0. 概述
分析web项目中spring的配置，初始化，bean解析和注入的过程，以及spring bean 的生命周期和扩展点，深入学习spring框架的使用。
示例项目: springinside
1. spring web 配置文件
项目常见的spring web 的web.xml， 这里去掉了一些无关的配置，尽量最小化配置
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath*:META-INF/spring/*.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<servlet>
<description>spring inside</description>
<display-name>spring inside</display-name>
<servlet-name>springinside</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>WEB-INF/applicationContext.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>springinside</servlet-name>
<url-pattern>/*</url-pattern>
</servlet-mapping>
</web-app>
- 配置context-param,contextConfigLocation 指定ContextLoaderListener使用的配置文件
- 配置了ContextLoaderListener，监听web容器的启动，用于做些初始化工作
- 配置了DispatcherServlet处理web请求，指定了DispatcherServlet要加载的配置文件
对于spring web 项目来说，其中1和2不是必须的，可以没有。但是，这里配置ContextLoaderListener，是为了和部门的项目一致。部门的sof开发框架在web容器启动的时候有一些初始化操作，就是在ContextLoaderListener这里触发的。接下来我们分析ContextLoaderListener和DispatcherServlet源码，来看看spring是怎么完成初始化的。
2. ContextLoaderListener分析
2.1 继承关系
首先，看一下ContextLoaderListener的继承关系
可以看到，ContextLoaderListener实现了ServletContextListener接口。ServletContextListener接口够监听 ServletContext 对象的生命周期，实际上就是监听 Web应用的生命周期。当Servlet 容器启动或终止Web 应用时，会触发ServletContextEvent 事件，该事件由ServletContextListener 来处理。
public interface ServletContextListener extends EventListener {
/**
** Notification that the web application initialization
** process is starting.
** All ServletContextListeners are notified of context
** initialization before any filter or servlet in the web
** application is initialized.
*/
public void contextInitialized ( ServletContextEvent sce );
/**
** Notification that the servlet context is about to be shut down.
** All servlets and filters have been destroy()ed before any
** ServletContextListeners are notified of context
** destruction.
*/
public void contextDestroyed ( ServletContextEvent sce );
}
当Servlet启动时就会调用contextInitialized方法；当Servlet销毁时会调用contextDestroyed方法。
2.2 ContextLoaderListener
接下来我们看ContextLoaderListener源码。
public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
public ContextLoaderListener() {
}
public ContextLoaderListener(WebApplicationContext context) {
super(context);
}
/**
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}
/**
* Close the root web application context.
*/
@Override
public void contextDestroyed(ServletContextEvent event) {
closeWebApplicationContext(event.getServletContext());
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}
}
ContextLoaderListener的代码很简单，在Servlet启动时，调用initWebApplicationContext方法初始化WebApplicationContext这个web应用上下文；在Servlet销毁时，关闭应用上下文。Spring是怎么初始化web应用上下文的，我们跟踪代码进入initWebApplicationContext方法一探究竟。
2.3 initWebApplicationContext 分析
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
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.
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);
cwac.setParent(parent);
}
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
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方法一共做了三件事：
1. 如果this.context为null, 则创建WebApplicationContext
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
我们在web.xml里，只配置了一个Listener，所以在ContextLoaderListener之前没有创建过WebApplicationContext，固这里this.context为null，所以会创建WebApplicationContext。
2. 如果this.context是ConfigurableWebApplicationContext的实例，并且没有刷新过WebApplicationContext，则配置并刷新WebApplicationContext。
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);
cwac.setParent(parent);
}
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
什么意思呢？首先，我们看看this.context的起继承关系 。
this.contexnt的类型是WebApplicationContext， 可以看到WebApplicationContext是个接口，它继承了BeanFactory。this.context是Spring的IoC容器，所以下面的配置和刷新应用上下文就不足为奇了，因为它要解析，注入bean。所以configureAndRefreshWebApplicationContext方法，会完成bean解析，加载，注入的过程。现在我们知道了WebApplicationContext是一个接口，这里的this.context到底会是什么那个具体的实例呢？其实，这里的this.context是XmlWebApplicationContext的具体实例。我们看一下XmlWebApplicationContext的继承关系。
可以看到，XmlWebApplicationContext实现了ConfigurableWebApplicationContext 接口，ConfigurableWebApplicationContext 继承了WebApplicationContext。继承关系理清楚了，这里的代码也就很容易看懂了。
3. 将WebApplicationContext放入servletContext中，供后续流程使用
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
这里讲this.context放入servletContext中，后续的DispatcherServlet会从servletContext取出该容器，作为其创建的容器的父容器。
2.4 configureAndRefreshWebApplicationContext 分析
现在我们进入configureAndRefreshWebApplicationContext方法，看看是如何配置和刷新上下文的。
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);
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);
wac.refresh();
}
方法进来首先检查是否设置过id，如果没有设置过id，那么就设置容器id。
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()));
}
}
容器id优先从配置文web.xml中获取，如果配置文件配置了容器id，那个就使用配置的容器id，否则使用默认生成的容器id。
然后是获取spring配置文件。配置文件也是我们在web.xml配置的context-param
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath*:META-INF/spring/*.xml</param-value>
</context-param>
Spring的环境配置，暂时不深入了解，后续在学习Spring的bean装配时在介绍。
接下来是一个扩展点————定制上下文，在容器刷新之前，给用户一个机会，可以做一些事情。
customizeContext(sc, wac);
跟进去看看
protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {
List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
determineContextInitializerClasses(sc);
for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {
Class<?> initializerContextClass =
GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);
if (initializerContextClass != null && !initializerContextClass.isInstance(wac)) {
throw new ApplicationContextException(String.format(
"Could not apply context initializer [%s] since its generic parameter [%s] " +
"is not assignable from the type of application context used by this " +
"context loader: [%s]", initializerClass.getName(), initializerContextClass.getName(),
wac.getClass().getName()));
}
this.contextInitializers.add(BeanUtils.instantiateClass(initializerClass));
}
AnnotationAwareOrderComparator.sort(this.contextInitializers);
for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) {
initializer.initialize(wac);
}
}
怎么定制上下文呢？看代码可以知道，要实现ApplicationContextInitializer接口
public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
/**
* Initialize the given application context.
* @param applicationContext the application to configure
*/
void initialize(C applicationContext);
}
接口就一个初始化方法，传入上下文。实现该接口，自己操作上下文，完成定制。实现了ApplicationContextInitializer接口之后，怎么配置让其生效呢？我们先看看是怎么找实现该接口的类的，进入determineContextInitializerClasses看看。
protected List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>
determineContextInitializerClasses(ServletContext servletContext) {
List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> classes =
new ArrayList<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>();
String globalClassNames = servletContext.getInitParameter(GLOBAL_INITIALIZER_CLASSES_PARAM);
if (globalClassNames != null) {
for (String className : StringUtils.tokenizeToStringArray(globalClassNames, INIT_PARAM_DELIMITERS)) {
classes.add(loadInitializerClass(className));
}
}
String localClassNames = servletContext.getInitParameter(CONTEXT_INITIALIZER_CLASSES_PARAM);
if (localClassNames != null) {
for (String className : StringUtils.tokenizeToStringArray(localClassNames, INIT_PARAM_DELIMITERS)) {
classes.add(loadInitializerClass(className));
}
}
return classes;
}
可以看到，这里是通过 servletContext.getInitParameter(GLOBAL_INITIALIZER_CLASSES_PARAM)和servletContext.getInitParameter(CONTEXT_INITIALIZER_CLASSES_PARAM) 获取类名，和Spring配置文件一样，也是在web.xml中配置的。
最后是刷新上下文。
这里的刷新上下文，其实就是Spring读取配置稳定，解析bean定义，注册BeanDefinition，实例化bean，完成注入的过程，在此暂不深入。
wac.refresh();
3. DispatcherServlet分析
3.1继承关系
DispatcherServlet间接实现了Servlet接口。在初始化阶段，会调用Servlet的init()方法，这是Servlet的入口，接下来我们找到入口，从入口分析。
3.2 追根溯源 Servlet的init()
本次分析的目的是研究DispatcherServlet的上下文和ContextLoaderListener的上下文是如何初始化和关联起来的，其它逻辑不是本文重点，将会被忽略。
在GenericServlet类中实现了Servlet接口的init方法
public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}
GenericServlet的init(ServletConfig config)方法除了设置了config并没有干其它的逻辑，而是调用了自身的init()方法，将具体逻辑委托给自己的子类处理。我们接着找谁实现了init()方法。在HttpServletBean类中找到了init()方法，看一下代码
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");
}
}
HttpServletBean的init()方法也没有看到有处理上下文的逻辑，但是看了initServletBean()这个方法，我们继续跟代码。
在FrameworkServlet类中重写了initServletBean()方法。
@Override
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 ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
catch (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");
}
}
在这里终于看到了与上下文有关的逻辑————initWebApplicationContext()方法，跟之。
protected WebApplicationContext initWebApplicationContext() {
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
cwac.setParent(rootContext);
}
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
wac = findWebApplicationContext();
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
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.
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;
}
我们看我们感兴趣的逻辑。
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
.
.
.
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
wac = createWebApplicationContext(rootContext);
}
这里获取根上下文rootContext，并将rootContext作为参数，创建WebApplicationContext。这里获取的rootContext就是在ContextLoaderListener设置的WebApplicationContext。Talk is cheap. Show me the code.
public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
}
看到 WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE，我们回忆一下ContextLoaderListener是怎么设置上下文到ServletContext的。
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
证据确凿。
再来看createWebApplicationContext(rootContext)
protected WebApplicationContext createWebApplicationContext(WebApplicationContext parent) {
return createWebApplicationContext((ApplicationContext) parent);
}
protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
Class<?> contextClass = getContextClass();
if (this.logger.isDebugEnabled()) {
this.logger.debug("Servlet with name '" + getServletName() +
"' will try to create custom WebApplicationContext context of class '" +
contextClass.getName() + "'" + ", using parent context [" + parent + "]");
}
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException(
"Fatal initialization error in servlet with name '" + getServletName() +
"': custom WebApplicationContext class [" + contextClass.getName() +
"] is not of type ConfigurableWebApplicationContext");
}
ConfigurableWebApplicationContext wac =
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
wac.setEnvironment(getEnvironment());
wac.setParent(parent);
wac.setConfigLocation(getContextConfigLocation());
configureAndRefreshWebApplicationContext(wac);
return wac;
}
看到createWebApplicationContext方法的代码，是不是似曾相识的感觉？这和ContextLoaderListener的 initWebApplicationContext方法的逻辑是不是很相似？
wac.setParent(parent);
我们将目光放在上面一行代码上。这行代码将ContextLoaderListener的创建的WebApplicationContext和DispatcherServlet创建的WebApplicationContext关联了起来，将Web应用上下文的层次体现出来，我们也知道WebApplicationContext其实也是Spring的BeanFactory，所以这里也是设置了Spring Ioc容器的父子关系。最后就是configureAndRefreshWebApplicationContext了，这里就不分析了。
作者：RayJY
链接：https://www.jianshu.com/p/3d3532f6647f
來源：简书
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