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启动Spring项目详细过程(小结)

2020年02月13日 编程语言 ⁄ 共 6679字 ⁄ 字号 评论关闭

1、Spring 项目放到web项目容器中(Tomcat、Jetty、JBoss)

本文以通用的Tomcat为例

2、项目容器启动时需要加载读取web.xml配置文件

如下图:

3、容器首先会去读取web.xml配置文件中的两个节点:<listener> </listener>和<context-param> </context-param>

说明:

tomcat在启动web容器的时候会启动一个叫ServletContextListener的监听器,每当在web容器中有ServletContextListener这个接口被实例化的时候,web容器会通知ServletContextListener被实例的对象去执行其contextInitialized()的方法进行相应的业务处理;

而spring框架在设计的过程中ContextLoadListener这个类实现了ServletContextListener这个接口,因此每当有ContextLoadListener这个类被实例化的时候,web容器会通知Spring执行contextInitialized()这个方法,从而进行spring容器的启动与创建的过程中;

4、ContextLoaderListener中的contextInitialized()进行了spring容器的启动配置,调用initWebApplicationContext初始化spring容器;

@Overridepublic void contextInitialized(ServletContextEvent event) { initWebApplicationContext(event.getServletContext());}

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) { //Spring 启动的句柄,spring容器开始启动的根目录 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!"); } else { 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 { //处理spring容器是否已经创建(只创建没有创建spring的各个bean) if(this.context == null) { this.context = this.createWebApplicationContext(servletContext); } if(this.context instanceof ConfigurableWebApplicationContext) { ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext)this.context; if(!cwac.isActive()) { if(cwac.getParent() == null) { ApplicationContext parent = this.loadParentContext(servletContext); cwac.setParent(parent); } //Spring容器创建完成后,加载spring容器的各个组件 this.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 var8) { logger.error("Context initialization failed", var8); servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, var8); throw var8; } catch (Error var9) { logger.error("Context initialization failed", var9); servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, var9); throw var9; } }}

5、spring容器创建完成后,准备开始实例化加载bean,Spring容器创建完成后,准备向spring容器中加载bean 使用configureAndRefreshWebApplicationContext(cwac, servletContext); 完成bean的加载;

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 informationString 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 #refreshConfigurableEnvironment env = wac.getEnvironment();if (env instanceof ConfigurableWebEnvironment) {((ConfigurableWebEnvironment) env).initPropertySources(sc, null);} customizeContext(sc, wac);wac.refresh();}

说明:

configureAndRefreshWebApplicationContext中加载spring的配置文件,即web.xml中读取<context-param></context-param>中加载到Spring的配置文件,即:classpath:/config/applicationContext.xml;

通过以下代码加载spring配置

public class Application{ public static void main(String[] args) { ClassPathXmlApplicationContext ctx = new ClassPathXmlApplicationContext("/context.xml"); ctx.start(); }}

此处略过如何调用DefaultResourceLoader

顶级接口ResourceLoader仅提供了一个getResource(String location)方法,可以根据一个资源地址加载资源文件,资源地址的表达式可以是以下几种:

--1. classpath:前缀开头的表达式,例如: classpath:smart-context.xml

--2.“/”开头的表达式,例如:/WEB-INF/classes/smart-context.xml

--3. 非“/”开头的表达,例如:WEB-INF/classes/smart-context.xml

--4. url协议,例如:file:/D:/ALANWANG-AIA/Horse-workspace/chapter3/target/classes/smart-context.xml

Spring提供了实现类DefaultResourceLoader,DefaultResourceLoader在实现了以上列举的功能基础上,还为开发者提供了自定义扩展接口ProtocolResolver,开发者可实现该接口定制个性化资源表达式,代码如下:

@Overridepublic Resource getResource(String location) {Assert.notNull(location, "Location must not be null");for (ProtocolResolver protocolResolver : this.protocolResolvers) { // 1Resource resource = protocolResolver.resolve(location, this);if (resource != null) {return resource;}} if (location.startsWith("/")) {return getResourceByPath(location);} //2else if (location.startsWith(CLASSPATH_URL_PREFIX)) { //3return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());}else {try {// Try to parse the location as a URL...URL url = new URL(location); //4return new UrlResource(url);}catch (MalformedURLException ex) {// No URL -> resolve as resource path.return getResourceByPath(location); //5}}}

步骤1,先用扩展协议解析器解析资源地址并返回。举个例子,咱们可以自定义资源解析器来完成带前缀“classpath:”的解析:

首先实现ProtocolResolver接口:

class ClasspathPreProtocolResolver implements ProtocolResolver{ private static String CLASS_PATH_PRE="classpath:"; public Resource resolve(String location, ResourceLoader resourceLoader) { if( location.startsWith(CLASS_PATH_PRE)) { return new ClassPathResource(location.substring(CLASS_PATH_PRE.length())); } return null; } }

步骤2,假设location以斜杠开头,则调用该类中 getResourceByPath(String path)方法 ,代码如下:

protected Resource getResourceByPath(String path) {return new ClassPathContextResource(path, getClassLoader());}

步骤三,假如资源表达式以classpath开头,则截取除前缀calsspath:的路径,并做为ClassPathResource的构造参数,生成ClassPathResource实例后返回。咱们可以在web.xml中做如下配置:

<context-param> <param-name>contextConfigLocation</param-name> <param-value>classpath:/config/applicationContext.xml</param-value></context-param>

6、通过refresh()内部的实现我们大致可以了解整个refresh()方法担负了整个Spring容器初始化和加载的所有逻辑,包括Bean工厂的初始化、post-processor的注册以及调用、bean的实例化、事件发布等。

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