本文基于Spring 5.2.2.RELEASE进行Spring启动流程的梳理，只讨论大致流程的实现，其中一些重要的具体细节实现在后续文章中介绍。
Step0. Spring启动从下面自定义的程序开始，我们的旅程也从这里开始。
public class SpringApplication {
public static void main(String[] args){
ApplicationContext context = new ClassPathXmlApplicationContext("application_context.xml");
User user = (User) context.getBean("user");
user.setName("雁阵惊寒");
System.out.println(user);
}
}
启动main方法，程序执行结果如下。
User{name='雁阵惊寒'}
程序中我们没有显式new一个新的User对象，而是通过一个xml配置文件application_context.xml进行配置，由Spring读取配置文件后创建的。这里相当于将本应该属于程序代码自己的对象初始化权限交给了Spring，所以称为控制反转（IoC，Inversion of Control），又因为是依赖于配置文件对对象进行初始化，也称为依赖注入（Dependency Injection）。两者本质上是相同的概念。
配置文件application_context.xml如下。
User类定义如下。
public class User {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "User{" +
"name='" + name + '\'' +
'}';
}
}
Step1. 创建ClassPathXmlApplicationContext上下文对象。
ApplicationContext context = new ClassPathXmlApplicationContext("application_context.xml");
进入到ClassPathXmlApplicationContext()构造函数中。setConfigLocations()函数将传入xml配置进行保存，代码中传入文件字符中的特殊意义符号，如${}也在这里进行处理转换。
public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
this(new String[] {configLocation}, true, null);
}
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
Step2. 进入refresh()函数，refresh()函数是Spring的核心函数，后起之秀Spring Boot也复用这里的逻辑，虽然具体实现类略有不同，但是思路大体一致。具体参看后续其他文章会详细讲解refresh()函数。这里只在源码中中文注释重要的函数功能，重要功能的界定原则是那些创建Bean的函数、为创建Bean初始化重要数据结构函数、进行AOP代理对象生成的函数。同时也标注Spring Boot相较于Spring的一些变动。
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
//获取beanFactory
//Spring会在这一步中解析xml文件转换为BeanDefinition结构
//Spring Boot会将beanFactory创建提前到与容器创建时一起，
//另外Spring Boot BeanDefinition的注册延后到了invokeBeanFactoryPostProcessors函数中完成
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.
//Spring激活beanFactory处理器
//Spring Boot BeanDefinition的注册在这里完成
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
//Spring注册拦截Bean创建的Bean处理器
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.
//Spring初始化剩余的非惰性加载的单例
//编写程序定义的Bean在这里创建
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();
}
}
}
Step3. 根据xml文件的配置，实际上走到这里示例代码中的User对象已经完成创建，我们具体看context.getBean("user")是如何获取对象的。
User user = (User) context.getBean("user");
Step4. 进入类AbstractApplicationContext中的getBean()函数。这里的getBeanFactory()函数走到AbstractRefreshableApplicationContext拿到beanFactory。beanFactory是在refresh()时进行的初始化。
@Override
public Object getBean(String name) throws BeansException {
assertBeanFactoryActive();
return getBeanFactory().getBean(name);
}
Step5. 类AbstractBeanFactory中的getBean()函数调用自身的doGetBean()函数。因为此时User对象已经创建（Step2中创建的），getSingleton()函数从容器缓存中获取Bean返回sharedInstance对象不为null，走if逻辑。
protected T doGetBean(final String name, @Nullable final Class requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
//getSingleton()函数从容器缓存中获取Bean
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
//...因为此时User对象已经创建，不会再走else逻辑，暂时略去...
}
// Check if required type matches the type of the actual bean instance.
//...略去暂时不用的代码...
return (T) bean;
}
getSingleton()调用类AbstractBeanFactory的父类DefaultSingletonBeanRegistry中的函数getSingleton()。singletonObjects是一个map结构，存储着创建后的单例Bean。singletonObject对象不为null，直接返回。
下面这段getSingleton()函数代码展示了Spring解决循环依赖的三级缓存结构。
- 一级缓存singletonObjects
- 二级缓存earlySingletonObjects
- 三级缓存singletonFactories
后续其他文章会有详细介绍Spring如何解决循环依赖问题。
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//singletonObjects是一个map结构，存储着创建后的单例Bean。
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
Step6. getObjectForBeanInstance()函数获取最终的对象。
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
getObjectForBeanInstance()函数中，因为User对象本身并不是FactoryBean，直接返回传入的对象本身。
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
if (!(beanInstance instanceof FactoryBean)) {
return beanInstance;
}
Step7. 到此为止，代码已经获取了Spring创建的User对象，与手动new的新对象一样。将对象创建移交给Spring处理，简化了代码编写复杂度，程序员不需要自己关注对象的初始化，也不需要编写复杂的代码逻辑对对象进行维护。Spring用容器维护对象，做到了随取随用。
对User对象的name字段进行赋值，然后打印。
User user = (User) context.getBean("user");
user.setName("雁阵惊寒");
System.out.println(user);