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https://www.cnblogs.com/lay2017/p/11775787.html

正文

springboot mvc自动配置的时候,获得了DispatcherServlet和DispatcherServletRegistrationBean。DispatcherServletRegistrationBean将DispatcherServlet给注册到了ServletContext当中。

注册到ServletContext中的Servlet将会触发其init方法,从而进行Servlet的初始化。本文将从Servlet的init方法开始,看看触发MVC各个组件初始化的代码

DispatcherServlet类图

我们先看看DispatcherServlet的类图

根据类图可以看到两部分的设计,第一部分是Servlet到HttpServlet,也就是Servlet容器相关的内部设计。第二部分是Spring在HttpServlet的基础上扩展了框架相关的内容,而最终DispatcherServlet将扩展springMVC的内容。

GenericServlet

我们跟进GenericServlet的init方法,看看它的实现

@Override
public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}

继续跟进init方法

public void init() throws ServletException {
// NOOP by default
}

没有实现逻辑,供子类去选择实现

HttpServletBean

httpServlet么有实现init方法,由HttpServletBean这个spring实现的类来扩展。跟进HttpServletBean的init方法

@Override
public final void init() throws ServletException { // Set bean properties from init parameters.
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
if (!pvs.isEmpty()) {
try {
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
initBeanWrapper(bw);
bw.setPropertyValues(pvs, true);
}
catch (BeansException ex) {
if (logger.isErrorEnabled()) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
}
throw ex;
}
} // Let subclasses do whatever initialization they like.
initServletBean();
}

HttpServletBean也是扩展了一个initServletBean接口来供子类实现

FrameworkServlet

跟进FrameworkServlet的initServletBean的方法

@Override
protected final void initServletBean() throws ServletException {
getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'");
if (logger.isInfoEnabled()) {
logger.info("Initializing Servlet '" + getServletName() + "'");
}
long startTime = System.currentTimeMillis(); try {
this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();
}
catch (ServletException | RuntimeException ex) {
logger.error("Context initialization failed", ex);
throw ex;
} // ...
}

initFrameworkServlet是一个空实现,核心逻辑落到了initWebApplicationContext中,跟进它

protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null; if (this.webApplicationContext != null) {
//...
}
if (wac == null) {
wac = findWebApplicationContext();
}
if (wac == null) {
wac = createWebApplicationContext(rootContext);
} if (!this.refreshEventReceived) {
synchronized (this.onRefreshMonitor) {
onRefresh(wac);
}
} if (this.publishContext) {
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
} return wac;
}

Springboot在启动过程中创建了ApplicationContext,这里将公用同一个ApplicationContext。

onRefresh方法提供了一个空实现,供子类去做初始化实现

protected void onRefresh(ApplicationContext context) {
// For subclasses: do nothing by default.
}

DispatcherServlet

跟进DispatcherServlet的onRefresh方法

@Override
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}

继续跟进initStrategies

protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}

我们看到,initStrategies包含了各种MVC组件的初始化方法

组件初始化

我们打开initMultipartResolver看看

private void initMultipartResolver(ApplicationContext context) {
try {
this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
// ...
catch (NoSuchBeanDefinitionException ex) {
// Default is no multipart resolver.
this.multipartResolver = null;
// ...
}
}

其实就是从Bean工厂当中获取对应的Bean对象。multipartResolver默认可能为空

再打开initViewResolvers看看

private void initViewResolvers(ApplicationContext context) {
this.viewResolvers = null; if (this.detectAllViewResolvers) {
Map<String, ViewResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false);
if (!matchingBeans.isEmpty()) {
this.viewResolvers = new ArrayList<>(matchingBeans.values());
AnnotationAwareOrderComparator.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.
}
} if (this.viewResolvers == null) {
this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);
}
}

一样的是从Bean工厂当中获取Bean,只不过这里是获取到一个集合,且如果没有将会获取默认的策略。

其它的MVC组件初始化类似,这里不每个打开看了。

总结

DispatcherServlet作为一个Servlet的实现,在Servlet被调用init方法以后最终将会调用DispatcherServlet的initStrategies方法,该方法将会初始化各个组件。

初始化组件基本就是把各个Bean对象从BeanFactory中拿出来组合到DispatcherServlet中,供后续使用。