优质博文：IT-BLOG-CN
一、Spring Boot应用启动
一个Spring Boot应用的启动通常如下：
@SpringBootApplication
@Slf4j
public class ApplicationMain {
public static void main(String[] args) {
ConfigurableApplicationContext ctx = SpringApplication.run(ApplicationMain.class, args);
}
}
执行如上代码，Spring Boot程序启动成功。事实上启动Spring Boot应用离不开SpringApplication。
所以，我们跟随SpringApplication的脚步，开始从源码角度分析Spring Boot的初始化过程。
btw，可参看例子一节，我对Spring Boot启动的拓展点都做了demo，可参照下面源码分析进行理解。
文档有一句话说了SpringApplication做了什么（目的）：
Create an appropriate ApplicationContext instance (depending on your classpath) Register a CommandLinePropertySource to expose command line arguments as Spring properties Refresh the application context, loading all singleton beans Trigger any CommandLineRunner beans
二、SpringApplication构造函数
启动代码先创建SpringApplication示例，在执行run方法：
public static ConfigurableApplicationContext run(Class<?>[] primarySources,
String[] args) {
return new SpringApplication(primarySources).run(args);
}
如下是SpringApplication的构造函数代码分析。
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
//通过Classloader探测不同web应用核心类是否存在，进而设置web应用类型
this.webApplicationType = WebApplicationType.deduceFromClasspath();
//找出所有spring.factories中声明的ApplicationContextInitializer并设置，
//ApplicationContextInitializer定义了回调接口，在refresh()前初始化调用（即在prepareContext的applyInitializers方法中调用）
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));
//找出所有spring.factories中声明的ApplicationListener（细节往后再叙），ApplicationListener继承了
//java.util.EventListener，实现了类似观察者模式的形式，通过实现ApplicationListener、SmartApplicationListener，能够监听Spring上下文的refresh、Prepared等事件或者是自定义事件
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
//找出主启动类（有趣的是，是通过new一个runtime异常然后在异常栈里面找出来的）
this.mainApplicationClass = deduceMainApplicationClass();
在构造期间，主要做了：
1、判定应用类型，为后面创建不同类型的spring context做准备。
2、初始化ApplicationContextInitializer和ApplicationListener。
3、找出启动类。
三、run()源码解析
介绍run()方法前，先说说贯穿run方法的ApplicationRunListener，它有助于理解整个run()的运行周期。
写在这里：Spring Application事件机制
run()方法分析如下：
//java.awt.headless，是J2SE的一种模式，用于在缺失显示屏、鼠标或者键盘时的系统配置。
configureHeadlessProperty();
//将spring.factories中的SpringApplicationRunListener接口实现类拖出来，塞到SpringApplicationRunListeners（一个集合）中，统一批量执行
SpringApplicationRunListeners listeners = getRunListeners(args);
//触发runlistener的starting
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
//spring.beaninfo.ignore如果没有设置值，则把它设为true，具体情况具体设置，
//如果没用的话，把它设为true可以ignore掉classloader对于不存在的BeanInfo的扫描，提高性能。
configureIgnoreBeanInfo(environment);
//banner打印。自定义banner挺好玩的
Banner printedBanner = printBanner(environment);
//根据webApplicationType（一开始推断的应用类型）去新建applicationContext
context = createApplicationContext();
//获取SpringBootExceptionReporter，回调接口类，提供启动时的异常报告
exceptionReporters = getSpringFactoriesInstances(
SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
//下面会说
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
refreshContext(context);
//do nothing
afterRefresh(context, applicationArguments);
//计时停止
stopWatch.stop();
//打日志
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
//启动
listeners.started(context);
//找出context的ApplicationRunner和CommandLineRunner，用AnnotationAwareOrderComparator排序，并执行
callRunners(context, applicationArguments);
下面再分别说说两个方法（prepareEnvironment、refreshContext）的代码。
四、prepareEnvironment
private ConfigurableEnvironment prepareEnvironment(
SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments) {
// Create and configure the environment
ConfigurableEnvironment environment = getOrCreateEnvironment();
configureEnvironment(environment, applicationArguments.getSourceArgs());
//发布environment prepared事件
listeners.environmentPrepared(environment);
//将获取到的environment中的spring.main配置绑定到SpringApplication中,
//使用的是Binder这个spring boot2.0开始有的类
bindToSpringApplication(environment);
if (!this.isCustomEnvironment) {
environment = new EnvironmentConverter(getClassLoader())
.convertEnvironmentIfNecessary(environment, deduceEnvironmentClass());
}
//附加的解析器将动态跟踪底层 Environment 属性源的任何添加或删除，
//关于ConfigurationPropertySourcesPropertySource和MutablePropertiySource
//将在Environment中作进一步讲解
ConfigurationPropertySources.attach(environment);
return environment;
}
五、prepareContext
private void prepareContext(ConfigurableApplicationContext context,
ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments, Banner printedBanner) {
//为上下文设置environment（配置、profile）
context.setEnvironment(environment);
//对application做一些处理，设置一些组件，
//比如BeanNameGenerator，ApplicationConversionService（包含一些默认的Converter和formatter）
postProcessApplicationContext(context);
// 加载并运行ApplicationContextInitializer
applyInitializers(context);
listeners.contextPrepared(context);
if (this.logStartupInfo) {
logStartupInfo(context.getParent() == null);
logStartupProfileInfo(context);
}
// Add boot specific singleton beans
ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
if (printedBanner != null) {
beanFactory.registerSingleton("springBootBanner", printedBanner);
}
if (beanFactory instanceof DefaultListableBeanFactory) {
((DefaultListableBeanFactory) beanFactory)
.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
}
// Load the sources
Set<Object> sources = getAllSources();
Assert.notEmpty(sources, "Sources must not be empty");
//Load beans（其实是由sources构建beanDefinition） into the application context.
//构建BeanDefinitionLoader并执行BeanDefinitionLoader.load()
load(context, sources.toArray(new Object[0]));
//执行contextLoaded事件
listeners.contextLoaded(context);
}
六、容器refresh（refreshContext）
private void refreshContext(ConfigurableApplicationContext context) {
refresh(context);
if (this.registerShutdownHook) {
try {
context.registerShutdownHook();
}
catch (AccessControlException ex) {
// Not allowed in some environments.
}
}
}
refreshContext会做两件事，
1、应用上下文刷新
2、注册shutdown钩子
我们来看看ServletWebServer的刷新。
// ServletWebServerApplicationContext
public final void refresh() throws BeansException, IllegalStateException {
try {
super.refresh();
}
catch (RuntimeException ex) {
//停止webserver
stopAndReleaseWebServer();
throw ex;
}
}
org.springframework.context.support.AbstractApplicationContext refresh()
public void refresh() throws BeansException, IllegalStateException {
// 单线程执行
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
// 1、设置Spring容器的启动时间，撤销关闭状态，开启活跃状态。2、初始化属性源信息(Property)3、验证环境信息里一些必须存在的属性
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
// 如果是RefreshtableApplicationContext会做了很多事情：
// 1、让子类刷新内部beanFactory ，创建IoC容器（DefaultListableBeanFactory--ConfigurableListableBeanFactory 的实现类）
// 2、加载解析XML文件（最终存储到Document对象中）
// 3、读取Document对象，并完成BeanDefinition的加载和注册工作
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
//从Spring容器获取BeanFactory(Spring Bean容器)并进行相关的设置为后续的使用做准备：
//1、设置classloader(用于加载bean)，设置表达式解析器(解析bean定义中的一些表达式)，添加属性编辑注册器(注册属性编辑器)
//2、添加ApplicationContextAwareProcessor这个BeanPostProcessor。取消ResourceLoaderAware、ApplicationEventPublisherAware、MessageSourceAware、ApplicationContextAware、EnvironmentAware这5个接口的自动注入。因为ApplicationContextAwareProcessor把这5个接口的实现工作做了
//3、设置特殊的类型对应的bean。BeanFactory对应刚刚获取的BeanFactory；ResourceLoader、ApplicationEventPublisher、ApplicationContext这3个接口对应的bean都设置为当前的Spring容器
//4、注入一些其它信息的bean，比如environment、systemProperties等
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context. 执行beanfactoryPostProcessor
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
// 注册beanPostProcessor
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context. 初始化messageSource
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();
}
}
}
七、postProcessBeanFactory()
设置BeanFactory之后再进行后续的一些BeanFactory操作。
不同的Context会进行不同的操作。 比如，AnnotationConfigServletWebServerApplicationContext
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
// 父类实现，会注册web应用特有的factory scope，
super.postProcessBeanFactory(beanFactory);
//查看basePackages属性，如果设置了会使用ClassPathBeanDefinitionScanner去扫描basePackages包下的bean并注
if (this.basePackages != null && this.basePackages.length > 0) {
this.scanner.scan(this.basePackages);
}
// 查看annotatedClasses属性，如果设置了会使用AnnotatedBeanDefinitionReader去注册这些bean
if (!this.annotatedClasses.isEmpty()) {
this.reader.register(ClassUtils.toClassArray(this.annotatedClasses));
}
}
八、invokeBeanFactoryPostProcessors()
/**
* Instantiate and invoke all registered BeanFactoryPostProcessor beans,
* respecting explicit order if given.
* <p>Must be called before singleton instantiation.
*/
protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
//执行AbstractContext持有的BeanFactory后置处理器
//这些处理器是之前ContextInitializer
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
// 如果通过-javaagent参数设置了LTW的织入器类包，那么增加LTW的BeanProcessor。
if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
}
}
从容器中找出BeanDefinitionRegistryPostProcessor、BeanFactoryPostProcessor（二者的区别是，一个使用BeanDefinitionRegistry作处理，一个使用BeanFactory做处理）， 并按一定的规则顺序执行。
ConfigurationClassPostProcessor的优先级为最高，它会对项目中的@Configuration注解修饰的类(@Component、@ComponentScan、@Import、@ImportResource修饰的类也会被处理)进行解析，解析完成之后把这些bean注册到BeanFactory中。 需要注意的是这个时候注册进来的bean还没有实例化。
ConfigurationClassPostProcessor的流程之后会独立进行分析。
九、registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory)方法
···java
/**
- Instantiate and invoke all registered BeanPostProcessor beans,
- respecting explicit order if given.
- Must be called before any instantiation of application beans.
*/
protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
//委派PostProcessorRegistrationDelegate去做
PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}
从Spring容器中按一定顺序（PriorityOrdered、Ordered、非PriorityOrdered非Ordered）找出实现了BeanPostProcessor接口的bean，并设置到BeanFactory的属性中。之后bean被实例化的时候会调用这个BeanPostProcessor。
## 十、initMessageSource()
初始化一些国际化相关的属性。
Spring boot的国际化配置可阅读MessageSourceAutoConfiguration。 默认情况会设置一个DelegatingMessageSource，是一个空实现，因为ApplicationContext接口拓展了MessageSource接口，所以Spring容器都有getMessage方法， 可是，在实现上又允许空MessageSource，所以，通过一个DelegatingMessageSource去适配。
## 十一、initApplicationEventMulticaster()
Initialize event multicaster for this context. 初始化事件广播器。默认实现是SimpleApplicationEventMulticaster。
onRefresh()
模板方法，给不同的Spring应用容器去实例化一些特殊的类。
比如，AnnotationConfigServletWebServerApplicationContext、AnnotationConfigReactiveWebServerApplicationContext会去创建web server（createWebServer()）。 spring boot的mvc内置支持有tomcat、Undertow、jetty三种server，而reactive web server则内置支持tomcat、jetty、netty三种。
java
// Unlike Jetty, all Tomcat threads are daemon threads. We create a
// blocking non-daemon to stop immediate shutdown
startDaemonAwaitThread();
btw，如果是tomcat server的话，spring boot会启动多一个线程防止退出。
十二、registerListeners()
把BeanFactory的ApplicationListener拿出来塞到事件广播器里。
如果ApplicationContext的earlyApplicationEvents属性有值，则广播该属性持有的early事件。
十三、finishBeanFactoryInitialization(beanFactory)
实例化BeanFactory中已经被注册但是未实例化的所有实例(懒加载的不需要实例化)。
比如invokeBeanFactoryPostProcessors方法中根据各种注解解析出来的类，在这个时候都会被初始化。
十四、finishRefresh()
// ReactiveWebServerApplicationContext
@Override
protected void finishRefresh() {
super.finishRefresh();
WebServer webServer = startReactiveWebServer();
if (webServer != null) {
publishEvent(new ReactiveWebServerInitializedEvent(webServer, this));
}
}
// AbstractApplicationContext
/**
* Finish the refresh of this context, invoking the LifecycleProcessor's
* onRefresh() method and publishing the
* {@link org.springframework.context.event.ContextRefreshedEvent}.
*/
protected void finishRefresh() {
// Clear context-level resource caches (such as ASM metadata from scanning).
// 容器完成刷新，清除资源缓存
clearResourceCaches();
// Initialize lifecycle processor for this context.
// 初始化lifeCycleProcessor, 默认实现是DefaultLifeCycleProcessor，实现了BeanFactoryAware接口，通过BeanFactory找出LifeCycle bean
// 可通过自定义实现LifeCycle接口的Bean，来监听容器的生命周期。
initLifecycleProcessor();
// Propagate refresh to lifecycle processor first.
//粗发生命周期处理器的onRefresh方法，顺带一说，在程序正常退出时，会粗发shutdownHook，那时会粗发生命周期处理器的onClose方法
getLifecycleProcessor().onRefresh();
// Publish the final event.
// 广播ContextRefreshed事件
publishEvent(new ContextRefreshedEvent(this));
// Participate in LiveBeansView MBean, if active.
// 将ApplicationContext注册到Spring tool suite里
LiveBeansView.registerApplicationContext(this);
}
十五、resetCommonCaches()
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
最后会在finally执行resetCommonCaches()，清除一些Spring core、beans加载和解析的Bean信息缓存（因为对于singleton bean来说已经不需要了）。
十六、流程整理
最后，按照启动阶段整理一幅全景图。
十七、例子
在github里，我把Spring Boot应用启动的拓展组件（自定义的应用初始器、监听器、事件、ApplicationRunner）都写了例子，可参照阅读。 代码在这 | spring-boot-none-startup
日志如下：
2020-05-20 18:30:11.625 INFO 81568 --- [ main] n.t.d.s.b.s.n.s.r.SimpleRunListener : environmentPrepared, env:StandardEnvironment {activeProfiles=[dev], defaultProfiles=[default], propertySources=[MapPropertySource {name='systemProperties'}, OriginAwareSystemEnvironmentPropertySource {name='systemEnvironment'}, RandomValuePropertySource {name='random'}, OriginTrackedMapPropertySource {name='applicationConfig: [classpath:/application-dev.yml]'}, OriginTrackedMapPropertySource {name='applicationConfig: [classpath:/application.yml]'}]}
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/ )| |)| | | | | || (| | ) ) ) )
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=========||==============|/=///_/
:: Spring Boot :: (v2.1.3.RELEASE)
2020-05-20 18:30:11.832 INFO 81568 --- [ main] n.t.d.s.b.s.n.s.r.SimpleRunListener : contextPrepared, ctx:org.springframework.context.annotation.AnnotationConfigApplicationContext@1d730606, started on Thu May 01 08:00:00 CST 1970
2020-05-20 18:30:11.838 INFO 81568 --- [ main] n.t.d.s.b.startup.none.ApplicationMain : Starting ApplicationMain on DESKTOP-OLDGHC1 with PID 81568 ( started by teash in )
2020-05-20 18:30:11.838 INFO 81568 --- [ main] n.t.d.s.b.startup.none.ApplicationMain : The following profiles are active: dev
2020-05-20 18:30:11.894 INFO 81568 --- [ main] n.t.d.s.b.s.n.s.r.SimpleRunListener : contextLoaded, context: org.springframework.context.annotation.AnnotationConfigApplicationContext@1d730606, started on Thu May 01 08:00:00 CST 1970
2020-05-20 18:30:12.404 INFO 81568 --- [ main] .s.b.s.n.s.SimpleApplicationContextAware : SimpleApplicationContextAware and send SimpleAppEvent
2020-05-20 18:30:12.441 INFO 81568 --- [ main] n.t.d.s.b.s.n.s.e.SimpleEventListener : event: net.teaho.demo.spring.boot.startup.none.spring.event.SimpleAppEvent[source=event source], source: event source
2020-05-20 18:30:12.444 INFO 81568 --- [ main] n.t.d.s.b.s.n.config.BeanConfiguration : [net.teaho.demo.spring.boot.startup.none.spring.spi.DemoSpringLoaderImpl@c96a4ea]
2020-05-20 18:30:12.484 INFO 81568 --- [ main] n.t.d.s.b.s.n.s.l.LoggingLifeCycle : In Life cycle bean start().
2020-05-20 18:30:12.496 INFO 81568 --- [ main] n.t.d.s.b.startup.none.ApplicationMain : Started ApplicationMain in 1.573 seconds (JVM running for 3.195)
2020-05-20 18:30:12.496 INFO 81568 --- [ main] n.t.d.s.b.s.n.s.r.SimpleRunListener : started, context: org.springframework.context.annotation.AnnotationConfigApplicationContext@1d730606, started on Mon May 25 18:30:11 CST 2020
2020-05-20 18:30:12.497 INFO 81568 --- [ main] n.t.d.s.b.s.n.s.r.EchoApplicationRunner : EchoApplicationRunner running, args:org.springframework.boot.DefaultApplicationArguments@45673f68
2020-05-20 18:30:12.497 INFO 81568 --- [ main] n.t.d.s.b.s.n.s.r.EchoCommandLineRunner : EchoCommandLineRunner running
2020-05-20 18:30:12.497 INFO 81568 --- [ main] n.t.d.s.b.s.n.s.r.SimpleRunListener : running, context: org.springframework.context.annotation.AnnotationConfigApplicationContext@1d730606, started on Mon May 25 18:30:11 CST 2020
2020-05-20 18:30:12.500 INFO 81568 --- [ Thread-3] n.t.d.s.b.s.n.s.l.LoggingLifeCycle : In Life cycle bean stop().
