简单分析springmvc是如何解析view视图，并返回页面给前端
SpringMVC配置视图解析器
<bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
<property key="prefix" value="/WEB-INF/jsp/"/>
<property key="suffix" value=".jsp" />
</bean>
配置的为jsp的解析器
ViewResolver接口
其内部只有一个接口方法，具体如下
View resolveViewName(String viewName, Locale locale) throws Exception;
由上可知其是通过解析ViewName来得到View视图对象
ViewResolver实现类-BeanNameViewResolver
其意图是参数viewName就是springmvc上下文中的beanName对象，具体源码如下
@Override
public View resolveViewName(String viewName, Locale locale) throws BeansException {
//获得springmvc上下文
ApplicationContext context = getApplicationContext();
if (!context.containsBean(viewName)) {
//viewName不存在，则直接返回null
return null;
}
if (!context.isTypeMatch(viewName, View.class)) {
//viewName对应的beanName不是View.class的实现类，则直接返回
return null;
}
return context.getBean(viewName, View.class);
}
BeanNameResolver表示返回的viewName必须是springmvc上下文中的beanName并且对应的实体类必须是View.class的实现类，否则返回null
ViewResolver实现类-AbstractCachingViewResolver
其是我们常用解析器的抽象类，比如Freemarker/Groovy，我们直接去看其实现的接口方法，具体源码如下
@Override
public View resolveViewName(String viewName, Locale locale) throws Exception {
//不采用缓存方案，则每次都进行创建View
if (!isCache()) {
return createView(viewName, locale);
}
else {
//此处为使用缓存情况下的获取View对象
Object cacheKey = getCacheKey(viewName, locale);
View view = this.viewAccessCache.get(cacheKey);
if (view == null) {
synchronized (this.viewCreationCache) {
view = this.viewCreationCache.get(cacheKey);
if (view == null) {
// Ask the subclass to create the View object.
view = createView(viewName, locale);
if (view == null && this.cacheUnresolved) {
//默认会返回一个null的View对象
view = UNRESOLVED_VIEW;
}
if (view != null) {
this.viewAccessCache.put(cacheKey, view);
this.viewCreationCache.put(cacheKey, view);
if (logger.isTraceEnabled()) {
logger.trace("Cached view [" + cacheKey + "]");
}
}
}
}
}
return (view != UNRESOLVED_VIEW ? view : null);
}
}
接以上代码我们接着分析AbstractCachingViewResolver#createView()方法
protected View createView(String viewName, Locale locale) throws Exception {
//此处的loadView便是模板方法，供子类去实现
return loadView(viewName, locale);
}
此处我们只分析其某个实现类UrlBasedViewResolver
UrlBasedViewResolver-对应请求url的解析器
- 常用的内部属性
public static final String REDIRECT_URL_PREFIX = "redirect:";
public static final String FORWARD_URL_PREFIX = "forward:";
//设置前缀
private String prefix = "";
//设置后缀
private String suffix = "";
//设置指定的viewName集合
private String[] viewNames ;
- createView()复写父类方法，即在创建view对象前做下跳转的请求检查
@Override
protected View createView(String viewName, Locale locale) throws Exception {
//viewNames集合为null或者对应的viewName在viewNames集合内则返回true
if (!canHandle(viewName, locale)) {
return null;
}
// Check for special "redirect:" prefix.
//检查handler返回的值为string类型时是否包含"redirect:"前缀
//此处处理的便是跳转请求，比如"redirect:/user/list"
if (viewName.startsWith(REDIRECT_URL_PREFIX)) {
String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());
return applyLifecycleMethods(viewName, view);
}
// Check for special "forward:" prefix.
//同"redirect:"请求，此处为服务端直接跳转
if (viewName.startsWith(FORWARD_URL_PREFIX)) {
String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
return new InternalResourceView(forwardUrl);
}
//通过父类再去调用loadView()方法，其实是本类也复写了此方法
return super.createView(viewName, locale);
}
- loadView()复写方法
@Override
protected View loadView(String viewName, Locale locale) throws Exception {
//创建View对象
AbstractUrlBasedView view = buildView(viewName);
//将view对象与viewName绑定注册至springmvc上下文中
View result = applyLifecycleMethods(viewName, view);
return (view.checkResource(locale) ? result : null);
}
- buildView()创建View对象主逻辑
protected AbstractUrlBasedView buildView(String viewName) throws Exception {
//获取类似FreemarkerView.class/GroovyView.class
AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(getViewClass());
//设置view对应的资源路径，此处便可知我们设置prefix和suffix的作用
view.setUrl(getPrefix() + viewName + getSuffix());
//下面都是设置与UrlBasedViewResolver的相关内部属性
String contentType = getContentType();
if (contentType != null) {
view.setContentType(contentType);
}
view.setRequestContextAttribute(getRequestContextAttribute());
view.setAttributesMap(getAttributesMap());
//是否暴露路径变量
Boolean exposePathVariables = getExposePathVariables();
if (exposePathVariables != null) {
view.setExposePathVariables(exposePathVariables);
}
//是否暴露springmvc的bean对象作为属性使用
Boolean exposeContextBeansAsAttributes = getExposeContextBeansAsAttributes();
if (exposeContextBeansAsAttributes != null) {
view.setExposeContextBeansAsAttributes(exposeContextBeansAsAttributes);
}
String[] exposedContextBeanNames = getExposedContextBeanNames();
if (exposedContextBeanNames != null) {
view.setExposedContextBeanNames(exposedContextBeanNames);
}
return view;
}
主要作用是设置prefix和suffix参数以及对应的内部属性，可自行查阅，并通过
buildView()方法创建ViewClass属性指定的View对象
AbstractTemplateViewResolver-UrlBasedViewResolver子类
添加另外的属性
- 内部属性
//是否暴露request对象的attributes属性给前端引擎
private boolean exposeRequestAttributes = false;
//是否允许请求处理过程中复写request对象的attributes
private boolean allowRequestOverride = false;
//是否暴露session对象的attributes属性给前端引擎
private boolean exposeSessionAttributes = false;
//是否允许请求处理过程中复写session对象的attributes
private boolean allowSessionOverride = false;
//是否使用暴露springMacroRequestContext属性
private boolean exposeSpringMacroHelpers = true;
- buildView()复写方法，主要是额外添加以上的属性
@Override
protected AbstractUrlBasedView buildView(String viewName) throws Exception {
//先调用父类的创建方法创建View对象
AbstractTemplateView view = (AbstractTemplateView) super.buildView(viewName);
//设置AbstractTemplateView的内部属性
view.setExposeRequestAttributes(this.exposeRequestAttributes);
view.setAllowRequestOverride(this.allowRequestOverride);
view.setExposeSessionAttributes(this.exposeSessionAttributes);
view.setAllowSessionOverride(this.allowSessionOverride);
view.setExposeSpringMacroHelpers(this.exposeSpringMacroHelpers);
return view;
}
在UrlBasedViewResolver的基础上设置额外的属性，属性集合见上文
FreeMarkerViewResolver-AbstractTemplateViewResolver子类
为方便理解，我们选取常用的模板引擎Freemarker，其他的引擎工具则供读者自行分析
- 构造函数-设置viewClass，满足上述的模板方法的viewClass的获取
public FreeMarkerViewResolver() {
//设置的为FreemarkView.class
setViewClass(requiredViewClass());
}
小结
本节只解析了ViewResolver的简单逻辑，其根据配置的ViewClass属性，将配置的其他属性都设置到ViewClass对应的实例中，具体的关于视图的渲染，也就是
view#render()方法我们放在下节讲解