专题五：Spring源码之初始化容器上下文

上一篇我们通过如下一段基础代码作为切入点，最终找到核心的处理是refresh方法，从今天开始正式进入refresh方法的解读。

public class Main {public static void main(String[] args) {ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");JmUser jmUser = (JmUser)context.getBean("jmUser");System.out.println(jmUser.getName());System.out.println(jmUser.getAge());}
}

初始化容器上下文

首先还是整体看下refresh方法

	@Overridepublic void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing. 1、初始化上下文信息，替换占位符、必要参数的校验prepareRefresh();// Tell the subclass to refresh the internal bean factory. 2、解析类Xml、初始化BeanFactoryConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); // 这一步主要是对初级容器的基础设计// Prepare the bean factory for use in this context. 	3、准备BeanFactory内容：prepareBeanFactory(beanFactory); // 对beanFactory容器的功能的扩展：try {// Allows post-processing of the bean factory in context subclasses. 4、扩展点加一：空实现，主要用于处理特殊Bean的后置处理器postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context. 	5、spring bean容器的后置处理器invokeBeanFactoryPostProcessors(beanFactory);// Register bean processors that intercept bean creation. 	6、注册bean的后置处理器registerBeanPostProcessors(beanFactory);// Initialize message source for this context.	7、初始化消息源initMessageSource();// Initialize event multicaster for this context.	8、初始化事件广播器initApplicationEventMulticaster();// Initialize other special beans in specific context subclasses. 9、扩展点加一：空实现；主要是在实例化之前做些bean初始化扩展onRefresh();// Check for listener beans and register them.	10、初始化监听器registerListeners();// Instantiate all remaining (non-lazy-init) singletons.	11、实例化：非兰加载BeanfinishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.	 12、发布相应的事件通知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();}}}

首先将目标聚焦在第一个方法prepareRefresh方法上，根据方法名称和注释，我们大概可以猜测到该方法是在容器初始化前做些准备工作。

有了这个想法我来具体看下这个方法到底干了什么？

/*** Prepare this context for refreshing, setting its startup date and* active flag as well as performing any initialization of property sources.* 一些初始化设置如：设置容器开始事件、容器状态active设置激活】初始化配置源等。* 1.1、其中关注初始化配置源：这个也是留给子类自己实现，扩展点加一* 1.2、容器初始化的时候，校验必须的配置是否为空，当我们自己对原框架修改的时候，可以通过这个属性加上必要的配置判断**/protected void prepareRefresh() {// Switch to active.this.startupDate = System.currentTimeMillis();this.closed.set(false);this.active.set(true);if (logger.isDebugEnabled()) {if (logger.isTraceEnabled()) {logger.trace("Refreshing " + this);}else {logger.debug("Refreshing " + getDisplayName());}}// Initialize any placeholder property sources in the context environment.// 初始化替换占位符为实际值initPropertySources();// Validate that all properties marked as required are resolvable:// see ConfigurablePropertyResolver#setRequiredProperties// 容器初始化的时候，校验必须的配置是否为空getEnvironment().validateRequiredProperties();// Store pre-refresh ApplicationListeners...if (this.earlyApplicationListeners == null) {this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);}else {// Reset local application listeners to pre-refresh state.this.applicationListeners.clear();this.applicationListeners.addAll(this.earlyApplicationListeners);}// Allow for the collection of early ApplicationEvents,// to be published once the multicaster is available...this.earlyApplicationEvents = new LinkedHashSet<>();}

可以看到，该方法大部分时间只是做了初始化的设置如开始时间、容器状态初始化等，聚焦下

initPropertySources方法

	/*** <p>Replace any stub property sources with actual instances.* @see org.springframework.core.env.PropertySource.StubPropertySource* @see org.springframework.web.context.support.WebApplicationContextUtils#initServletPropertySources*/protected void initPropertySources() {// For subclasses: do nothing by default.}

Spring最经典的设计之一，空实现方法方法的权限级别为protected。给子类自己实现，扩展点加一。这里单独提出来和大家看看，因为后面我们能看到很多类似的代码。这也是Spring是一个易扩展框架的原因之一。

说完这个方法的设计，下面再来看看这个方法具体干了什么。看注释说是为了替换占位符。既然这样我们自己来重写这个方法试试看就知道啦。重写代码如下：

public class MyselfClassPathXmlApplicationContext extends ClassPathXmlApplicationContext {/*** Create a new ClassPathXmlApplicationContext, loading the definitions* from the given XML file and automatically refreshing the context.* @param configLocations resource location* @throws BeansException if context creation failed*/public MyselfClassPathXmlApplicationContext(String... configLocations) throws BeansException {super(configLocations);}@Overrideprotected void initPropertySources() {System.out.println("自定义 initPropertySources");getEnvironment().getSystemProperties().put("systemOS", "mac");}public class Main {public static void main(String[] args) {ApplicationContext context = new MyselfClassPathXmlApplicationContext("applicationContext.xml");JmUser jmUser = (JmUser)context.getBean("jmUser");System.out.println(jmUser.getName());System.out.println(jmUser.getAge());}
}

执行完initPropertySources方法以后，发现环境变量多了我们设置的代码systemOS，后续在需要的地方可以替换成我们所需要的值。

接着我们来看prepareRefresh下一个方法：

getEnvironment().validateRequiredProperties();

老样子根据注释和方法名称简答猜测一下，应该是用来校验是否需要检验某个必须的属性。猜测后进入代码验证一波。

看代码是自己的成员属性propertyResolver进行调用的，在进入方法看下：

@Overridepublic void validateRequiredProperties() {MissingRequiredPropertiesException ex = new MissingRequiredPropertiesException();for (String key : this.requiredProperties) {if (this.getProperty(key) == null) {ex.addMissingRequiredProperty(key);}}if (!ex.getMissingRequiredProperties().isEmpty()) {throw ex;}}

上述代码主要是遍历requirePrpperties属性，将不存在的key存入ex中，待循环结束以后抛出异常。目前我们的代码属性为空。我们再改写下上述代码看看。

package org.springframework;import org.springframework.beans.BeansException;
import org.springframework.context.support.ClassPathXmlApplicationContext;/*** @author Jeremy* @version 1.0* @description: 自定义容器* @date 2024/7/1 20:17*/
public class MyselfClassPathXmlApplicationContext extends ClassPathXmlApplicationContext {/*** Create a new ClassPathXmlApplicationContext, loading the definitions* from the given XML file and automatically refreshing the context.* @param configLocations resource location* @throws BeansException if context creation failed*/public MyselfClassPathXmlApplicationContext(String... configLocations) throws BeansException {super(configLocations);}@Overrideprotected void initPropertySources() {System.out.println("自定义 initPropertySources");
//		getEnvironment().getSystemProperties().put("systemOS", "mac");getEnvironment().setRequiredProperties("systemOS");}
}

堆栈日志如下

自定义 initPropertySources
Disconnected from the target VM, address: 'localhost:50403', transport: 'socket'
Exception in thread "main" org.springframework.core.env.MissingRequiredPropertiesException: The following properties were declared as required but could not be resolved: [systemOS]
   at org.springframework.core.env.AbstractPropertyResolver.validateRequiredProperties(AbstractPropertyResolver.java:145)
   at org.springframework.core.env.AbstractEnvironment.validateRequiredProperties(AbstractEnvironment.java:519)
   at org.springframework.context.support.AbstractApplicationContext.prepareRefresh(AbstractApplicationContext.java:602)
   at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:522)
   at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:148)
   at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:95)
   at org.springframework.MyselfClassPathXmlApplicationContext.<init>(MyselfClassPathXmlApplicationContext.java:20)
   at org.springframework.Main.main(Main.java:15)

放开上面注释：正常运行。通过上述两个简单的实例我们可以通过重写上述代码为我们Spring容器提供基础的校验和设置对应的值。方便后续开发。到这里我们初始化容器上下文prepareRefresh方法告一段落。