行根据ModelAndView渲染视图

1259根据视图名称解析成View对象

1282行调用AbstractView的render方法进行渲染

点进去，这里以解析ftl做测试

301行创建包含动态值和静态属性的Map

302为待渲染的响应做准备

303行调用子类实现方法渲染这个包含动态值和静态属性的Map

接下来会进入AbstractTemplateView的renderMergedOutputModel方法，AbstractTemplateView是AbstractView的子类

AbstractTemplateView的renderMergedOutputModel方法会调用内部renderMergedTemplateModel方法，该方法被子类FreemarkerView实现

打开FreemarkerView的renderMergedTemplateModel方法

进入doRender方法，该方法根据response渲染FreeMarker视图

275行exposeModelAsRequestAttributes方法在父类AbstractView被实现，该方法把Model设置到request中

277行创建Freemarker模板模型

284行处理、加工Freemarker模板给response

再看看FreeMarkerViewResolver做了什么，打开类接口继承图

构造函数设置了前缀和后缀，再看看AbstractTemplateViewResolver

104~108行给视图设置属性

我们发现103行super引用了父类，点开进入UrlBasedViewResolver的buildView方法

532行拼接前缀+视图名称+后缀，例如"employee/form.ftl"，并设置url

534~553行判断属性是否为空，非空则设置到View中

再看createView方法

460行如果以"redirect:"开头，交给RedirectView处理

467行如果以"forward:"开头，交给InternalResourceView处理

Freemarker解析逻辑至此分析完毕

现在看JSP的解析流程

回到AbstractView类的render方法，刚刚我们分析过

304行此时renderMergedOutputModel方法被子类InternalResourceView实现

139行将Model设置到Request中

142将辅助性参数设置到Request中

145行获取视图文件路径

148行获取请求分发器

155到170行如果response已经提交，则把资源文件纳入到response中，否则调用forward方法转发

再看下InternalResourceViewResolver，该类辅助InternalResourceView,为其设置属性，其余的ViewResolver的实现类分析同Freemarker

3.自定义视图解析

3.1 自定义支持Freemarker和Jsp的视图解析器

自定义视图解析器，"jsp:"开头构造InternalResourceView解析jsp，以"freemarker:"开头则构造FreemarkerView解析ftl

这样做的好处是可以区分同名的jsp和ftl

public class CustomViewResolver extends UrlBasedViewResolver {

    public static final String JSP_URL_PREFIX = "jsp:";
    public static final String FTL_URL_PREFIX = "freemarker:";

    private static final boolean jstlPresent = ClassUtils.isPresent(
            "javax.servlet.jsp.jstl.core.Config", CustomViewResolver.class.getClassLoader());

    private Boolean exposePathVariables = false;

    private boolean exposeRequestAttributes = false;

    private boolean allowRequestOverride = false;

    private boolean exposeSessionAttributes = false;

    private boolean allowSessionOverride = false;

    private boolean exposeSpringMacroHelpers = true;

    public CustomViewResolver() {
        this.setViewClass(FreeMarkerView.class);
    }

    @Override
    protected AbstractUrlBasedView buildView(String viewName) throws Exception {
            if(viewName.startsWith(FTL_URL_PREFIX)) {
            return buildFreemarkerView(viewName.substring(FTL_URL_PREFIX.length()));
        } else if(viewName.startsWith(JSP_URL_PREFIX)) {
            Class viewCls = jstlPresent ? JstlView.class : InternalResourceView.class;
            return build(viewCls, viewName.substring(JSP_URL_PREFIX.length()), getPrefix(), ".jsp");
        } else {
            //默认以freemarker处理
            return buildFreemarkerView(viewName);
        }
    }

    /**
     * @Author 谷天乐
     * @Description 使用UrlBasedViewResolver的buildView方法
     * 因为CustomViewResolver重写了buildView，不再执行UrlBasedViewResolver的buildView方法
     * @Date 2019/1/17 11:26
     * @Param [viewClass, viewName, prefix, suffix]
     * @return org.springframework.web.servlet.view.AbstractUrlBasedView
     **/
    private AbstractUrlBasedView build(Class viewClass, String viewName, String prefix, String suffix) {
        AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(viewClass);
        view.setUrl(prefix + viewName + suffix);
        String contentType = getContentType();
        if (contentType != null) {
            view.setContentType(contentType);
        }
        view.setRequestContextAttribute(getRequestContextAttribute());
        view.setAttributesMap(getAttributesMap());
        if (this.exposePathVariables != null) {
            view.setExposePathVariables(exposePathVariables);
        }
        return view;
    }

    /**
     * @Author 谷天乐
     * @Description 使用AbstractTemplateViewResolver的buildView,为view设置属性
     * Freemarker解析所需
     *
     * @Date 2019/2/7 10:22
     * @Param [viewName]
     * @return org.springframework.web.servlet.view.AbstractUrlBasedView
     **/
    private AbstractUrlBasedView buildFreemarkerView(String viewName) throws Exception {
        AbstractTemplateView view = (AbstractTemplateView) build(FreeMarkerView.class, viewName, "", getSuffix());
        view.setExposeRequestAttributes(this.exposeRequestAttributes);
        view.setAllowRequestOverride(this.allowRequestOverride);
        view.setExposeSessionAttributes(this.exposeSessionAttributes);
        view.setAllowSessionOverride(this.allowSessionOverride);
        view.setExposeSpringMacroHelpers(this.exposeSpringMacroHelpers);
        return view;
    }

    public boolean isExposeRequestAttributes() {
        return exposeRequestAttributes;
    }

    public void setExposeRequestAttributes(boolean exposeRequestAttributes) {
        this.exposeRequestAttributes = exposeRequestAttributes;
    }

    public boolean isExposeSessionAttributes() {
        return exposeSessionAttributes;
    }

    public void setExposeSessionAttributes(boolean exposeSessionAttributes) {
        this.exposeSessionAttributes = exposeSessionAttributes;
    }

    public boolean isExposeSpringMacroHelpers() {
        return exposeSpringMacroHelpers;
    }

    public void setExposeSpringMacroHelpers(boolean exposeSpringMacroHelpers) {
        this.exposeSpringMacroHelpers = exposeSpringMacroHelpers;
    }
}

在dispatcher-servlet.xml加入自定义视图解析器

   <bean class="org.format.demo.custom.CustomViewResolver">
        <property name="prefix" value="/WEB-INF/view/"/>
        <property name="suffix" value=".ftl"/>
        <property name="contentType" value="text/html;charset=utf-8"/>
        <property name="exposeRequestAttributes" value="true"/>
        <property name="exposeSessionAttributes" value="true"/>
        <property name="exposeSpringMacroHelpers" value="true"/>
        <property name="requestContextAttribute" value="request"/>
    </bean>

引入freemarker和jsp的配置，配置在前遇到同名情况具备更高的优先级，比如index.ftl和index.jsp会优先解析index.ftl

    <!--遇到同名的ftl和jsp文件，配置在前的解析器优先级更高-->
    <import resource="classpath:springConfig/viewConfig/freemarker.xml"/>
    <import resource="classpath:springConfig/viewConfig/jsp.xml"/>

测试Controller

@Controller
@RequestMapping(value = "/tvrc")
public class TestViewResolverController {

    @RequestMapping("jsp")
    public ModelAndView jsp(ModelAndView view) {
        view.setViewName("jsp:tvrc/test");
        return view;
    }

    @RequestMapping("/ftl")
    public ModelAndView freemarker(ModelAndView view) {
        view.setViewName("freemarker:tvrc/test");
        return view;
    }

}

浏览器输入http://localhost:8080/springmvcdemo/tvrc/jsp

浏览器输入http://localhost:8080/springmvcdemo/tvrc/ftl

3.2 自定义Pdf视图

测试Controller

@Controller
@RequestMapping("otherview")
public class OtherViewController {
    @RequestMapping("/pdf")
    public ModelAndView mypdf() {
        ModelAndView mav = new ModelAndView();
        //添加自定义视图
        mav.setView(new MyPdfView());
        List<String> list = new ArrayList<String>();
        for (int i = 0; i < 10; i++) {
            list.add(i+"");
        }
        mav.addObject("list", list);
        return mav;
    }

    @RequestMapping("/excel")
    public ModelAndView myexcle() {
        ModelAndView mav = new ModelAndView();
        //添加视图
        mav.setView(new MyExcelView());
        List list = new ArrayList<String>();
        for (int i = 0; i < 10; i++) {
            list.add(i+"");
        }
        mav.addObject("list", list);
        return mav;
    }
}

创建测试类继承AbstractPdfView，重写buildPdfDocument方法

public class MyPdfView extends AbstractPdfView {

    @Override
    protected void buildPdfDocument(Map<String, Object> model, Document document, PdfWriter writer,
                                    HttpServletRequest request, HttpServletResponse response) throws Exception {
        List list = (List) model.get("list");
        for (int i = 0; i < list.size(); i++) {
            //将数据加载到视图上
            document.add(new Paragraph((String)(list.get(i))));
        }
    }
}

浏览器输入http://localhost:8080/springmvcdemo/otherview/pdf

3.3 自定义Excel视图

创建测试类继承AbstractExcelView，重写buildExcelDocument方法

public class MyExcelView extends AbstractExcelView {

    @Override
    protected void buildExcelDocument(Map<String, Object> model, HSSFWorkbook workbook, HttpServletRequest request,
                                      HttpServletResponse response) throws Exception {
        HSSFSheet sheet;//工作簿的名字
        HSSFCell cell;//单元格

        sheet = workbook.createSheet("spring");
        sheet.setDefaultColumnWidth(12);

        List list = (List) model.get("list");
        for (int i = 0; i < list.size(); i++) {
            cell = getCell(sheet, 0, i);
            setText(cell, (String) list.get(i));
        }
    }
}

浏览器输入http://localhost:8080/springmvcdemo/otherview/excel

4.总结

View作为视图接口，AbstractView的render方法创建待渲染的model，调用子类，如果待解析视图类型是jsp，则调用InternalResourceView，如果是ftl，则调用FreemarkerView，执行渲染

ViewResolver作为视图解析接口，主要为View提供支持，createView方法创建View,buildView根据路径构造View,并为View设置一系列属性

DispatcherServlet根据ModelAndView调用View实现类的render方法进行视图渲染，ViewResolver实现类起到辅助作用，为View设置属性（前缀、后缀等）

5.参考

https://docs.spring.io/spring/docs/4.3.7.RELEASE/spring-framework-reference/htmlsingle/#beans-beans-conversion

https://docs.spring.io/spring/docs/current/javadoc-api/

https://github.com/spring-projects/spring-framework

文中难免有不足，欢迎指出