Spring 启动流程源码解析

落日映苍穹つ 2024-04-18 22:35 16阅读 0赞

#### 文章目录 ####

*  1. Spring 启动配置
 *  2. Spring 启动流程
 *   *   *  2.1 Spring 基于 ServletContext 创建 RootContext
         *  2.2 Spring servlet 基于 RootContext 创建 WebApplicationContext
 *  3. 总结

## 1. Spring 启动配置 ##

Spring的启动是基于 web 容器的，所有 web工程的初始配置都写在 web.xml 中，该文件一般配置了context 参数，servlet 和监听器（listener）。< context-param >是初始化 Context 的配置，< listener >调用 Spring 包中的 `ContextLoaderListener` ，用于监听 web 容器初始化事件，并加载相关配置

<context-param>
    		<param-name>contextConfigLocation</param-name>
    		<param-value>classpath:spring-mybatis.xml</param-value>
    	</context-param>
    
    	<listener>
    		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    	</listener>
    
    
    	<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>
    		<load-on-startup>1</load-on-startup>
    		<async-supported>true</async-supported>
    	</servlet>

## 2. Spring 启动流程 ##

Spring 的启动流程可以分为两部分：

1.  基于 web 容器的全局域 ServletContext 创建 WebApplicationContext作为 RootContext，也就是整个框架的核心容器
2.  配置的其他 Spring servlet 基于 RootContext 创建自己的 WebApplicationContext，从而持有自己的 bean 空间

#### 2.1 Spring 基于 ServletContext 创建 RootContext ####

1.  Spring 的启动其实就是 IoC 容器的启动过程，其核心监听器 ContextLoaderListener 父类是 ContextLoader，实现了 ServletContextListener 接口，在容器启动时会触发其 contextInitialized 初始化方法
    
        public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
              
         /**
          * Initialize the root web application context.
          */
         @Override
         public void contextInitialized(ServletContextEvent event) {
              
         	initWebApplicationContext(event.getServletContext());
         }
        }
2.  此处 initWebApplicationContext() 是 ContextLoader 中的方法， 该方法用于对 整个Spring 框架的ApplicationContext 进行初始化，在这里进入了spring IoC的初始化。  
    这个方法主要做了三件事：  
    【1】`createWebApplicationContext()`实际创建 XmlWebApplicationContext 作为 RootContext  
    【2】`configureAndRefreshWebApplicationContext()`加载 Spring 配置文件中的配置并创建 bean  
    【3】`servletContext.setAttribute()`将 WebApplicationContext 放入 ServletContext 全局域
    
        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!");
           }
           
           ······
           
           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);
           	}
        
           	······
           	
           	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;
           }
        }
3.  `configureAndRefreshWebApplicationContext(cwac, servletContext)`会从 web.xml 中读取 contextConfigLocation 配置，也就是spring xml文件配置，将其存入 WebApplicationContext 中，最后调用`refresh()` 方法执行所有Java对象的创建工作。
    
        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 contextConfigLocation 配置文件
        	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(); // Spring IOC 创建 Bean
        }
4.  `refresh()` 方法的实现在 `AbstractApplicationContext`类中，其主要方法功能如图所示。
    
        public void refresh() throws BeansException, IllegalStateException {
              
        	synchronized (this.startupShutdownMonitor) {
              
        		// Prepare this context for refreshing.
        		prepareRefresh();
        
        		// Tell the subclass to refresh the internal bean factory.
        		ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
        
        		// Prepare the bean factory for use in this context.
        		prepareBeanFactory(beanFactory);
        
        		try {
              
        			// Allows post-processing of the bean factory in context subclasses.
        			postProcessBeanFactory(beanFactory);
        
        			// Invoke factory processors registered as beans in the context.
        			invokeBeanFactoryPostProcessors(beanFactory);
        
        			// Register bean processors that intercept bean creation.
        			registerBeanPostProcessors(beanFactory);
        
        			// Initialize message source for this context.
        			initMessageSource();
        
        			// Initialize event multicaster for this context.
        			initApplicationEventMulticaster();
        
        			// Initialize other special beans in specific context subclasses.
        			onRefresh();
        
        			// Check for listener beans and register them.
        			registerListeners();
        
        			// Instantiate all remaining (non-lazy-init) singletons.
        			finishBeanFactoryInitialization(beanFactory);
        
        			// Last step: publish corresponding event.
        			finishRefresh();
        		}
        
        		catch (BeansException ex) {
              
        			if (logger.isWarnEnabled()) {
              
        				logger.warn("Exception encountered during context initialization - " +
        						"cancelling refresh attempt: " + ex);
        			}
        
        			// Destroy already created singletons to avoid dangling resources.
        			destroyBeans();
        
        			// Reset 'active' flag.
        			cancelRefresh(ex);
        
        			// Propagate exception to caller.
        			throw ex;
        		}
        
        		finally {
              
        			// Reset common introspection caches in Spring's core, since we
        			// might not ever need metadata for singleton beans anymore...
        			resetCommonCaches();
        		}
        	}
        }

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#### 2.2 Spring servlet 基于 RootContext 创建 WebApplicationContext ####

1.  contextLoaderListener 监听器初始化完毕后，开始初始化web.xml中配置的 servlet。servlet可以配置多个，以最常见的DispatcherServlet为例，DispatcherServlet 在初始化的时候会建立自己的 IoC context，用以持有spring mvc相关的bean。DispatcherServlet 继承关系如下图，web容器启动时 servlet 的调用链如下：

GenericServlet#init()->HttpServletBean#init()->FrameworkServlet#initServletBean()->initWebApplicationContext()

此处 FrameworkServlet \# initWebApplicationContext() 方法与 Spring框架创建 RootContext 流程大致相同，分为以下几步，只不过设置的 parent context 不是 ServletConext 而是 Spring 核心容器 RootContext  
【1】`WebApplicationContextUtils.getWebApplicationContext`获取 RootContext  
【2】使用已有或者新建的 WebApplicationContext 加载 SpringMVC 配置文件中的配置并创建 bean  
【3】`onRefresh()`调用 `DispatcherServlet#initStrategies()` 进行 servlet 初始化  
【4】将 servlet 自己的 WebApplicationContext 存入 ServletContext

protected WebApplicationContext initWebApplicationContext() {
        
    		// 获取 root context
    		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;
    	}

![在这里插入图片描述][2019091716405591.png_pic_center]

## 3. 总结 ##

1.  一个web应用部署在 web 容器中，web容器为其提供一个全局域 ServletContext，作为spring IoC容器 WebApplicationContext 的宿主环境
2.  在web.xml 中配置的 contextLoaderListener 会在容器启动时初始化 一个WebApplicationContext 作为 RootContext。这是一个接口类，其实际的实现类是 XmlWebApplicationContext (在 `ContextLoader# determineContextClass()`方法中决定)。这个就是 spring 的核心 IoC 容器，其对应 bean 定义的配置由web.xml 中的 context-param 标签指定，并通过 `refresh()方法`完成 bean 创建。IoC容器初始化完毕后，Spring 将以 `WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE`作为 Key，将创建的 XmlWebApplicationContext 对象存储到 ServletContext 中，便于之后作为 RootContext 使用
    
        protected Class<?> determineContextClass(ServletContext servletContext) {
              
            // 如果在 web.xml 中直接指定了 ContextClass
         	String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
         	if (contextClassName != null) {
              
         		try {
              
         			return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
         		}
         		catch (ClassNotFoundException ex) {
              
         			throw new ApplicationContextException(
         					"Failed to load custom context class [" + contextClassName + "]", ex);
         		}
         	}
         	else {
               
         	// 没有直接指定，则读取属性文件 ContextLoader.properties 的配置
         	/** org.springframework.web.context.WebApplicationContext=
             org.springframework.web.context.support.XmlWebApplicationContext
             */
         		contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
         		try {
              
         			return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
         		}
         		catch (ClassNotFoundException ex) {
              
         			throw new ApplicationContextException(
         					"Failed to load default context class [" + contextClassName + "]", ex);
         		}
         	}
         }
3.  contextLoaderListener 监听器初始化完毕后，开始初始化 web.xml 中配置的servlet。以DispatcherServlet为例，DispatcherServlet 在初始化的时候会建立自己的 context，用以持有spring mvc相关的 bean，并完成 bean 的创建。初始化时设置其 parent context 为 Spring 的核心容器 RootContext，这样每个 servlet 都拥有自己的 context，即拥有自己独立的bean空间，同时又共享 RootContext 中定义的那些bean。当 Spring 组件在执行 getBean 时，如果在自己的 context 中找不到对应的bean，则会在父ApplicationContext (通常为Spring 核心容器 RootContext)中去找，这也就解释了在 DispatcherServlet 中为何可以获取到由 ContextLoaderListener 创建的ApplicationContext中的bean。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NTUwNTMxMw_size_16_color_FFFFFF_t_70 1]

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[2019091716405591.png_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/18/a4046dbc3e8b47e3bb5cecbefb3a3719.png
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