我們來看看SpringBoot啟動的全過程。

1.SpringBoot的啟動類是**application，注解@SpringBootApplication。

@SpringBootApplication
public class CmsApplication {
 public static void main(String[] args) {
  SpringApplication.run(CmsApplication.class, args);
 }
}

SpringBoot 應用注解是@Configuration、@Enable AutoConfiguration 和@ComponentScan 注解的集成。它們代表Spring bean的配置bean，開啟自動配置spring的上下文，組件掃描的路徑。這就是為什么 * 應用程序。Java需要放在根路徑下，所以@ComponentScan掃描就是整個項目。

其次，啟動類默認只有一個main方法，調用SpringApplication。運行方法。讓我們看一下 SpringApplication 類。

public static ConfigurableApplicationContext run(Object source, String... args) {
  return run(new Object[]{source}, args);
 }
...
public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
  Return (new Spring Application (sources). run (args); //sources is the specific CmsApplication. class class class class
 }
...

通過提取兩個直接調用的run方法，可以看到靜態方法SpringApplication。run 最后創建一個 SpringApplication 并在其中運行 run 方法。

查看構造方法：

public SpringApplication(Object... sources) {
  this.bannerMode = Mode.CONSOLE;
  this.logStartupInfo = true;
  this.addCommandLineProperties = true;
  this.headless = true;
  this.registerShutdownHook = true;
  this.additionalProfiles = new HashSet();
  this.initialize(sources);
 }
...

設置基值后調用initialize方法初始化構造函數，如下：

private void initialize(Object[] sources) {
  if (sources != null && sources.length > 0) {
   this.sources.addAll(Arrays.asList(sources));
  }
  this.webEnvironment = this.deduceWebEnvironment();
  this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
  this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
  this.mainApplicationClass = this.deduceMainApplicationClass();
 }
...

2.初始化方法主要做了幾個步驟：

(1)將source放在SpringApplication的source屬性中，source是一個LinkedHashSet()，也就是說我們可以同時創建多個自定義的非重復應用，但是目前只有一個。

(2)判斷是否為web程序(?javax.servlet.Servlet且org.springframework.web.context.ConfigurableWebApplicationContextAll必須存在于類加載器中并設置為webEnvironmentAttribute.

(3)從spring中找到Application Context Initializer。工廠并將其設置為初始化程序。

(4)從spring中找到Application Listener。工廠并將其實例化為 Spring Application 的 listener listeners 屬性。這個過程是找到所有的應用程序事件監聽器。

(5)找出主方法類(這里是CmsApplication)，返回Class對象。

默認情況下，initialize 方法從 spring 中查找鍵為 ApplicationContextInitializer 的類。工廠文件：

org.springframework.boot.context.config.DelegatingApplicationContextInitializer

org.springframework.boot.context.ContextIdApplicationContextInitializer

org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer

org.springframework.boot.context.web.ServerPortInfoApplicationContextInitializer

org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

關鍵是ApplicationListener。

org.springframework.boot.context.config.ConfigFileApplicationListener

org.springframework.boot.context.config.AnsiOutputApplicationListener

org.springframework.boot.logging.LoggingApplicationListener

org.springframework.boot.logging.ClasspathLoggingApplicationListener

org.springframework.boot.autoconfigure.BackgroundPreinitializer

org.springframework.boot.context.config.DelegatingApplicationListener

org.springframework.boot.builder.ParentContextCloserApplicationListener

org.springframework.boot.context.FileEncodingApplicationListener

org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

3.SpringApplication的構建和初始化完成后，運行run方法。

public ConfigurableApplicationContext run(String... args) {
  StopWatch stopWatch = new StopWatch ();// Construct a Task Execution Observer
  StopWatch. start ();// Start execution, record start time
  ConfigurableApplicationContext context = null;
  FailureAnalyzers analyzers = null;
  this.configureHeadlessProperty();
  // Get Spring Application RunListeners with only one Event Publishing RunListener inside
  SpringApplicationRunListeners listeners = this.getRunListeners(args);
  // Encapsulate Spring Application Event events and broadcast them to listeners in Spring Application to start listening
  listeners.starting();
  try {
   // Construct an application parameter holder class
   ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
   // Load Configuration Environment
   ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
   Banner printedBanner = this.printBanner(environment);
   // Create Spring containers (using BeanUtils. instantiate)
   context = this.createApplicationContext();
   // If container creation fails, analyze the cause of output failure
   new FailureAnalyzers(context);
   // Setting up container configuration environment, monitoring, etc.
   this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
   // Refresh containers
   this.refreshContext(context);
   this.afterRefresh(context, applicationArguments);
   // Broadcast the Application ReadyEvent event to the corresponding listener for execution
   listeners.finished(context, (Throwable)null);
   StopWatch. stop ();// End of execution, record execution time
   if (this.logStartupInfo) {
    (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
   }
   Return context; // Return Spring container
  } catch (Throwable var9) {
   this.handleRunFailure(context, listeners, (FailureAnalyzers)analyzers, var9);
   throw new IllegalStateException(var9);
  }
 }

run方法的流程分析如下。該方法的幾個關鍵步驟如下：

(1)創建應用監聽Spring Application RunListeners并開始監聽

(2)加載SpringBook可配置環境。如果通過 Web 容器發布，它會加載標準環境，最終繼承可配置環境。類圖如下。

可以看到，*Environment最終實現了PropertyResolver接口，而我們通常在通過環境對象獲取配置文件中Key對應的value方法時，會調用PropertyResolver接口的getProperty方法。

(3)配置環境添加到監聽器對象(Spring Application RunListeners)

(4)創建Spring Container：可配置應用上下文(Application Configuration Context)，我們可以看看創建方法

protected ConfigurableApplicationContext createApplicationContext() {
  Class<?> contextClass = this.applicationContextClass;
  if (contextClass == null) {
   try {
    contextClass = Class.forName(this.webEnvironment ? "org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext" : "org.springframework.context.annotation.AnnotationConfigApplicationContext");
   } catch (ClassNotFoundException var3) {
    throw new IllegalStateException("Unable create a default ApplicationContext, please specify an ApplicationContextClass", var3);
   }
  }
  return (ConfigurableApplicationContext)BeanUtils.instantiate(contextClass);
 }

該方法首先獲取顯式設置的應用上下文類，如果不存在，則加載默認環境配置(通過判斷是否為web環境)，默認Annotation Config應用上下文注解上下文(通過掃描所有注解類加載bean) ，最后通過BeanUtils實例化上下文對象，并返回，Configurab。leApplicationContext類圖如下

主要取決于其繼承的兩個方向：

LifeCycle：生命周期類，定義了start start、stop、isRunning是否運行一個中等生命周期的空值方法

ApplicationContext：應用上下文類，主要繼承beanFactory類。

(5)回到run方法，設置容器prepareContext方法，將監聽器、環境、應用Arguments、banner等重要組件與上下文對象關聯起來。

(6)刷新容器。refresh()方法：初始化方法如下：

public void refresh() throws BeansException, IllegalStateException {
  Object var1 = this.startupShutdownMonitor;
  synchronized(this.startupShutdownMonitor) {
   this.prepareRefresh();
   ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
   this.prepareBeanFactory(beanFactory);
   try {
    this.postProcessBeanFactory(beanFactory);
    this.invokeBeanFactoryPostProcessors(beanFactory);
    this.registerBeanPostProcessors(beanFactory);
    this.initMessageSource();
    this.initApplicationEventMulticaster();
    this.onRefresh();
    this.registerListeners();
    this.finishBeanFactoryInitialization(beanFactory);
    this.finishRefresh();
   } catch (BeansException var9) {
    if (this.logger.isWarnEnabled()) {
     this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
    }
    this.destroyBeans();
    this.cancelRefresh(var9);
    throw var9;
   } finally {
    this.resetCommonCaches();
   }
  }
 }

refresh()Method做了很多核心工作，比如設置BeanFactory，執行BeanFactory PostProcessor接口，執行BeanFactory Processor接口，解析自動化配置類，加載spring。工廠，實例化bean，解析條件注釋，國際化初始化等等。這部分將在以后的文章中分析。

(7)廣播Application ReadyEvent并在執行結束時返回Configurable Application Context。

至此，SpringBoot啟動完成，回顧一下整體流程，Springboot啟動，主要創建配置環境(environment)，監聽器，應用上下文，根據以上條件，我們開始實例化我們需要的bean容器。如果大家想了解更多相關知識，可以關注一下動力節點的Spring框架教程，里面有更豐富的知識等著大家去學習，希望對大家能夠有所幫助。