首先我们先看看ModelAndView中重要的View接口。
View接口:
再看下ViewResolver接口:
它是对给定的viewName找到对应的View对象,然后使用该view对象的render方法将本身的内容写到response中。
然后就看下,当我们的处理函数返回一个viewName时,SpringMVC是如何渲染的。
继续看下processDispatchResult是如何来渲染的
这里可以看到整体的处理流程。首先判断view是不是一个视图的名称,若是需要找到这个视图名称对应的View对象,然后便是调用view对象的render方法,渲染到response中。
由于我们的处理函数经常仅仅是返回一个view名称,所以我们重点要看看它是如何根据视图名称来找到对应的View对象的,即resolveViewName方法内容。其实上文已经说明了View接口和ViewResolver 接口,ViewResolver 接口就是根据view名称来找到对应的View对象的,所以看下面就会很清晰明白
这里就是对DispatcherServlet的private List<ViewResolver> viewResolvers属性进行遍历找到一个能够获取View对象的ViewResolver,并返回这个view对象。
至此整个流程便走通了,接下来就是要看看有哪些ViewResolver以及它们的注册来源是什么?
常用的ViewResolver有:FreeMarkerViewResolver、InternalResourceViewResolver、VelocityViewResolver等。
接下来就是如何来注册这些ViewResolver:
还是在DispatcherServlet的初始化策略中,调用了initViewResolvers,如下:
这和HandleMapping和HandlerAdapter的初始化过程基本类似。this.detectAllViewResolvers是DispatcherServlet的一个boolean属性,可以在web.xml文件中修改这个值,默认是true。
当detectAllViewResolvers为true,意味着就会获取从xml文件中解析出来的ViewResolver。如果为false,则直接去找bean name为"viewResolver"并且是ViewResolver类型的作为DispatcherServlet的ViewResolver。
当上述两种情况都没有找到,则会启用默认的ViewResolver,在this.viewResolvers = getDefaultStrategies(context, ViewResolver.class)中,这个过程已经多次说过,可以见本系列第一篇HandleMapping的来源。它就是依据DispatcherServlet.properties文件中所配置的ViewResolver,如下:
也就是默认采用的是InternalResourceViewResolver。
再说说在xml文件中配置ViewResolver的情况,如下:
这里是以FreeMarkerViewResolver为例来说明,它的配置内容还是需要有待继续研究。这里只是粗略的说下它的继承情况。
FreeMarkerViewResolver继承AbstractTemplateViewResolver继承UrlBasedViewResolver继承AbstractCachingViewResolver。
首先是抽象类AbstractCachingViewResolver:它加入了缓存功能,它有几个重要的属性。
属性一:cacheLimit 最大的缓存数量,默认为1024。
属性二:viewAccessCache 是ConcurrentHashMap类型的,适合高并发。
属性三:viewCreationCache是LinkedHashMap类型的
我们再来看下,由view名称来解析到view视图对象的具体过程:
对于Object cacheKey = getCacheKey(viewName, locale);默认为viewName + "_" + locale;
但是可以被子类覆盖,子类UrlBasedViewResolver覆盖了它,变成只有viewName。
先从viewAccessCache中看能否找到已缓存的view视图,若能找到则返回。若未找到则加上同步锁synchronized (this.viewCreationCache),进入这个方法之后,最关键的是仍需要进行一次判断view = this.viewCreationCache.get(cacheKey),看看是否已经创建过了,并不是viewAccessCache和viewCreationCache他们所缓存的内容不一样而是如果没有这个判断,则会有多线程问题。
如线程1和线程2同时要解析相同的view名称,他们都来到同步锁synchronized (this.viewCreationCache)之前,线程2先拿到锁,线程1等待,线程2创建好view视图后,加入viewCreationCache和viewAccessCache,并释放锁。此时线程1获得锁,进入同步锁synchronized (this.viewCreationCache)内部,若不进行判断,则线程1又会去创建一次view视图。所以view = this.viewCreationCache.get(cacheKey)并判断view是否为null这一步骤是十分有用的。
创建View视图的任务就交给了子类来实现。resolveViewName这个方法基本上就分析完了,应该还会想到,它的那个cacheLimit限制好像还没发挥出作用。
继续回看
viewCreationCache 的类型是LinkedHashMap,但是它复写了protected boolean removeEldestEntry(Map.Entry<Object, View> eldest)方法,当该方法返回true时,LinkedHashMap则会删除最老的key。在这里我们可以看到,当viewCreationCache 的所存的View数量达到cacheLimit时,就会删除最老的那个key和value,同时也会使viewAccessCache删除这个key和value。
viewAccessCache主要是用来高并发的访问,viewCreationCache 则是用来统计最老的key。他们所存储的view都是一样的。
View接口:
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String getContentType();
/**
* Render the view given the specified model.
* <p>The first step will be preparing the request: In the JSP case,
* this would mean setting model objects as request attributes.
* The second step will be the actual rendering of the view,
* for example including the JSP via a RequestDispatcher.
* @param model Map with name Strings as keys and corresponding model
* objects as values (Map can also be {@code null} in case of empty model)
* @param request current HTTP request
* @param response HTTP response we are building
* @throws Exception if rendering failed
*/
//上面说的很清楚,对于jsp来说,第一步就是将model作为request的attributes;第二步才开始渲染view
void
render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response)
throws
Exception;
|
再看下ViewResolver接口:
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View resolveViewName(String viewName, Locale locale)
throws
Exception;
|
它是对给定的viewName找到对应的View对象,然后使用该view对象的render方法将本身的内容写到response中。
然后就看下,当我们的处理函数返回一个viewName时,SpringMVC是如何渲染的。
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try
{
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
}
finally
{
if
(asyncManager.isConcurrentHandlingStarted()) {
return
;
}
}
applyDefaultViewName(request, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch
(Exception ex) {
dispatchException = ex;
}
//这里是我们的关注重点,就是进行视图渲染的过程
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch
(Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch
(Error err) {
triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
}
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继续看下processDispatchResult是如何来渲染的
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private
void
processDispatchResult(HttpServletRequest request, HttpServletResponse response,
HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception)
throws
Exception {
boolean
errorView =
false
;
if
(exception !=
null
) {
if
(exception
instanceof
ModelAndViewDefiningException) {
logger.debug(
"ModelAndViewDefiningException encountered"
, exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else
{
Object handler = (mappedHandler !=
null
? mappedHandler.getHandler() :
null
);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv !=
null
);
}
}
// Did the handler return a view to render?
//这里是我们关注的重点
if
(mv !=
null
&& !mv.wasCleared()) {
render(mv, request, response);
if
(errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else
{
if
(logger.isDebugEnabled()) {
logger.debug(
"Null ModelAndView returned to DispatcherServlet with name '"
+ getServletName() +
"': assuming HandlerAdapter completed request handling"
);
}
}
if
(WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return
;
}
if
(mappedHandler !=
null
) {
mappedHandler.triggerAfterCompletion(request, response,
null
);
}
}
|
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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.resolveLocale(request);
response.setLocale(locale);
View view;
if
(mv.isReference()) {
// We need to resolve the view name.
view = resolveViewName(mv.getViewName(), 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.
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.isDebugEnabled()) {
logger.debug(
"Rendering view ["
+ view +
"] in DispatcherServlet with name '"
+ getServletName() +
"'"
);
}
try
{
view.render(mv.getModelInternal(), request, response);
}
catch
(Exception ex) {
if
(logger.isDebugEnabled()) {
logger.debug(
"Error rendering view ["
+ view +
"] in DispatcherServlet with name '"
+
getServletName() +
"'"
, ex);
}
throw
ex;
}
}
|
这里可以看到整体的处理流程。首先判断view是不是一个视图的名称,若是需要找到这个视图名称对应的View对象,然后便是调用view对象的render方法,渲染到response中。
由于我们的处理函数经常仅仅是返回一个view名称,所以我们重点要看看它是如何根据视图名称来找到对应的View对象的,即resolveViewName方法内容。其实上文已经说明了View接口和ViewResolver 接口,ViewResolver 接口就是根据view名称来找到对应的View对象的,所以看下面就会很清晰明白
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protected
View resolveViewName(String viewName, Map<String, Object> model, Locale locale,
HttpServletRequest request)
throws
Exception {
for
(ViewResolver viewResolver :
this
.viewResolvers) {
View view = viewResolver.resolveViewName(viewName, locale);
if
(view !=
null
) {
return
view;
}
}
return
null
;
}
|
这里就是对DispatcherServlet的private List<ViewResolver> viewResolvers属性进行遍历找到一个能够获取View对象的ViewResolver,并返回这个view对象。
至此整个流程便走通了,接下来就是要看看有哪些ViewResolver以及它们的注册来源是什么?
常用的ViewResolver有:FreeMarkerViewResolver、InternalResourceViewResolver、VelocityViewResolver等。
接下来就是如何来注册这些ViewResolver:
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protected
void
initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
//我们关注的重点
initViewResolvers(context);
initFlashMapManager(context);
}
|
还是在DispatcherServlet的初始化策略中,调用了initViewResolvers,如下:
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private
void
initViewResolvers(ApplicationContext context) {
this
.viewResolvers =
null
;
if
(
this
.detectAllViewResolvers) {
// Find all ViewResolvers in the ApplicationContext, including ancestor contexts.
Map<String, ViewResolver> matchingBeans =
BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.
class
,
true
,
false
);
if
(!matchingBeans.isEmpty()) {
this
.viewResolvers =
new
ArrayList<ViewResolver>(matchingBeans.values());
// We keep ViewResolvers in sorted order.
OrderComparator.sort(
this
.viewResolvers);
}
}
else
{
try
{
ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.
class
);
this
.viewResolvers = Collections.singletonList(vr);
}
catch
(NoSuchBeanDefinitionException ex) {
// Ignore, we'll add a default ViewResolver later.
}
}
// Ensure we have at least one ViewResolver, by registering
// a default ViewResolver if no other resolvers are found.
if
(
this
.viewResolvers ==
null
) {
this
.viewResolvers = getDefaultStrategies(context, ViewResolver.
class
);
if
(logger.isDebugEnabled()) {
logger.debug(
"No ViewResolvers found in servlet '"
+ getServletName() +
"': using default"
);
}
}
}
|
这和HandleMapping和HandlerAdapter的初始化过程基本类似。this.detectAllViewResolvers是DispatcherServlet的一个boolean属性,可以在web.xml文件中修改这个值,默认是true。
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/** Detect all ViewResolvers or just expect "viewResolver" bean? */
private
boolean
detectAllViewResolvers =
true
;
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当detectAllViewResolvers为true,意味着就会获取从xml文件中解析出来的ViewResolver。如果为false,则直接去找bean name为"viewResolver"并且是ViewResolver类型的作为DispatcherServlet的ViewResolver。
当上述两种情况都没有找到,则会启用默认的ViewResolver,在this.viewResolvers = getDefaultStrategies(context, ViewResolver.class)中,这个过程已经多次说过,可以见本系列第一篇HandleMapping的来源。它就是依据DispatcherServlet.properties文件中所配置的ViewResolver,如下:
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org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver
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也就是默认采用的是InternalResourceViewResolver。
再说说在xml文件中配置ViewResolver的情况,如下:
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<bean
class
=
"org.springframework.web.servlet.view.freemarker.FreeMarkerConfigurer"
>
<property name=
"templateLoaderPath"
value=
"/WEB-INF/views"
/>
<property name=
"defaultEncoding"
value=
"utf-8"
/>
<property name=
"freemarkerSettings"
>
<props>
<prop key=
"locale"
>zh_CN</prop>
</props>
</property>
</bean>
<bean
class
=
"org.springframework.web.servlet.view.freemarker.FreeMarkerViewResolver"
>
<property name=
"suffix"
value=
".html"
/>
<property name=
"contentType"
value=
"text/html;charset=utf-8"
/>
<property name=
"requestContextAttribute"
value=
"request"
/>
<property name=
"exposeRequestAttributes"
value=
"true"
/>
<property name=
"exposeSessionAttributes"
value=
"true"
/>
</bean>
|
这里是以FreeMarkerViewResolver为例来说明,它的配置内容还是需要有待继续研究。这里只是粗略的说下它的继承情况。
FreeMarkerViewResolver继承AbstractTemplateViewResolver继承UrlBasedViewResolver继承AbstractCachingViewResolver。
首先是抽象类AbstractCachingViewResolver:它加入了缓存功能,它有几个重要的属性。
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/** Default maximum number of entries for the view cache: 1024 */
public
static
final
int
DEFAULT_CACHE_LIMIT =
1024
;
/** The maximum number of entries in the cache */
private
volatile
int
cacheLimit = DEFAULT_CACHE_LIMIT;
/** Fast access cache for Views, returning already cached instances without a global lock */
private
final
Map<Object, View> viewAccessCache =
new
ConcurrentHashMap<Object, View>(DEFAULT_CACHE_LIMIT);
/** Map from view key to View instance, synchronized for View creation */
@SuppressWarnings
(
"serial"
)
private
final
Map<Object, View> viewCreationCache =
new
LinkedHashMap<Object, View>(DEFAULT_CACHE_LIMIT,
0
.75f,
true
) {
@Override
protected
boolean
removeEldestEntry(Map.Entry<Object, View> eldest) {
if
(size() > getCacheLimit()) {
viewAccessCache.remove(eldest.getKey());
return
true
;
}
else
{
return
false
;
}
}
};
|
属性一:cacheLimit 最大的缓存数量,默认为1024。
属性二:viewAccessCache 是ConcurrentHashMap类型的,适合高并发。
属性三:viewCreationCache是LinkedHashMap类型的
我们再来看下,由view名称来解析到view视图对象的具体过程:
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public
View resolveViewName(String viewName, Locale locale)
throws
Exception {
//这里进行了是否进行缓存的判断,即cacheLimit是否大于0
if
(!isCache()) {
//不进行缓存,始终每次都创建
return
createView(viewName, locale);
}
else
{
//viewAccessCache viewCreationCache两者的key
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) {
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
);
}
}
|
对于Object cacheKey = getCacheKey(viewName, locale);默认为viewName + "_" + locale;
但是可以被子类覆盖,子类UrlBasedViewResolver覆盖了它,变成只有viewName。
先从viewAccessCache中看能否找到已缓存的view视图,若能找到则返回。若未找到则加上同步锁synchronized (this.viewCreationCache),进入这个方法之后,最关键的是仍需要进行一次判断view = this.viewCreationCache.get(cacheKey),看看是否已经创建过了,并不是viewAccessCache和viewCreationCache他们所缓存的内容不一样而是如果没有这个判断,则会有多线程问题。
如线程1和线程2同时要解析相同的view名称,他们都来到同步锁synchronized (this.viewCreationCache)之前,线程2先拿到锁,线程1等待,线程2创建好view视图后,加入viewCreationCache和viewAccessCache,并释放锁。此时线程1获得锁,进入同步锁synchronized (this.viewCreationCache)内部,若不进行判断,则线程1又会去创建一次view视图。所以view = this.viewCreationCache.get(cacheKey)并判断view是否为null这一步骤是十分有用的。
创建View视图的任务就交给了子类来实现。resolveViewName这个方法基本上就分析完了,应该还会想到,它的那个cacheLimit限制好像还没发挥出作用。
继续回看
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private
final
Map<Object, View> viewAccessCache =
new
ConcurrentHashMap<Object, View>(DEFAULT_CACHE_LIMIT);
private
final
Map<Object, View> viewCreationCache =
new
LinkedHashMap<Object, View>(DEFAULT_CACHE_LIMIT,
0
.75f,
true
) {
@Override
protected
boolean
removeEldestEntry(Map.Entry<Object, View> eldest) {
if
(size() > getCacheLimit()) {
viewAccessCache.remove(eldest.getKey());
return
true
;
}
else
{
return
false
;
}
}
};
|
viewCreationCache 的类型是LinkedHashMap,但是它复写了protected boolean removeEldestEntry(Map.Entry<Object, View> eldest)方法,当该方法返回true时,LinkedHashMap则会删除最老的key。在这里我们可以看到,当viewCreationCache 的所存的View数量达到cacheLimit时,就会删除最老的那个key和value,同时也会使viewAccessCache删除这个key和value。
viewAccessCache主要是用来高并发的访问,viewCreationCache 则是用来统计最老的key。他们所存储的view都是一样的。