一直很好奇SpringBoot这么一个大怪物，启动的时候做了哪些事情，然后看了很多老师讲的教学视频，然后自己想好好整理一下，做下学习笔记下次也方便自己阅读

1.执行main方法

public static void main(String[] args) {
//代码很简单SpringApplication.run();
SpringApplication.run(ConsumerApp.class, args);
}

public static ConfigurableApplicationContext run(Class<?> primarySource,
String... args) {
//这个里面调用了run() 方法，我们转到定义
return run(new Class<?>[] { primarySource }, args);
}
//这个run方法代码也很简单，就做了两件事情
//1、new了一个SpringApplication() 这么一个对象
//2、执行new出来的SpringApplication()对象的run()方法
public static ConfigurableApplicationContext run(Class<?>[] primarySources,
String[] args) {
return new SpringApplication(primarySources).run(args);
}

上面代码主要做了两件事情。第一步new了一个SpringApplication对象，第二部调用了run()方法。接下来我们一起看下new SpringApplication() 主要做了什么事情。

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
//1、先把主类保存起来
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
//2、判断运行项目的类型
this.webApplicationType = WebApplicationType.deduceFromClasspath();
//3、扫描当前路径下META-INF/spring.factories文件的，加载ApplicationContextInitializer接口实例
setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
//4、扫描当前路径下META-INF/spring.factories文件的，加载ApplicationListener接口实例
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
this.mainApplicationClass = deduceMainApplicationClass();
}

利用SPI机制扫描 META-INF/spring.factories 这个文件，并且加载 ApplicationContextInitializer、ApplicationListener 接口实例。

1、ApplicationContextInitializer 这个类当springboot上下文Context初始化完成后会调用

2、ApplicationListener 当springboot启动时事件change后都会触发

下面我们来自定义ApplicationContextInitializer、ApplicationListener 接口实现类，然后Debug来看下效果。

/**
* Context初始化后调用类
* @author ShiMinChen
*
*/
public class StarterApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext> {
@Override
public void initialize(ConfigurableApplicationContext applicationContext) {
System.out.println("applicationContext 初始化完成 ... ");
}
}

public class StarterApplicationListener implements ApplicationListener {
@Override
public void onApplicationEvent(ApplicationEvent event) {
System.out.println(event.toString());
System.out.println("ApplicationListener .... " + System.currentTimeMillis());
}
}

我们需要把这两个类集成到springboot里面去，其实操作也挺简单的

然后在META-INF/spring.factories 文件配置那两个类

org.springframework.context.ApplicationContextInitializer=\
org.admin.starter.test.listener.StarterApplicationContextInitializer
org.springframework.context.ApplicationListener=\
org.admin.starter.test.listener.StarterApplicationListener

4、我们代码DEBUG一下，在loadSpringFactories() 方法打一个断点

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = cache.get(classLoader);
if (result != null)
return result;
try {
Enumeration<URL> urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
result = new LinkedMultiValueMap<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry<?, ?> entry : properties.entrySet()) {
List<String> factoryClassNames = Arrays.asList(
StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
result.addAll((String) entry.getKey(), factoryClassNames);
}
}
cache.put(classLoader, result);
// 端点打在这里就行了
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}

总结：上面就是SpringApplication初始化的代码，new SpringApplication()没做啥事情，利用SPI机制主要加载了META-INF/spring.factories 下面定义的事件监听器接口实现类

`2.执行run() 方法`

public ConfigurableApplicationContext run(String... args) {

StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();

configureHeadlessProperty();

SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
try {

ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);

ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);

configureIgnoreBeanInfo(environment);

Banner printedBanner = printBanner(environment);

context = createApplicationContext();

exceptionReporters = getSpringFactoriesInstances(
SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);

prepareContext(context, environment, listeners, applicationArguments,
printedBanner);

//这个refreshContext()加载了bean，还启动了内置web容器，需要细细的去看看
refreshContext(context);

afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}

listeners.started(context);

callRunners(context, applicationArguments);
}
catch (Throwable ex) {
handleRunFailure(context, listeners, exceptionReporters, ex);
throw new IllegalStateException(ex);
}
listeners.running(context);
return context;
}

`2.1 createApplicationContext()`

一起来看下context = createApplicationContext(); 这段代码，这段代码主要是根据项目类型创建上下文，并且会注入几个核心组件类。

protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
switch (this.webApplicationType) {
case SERVLET:
contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
break;
case REACTIVE:
contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
break;
default:
contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
}
}
catch (ClassNotFoundException ex) {
throw new IllegalStateException(
"Unable create a default ApplicationContext, please specify an ApplicationContextClass", ex);
}
}
return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

public AnnotationConfigServletWebServerApplicationContext(DefaultListableBeanFactory beanFactory) {
super(beanFactory);
//1：会去注入一些spring核心组件
this.reader = new AnnotatedBeanDefinitionReader(this);
this.scanner = new ClassPathBeanDefinitionScanner(this);
}

Web类型项目创建上下文对象 AnnotationConfigServletWebServerApplicationContext 。这里会把 ConfigurationClassPostProcessor 、AutowiredAnnotationBeanPostProcessor 等一些核心组件加入到Spring容器（扯淡有点远，这是spring容器启动的一些知识点，去了解一下对理解springboot有很大的帮助）

`2.2 refreshContext()`

下面一起来看下refreshContext(context) 这个方法，这个方法启动spring的代码加载了bean，还启动了内置web容器

private void refreshContext(ConfigurableApplicationContext context) {
// 转到定义看看
refresh(context);
if (this.registerShutdownHook) {
try {
context.registerShutdownHook();
}
catch (AccessControlException ex) {
// Not allowed in some environments.
}
}
}

protected void refresh(ApplicationContext applicationContext) {
Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
//看看refresh()方法去
((AbstractApplicationContext) applicationContext).refresh();
}

转到AbstractApplicationContext - >refresh()方法里面发现这是spring容器启动代码

@Override
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();
}
}
}

spring容器启动代码就不说了，这里主要看一下onRefresh() 这个方法。转到定义发现这个方法里面啥都没有，这明显是一个钩子方法，它会钩到它子类重写onRefresh()方法。所以去看子类里面的onRefresh()

protected void onRefresh() throws BeansException {
//这是一个空方法，AbstractApplicationContext 这个类是一个抽象类，
//所以我们要找到集成AbstractApplicationContext的子类，去看子类里面的onRefresh()
// For subclasses: do nothing by default.
}

我们这里是一个Web项目，所以我们就去看 ServletWebServerApplicationContext 这个类，我还是把类的关系图贴一下

我们就去看 ServletWebServerApplicationContext 这个类下面的 onRefresh() 方法

protected void onRefresh() {
super.onRefresh();
try {
//看到内置容器的影子了，进去看看
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}

private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
//1、这个获取webServerFactory还是要进去看看
ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
}
else if (servletContext != null) {
try {
getSelfInitializer().onStartup(servletContext);
}
catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context",
ex);
}
}
initPropertySources();
}

我们继续看下getWebServletFactory() 这个方法，这个里面其实就是选择出哪种类型的web容器了

protected ServletWebServerFactory getWebServerFactory() {
// Use bean names so that we don't consider the hierarchy
String[] beanNames = getBeanFactory()
.getBeanNamesForType(ServletWebServerFactory.class);
if (beanNames.length == 0) {
throw new ApplicationContextException(
"Unable to start ServletWebServerApplicationContext due to missing "
+ "ServletWebServerFactory bean.");
}
if (beanNames.length > 1) {
throw new ApplicationContextException(
"Unable to start ServletWebServerApplicationContext due to multiple "
+ "ServletWebServerFactory beans : "
+ StringUtils.arrayToCommaDelimitedString(beanNames));
}
return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
}

我们再回头去看factory.getWebServer(getSelfInitializer()) ，转到定义就会看到很熟悉的名字tomcat

public WebServer getWebServer(ServletContextInitializer... initializers) {
//tomcat这位大哥出现了
Tomcat tomcat = new Tomcat();
File baseDir = (this.baseDirectory != null ? this.baseDirectory
: createTempDir("tomcat"));
tomcat.setBaseDir(baseDir.getAbsolutePath());
Connector connector = new Connector(this.protocol);
tomcat.getService().addConnector(connector);
customizeConnector(connector);
tomcat.setConnector(connector);
tomcat.getHost().setAutoDeploy(false);
configureEngine(tomcat.getEngine());
for (Connector additionalConnector : this.additionalTomcatConnectors) {
tomcat.getService().addConnector(additionalConnector);
}
prepareContext(tomcat.getHost(), initializers);
return getTomcatWebServer(tomcat);
}

内置的Servlet容器就是在onRefresh() 方法里面启动的，至此一个Servlet容器就启动OK了。

总结：

1、new了一个SpringApplication对象，使用SPI技术加载加载 ApplicationContextInitializer、ApplicationListener 接口实例

2、调用SpringApplication.run() 方法

3、调用createApplicationContext()方法创建上下文对象，创建上下文对象同时会注册spring的核心组件类（ConfigurationClassPostProcessor 、AutowiredAnnotationBeanPostProcessor 等）。

4、调用refreshContext() 方法启动Spring容器和内置的Servlet容器

本人也是一个刚入门的小白，也是看了很多大神写的这类文章，按照自己思路整理一下，就是为了加深下印象。但也希望对大家有帮助

SpringBoot启动流程总结

1.执行main方法

2.执行run() 方法

2.1 createApplicationContext()

2.2 refreshContext()

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`2.执行run() 方法`

`2.1 createApplicationContext()`

`2.2 refreshContext()`