Spring MVC启动流程及源码分析 - morris131/morris-book GitHub Wiki
<?xml version="1.0" encoding="UTF-8"?>
<web-app
version="3.0"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
<display-name>Web Application</display-name>
<!--全局变量配置-->
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:spring.xml</param-value>
</context-param>
<!--监听器-->
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<!--解决乱码问题的filter-->
<filter>
<filter-name>CharacterEncodingFilter</filter-name>
<filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
<init-param>
<param-name>encoding</param-name>
<param-value>utf-8</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>CharacterEncodingFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<!--Restful前端控制器-->
<servlet>
<servlet-name>springMVC</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:spring-mvc.xml</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>springMVC</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
</web-app>
下面是一个web应用启动后初始化的步骤：
- 创建和初始化元素标记的listener
- 对于所有事件监听器，如果实现了ServletContextListener接口，将会执行其实现的contextInitialized()方法
- 创建和初始化元素标记的filter，并调用其init()方法
- 根据的权值按顺序创建和初始化元素标记的servlet，并调用其init()方法
web.xml中首先定义了标签，用于配置一个全局变量，标签的内容读取后会被放进application中，做为Web应用的全局变量使用，接下来创建listener时会使用到这个全局变量，因此，Web应用在容器中部署后，进行初始化时会先读取这个全局变量，之后再进行上述讲解的初始化启动过程。
接着定义了一个ContextLoaderListener类的listener。查看ContextLoaderListener的类声明源码如下:
public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
ContextLoaderListener类继承了ContextLoader类并实现了ServletContextListener接口，再看一下ServletContextListener接口源码:
default public void contextInitialized(ServletContextEvent sce) {}
default public void contextDestroyed(ServletContextEvent sce) {}
}
该接口只有两个方法contextInitialized和contextDestroyed，这里采用的是观察者模式，也称为为订阅-发布模式，实现了该接口的listener会向发布者进行订阅，当Web应用初始化或销毁时会分别调用上述两个方法。
继续看ContextLoaderListener，该listener实现了ServletContextListener接口，因此在Web应用初始化时会调用该方法，该方法的具体实现如下：
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
/*
首先通过WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
这个String类型的静态变量获取一个根IoC容器，根IoC容器作为全局变量
存储在application对象中，如果存在则有且只能有一个
如果在初始化根WebApplicationContext即根IoC容器时发现已经存在
则直接抛出异常，因此web.xml中只允许存在一个ContextLoader类或其子类的对象
*/
if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
throw new IllegalStateException(
"Cannot initialize context because there is already a root application context present - " +
"check whether you have multiple ContextLoader* definitions in your web.xml!");
}
Log logger = LogFactory.getLog(ContextLoader.class);
servletContext.log("Initializing Spring root WebApplicationContext");
if (logger.isInfoEnabled()) {
logger.info("Root WebApplicationContext: initialization started");
}
long startTime = System.currentTimeMillis();
try {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
// 如果当前成员变量中不存在WebApplicationContext则创建一个根WebApplicationContext
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent ->
// determine parent for root web application context, if any.
ApplicationContext parent = loadParentContext(servletContext);
//为根WebApplicationContext设置一个父容器
cwac.setParent(parent);
}
//配置并刷新整个根IoC容器，在这里会进行Bean的创建和初始化
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
//将创建好的IoC容器放入到application对象中，并设置key为WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
}
else if (ccl != null) {
currentContextPerThread.put(ccl, this.context);
}
if (logger.isDebugEnabled()) {
logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
}
if (logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
}
return this.context;
}
catch (RuntimeException ex) {
logger.error("Context initialization failed", ex);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
throw ex;
}
catch (Error err) {
logger.error("Context initialization failed", err);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
throw err;
}
}
initWebApplicationContext()方法如上注解讲述，主要目的就是创建root WebApplicationContext对象即根IoC容器，其中比较重要的就是，整个Web应用如果存在根IoC容器则有且只能有一个，根IoC容器作为全局变量存储在ServletContext即application对象中。将根IoC容器放入到application对象之前进行了IoC容器的配置和刷新操作，调用了configureAndRefreshWebApplicationContext()方法，该方法源码如下:
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
// The application context id is still set to its original default value
// -> assign a more useful id based on available information
String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
if (idParam != null) {
wac.setId(idParam);
}
else {
// Generate default id...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(sc.getContextPath()));
}
}
wac.setServletContext(sc);
//获取web.xml中<context-param>标签配置的全局变量，也就是获取spring.xml，并将其放入到WebApplicationContext中
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}
// The wac environment's #initPropertySources will be called in any case when the context
// is refreshed; do it eagerly here to ensure servlet property sources are in place for
// use in any post-processing or initialization that occurs below prior to #refresh
ConfigurableEnvironment env = wac.getEnvironment();
if (env instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
}
customizeContext(sc, wac);
// 调用父类AbstractApplicationContext创建并初始化spring.xml中的Bean
wac.refresh();
}
在监听器listener初始化完成后，按照文章开始的讲解，接下来会进行filter的初始化操作，filter的创建和初始化中没有涉及IoC容器的相关操作，因此不是本文讲解的重点，本文举例的filter是一个用于编码用户请求和响应的过滤器，采用utf-8编码用于适配中文。
Web应用启动的最后一个步骤就是创建和初始化相关Servlet，在开发中常用的Servlet就是DispatcherServlet类前端控制器，前端控制器作为中央控制器是整个Web应用的核心，用于获取分发用户请求并返回响应。
通过类的继承关系可以看出DispatcherServlet类间接实现了Servlet接口，因此其本质上依旧是一个Servlet。DispatcherServlet类的设计很巧妙，上层父类不同程度的实现了相关接口的部分方法，并留出了相关方法用于子类覆盖，将不变的部分统一实现，将变化的部分预留方法用于子类实现。 在Web应用部署到容器后进行Servlet初始化时会调用相关的init(ServletConfig)方法，此方法的实现实在父类HttpServletBean中，源码如下：
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
// 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();
if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}
init()中调用了子类FrameworkServlet的initServletBean()方法，
protected final void initServletBean() throws ServletException {
getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
if (this.logger.isInfoEnabled()) {
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
}
long startTime = System.currentTimeMillis();
try {
this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();
}
catch (ServletException | RuntimeException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
if (this.logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
elapsedTime + " ms");
}
}
该方法中比较重要的就是initWebApplicationContext()方法的调用，该方法仍由FrameworkServlet抽象类实现，继续查看其源码如下所示:
protected WebApplicationContext initWebApplicationContext() {
// 获取之前由ContextLoaderListener创建的根IoC容器
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent -> set
// the root application context (if any; may be null) as the parent
// 如果当前Servelt存在一个WebApplicationContext即子IoC容器,则将根IoC容器作为子IoC容器的父容器
cwac.setParent(rootContext);
}
// 读取spring-mvc.xml文件，配置并刷新当前的子IoC容器，功能与前文讲解根IoC容器时的配置刷新一致，用于构建相关Bean
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
// No context instance was injected at construction time -> see if one
// has been registered in the servlet context. If one exists, it is assumed
// that the parent context (if any) has already been set and that the
// user has performed any initialization such as setting the context id
// 如果当前Servlet不存在一个子IoC容器则去查找一下
wac = findWebApplicationContext();
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
// 如果仍旧没有查找到子IoC容器则创建一个子IoC容器
wac = createWebApplicationContext(rootContext);
}
if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here.
//调用子类DispatcherServlet覆盖的onRefresh方法完成“可变”的初始化过程
onRefresh(wac);
}
if (this.publishContext) {
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
if (this.logger.isDebugEnabled()) {
this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
"' as ServletContext attribute with name [" + attrName + "]");
}
}
return wac;
}
当IoC子容器构造完成后调用了onRefresh()方法，
/**
* This implementation calls {@link #initStrategies}.
*/
@Override
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}
/**
* Initialize the strategy objects that this servlet uses.
* <p>May be overridden in subclasses in order to initialize further strategy objects.
*/
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}
onRefresh()方法直接调用了initStrategies()方法，源码如上，通过函数名可以判断，该方法用于初始化创建multipartResovle来支持图片等文件的上传、本地化解析器、主题解析器、HandlerMapping处理器映射器、HandlerAdapter处理器适配器、异常解析器、视图解析器、flashMap管理器等，这些组件都是SpringMVC开发中的重要组件，九大组件的初始化创建过程均在此完成。