众所周知，类上面带有 @SpringBootApplication 注解的类，即为 springboot 的启动类。一个 springboot 项目只能有一个启动类。我们来分析一下 SpringBoot 项目的启动过程，首先看看启动类里面都包含什么

@SpringBootApplication
public class HelloWorldMainApplication {public static void main(String[] args) {
        //spring 应用启动起来
        SpringApplication.run(HelloWorldMainApplication.class,args);

    }
}

  从上面的代码中可以看出真正起作用的是 SpringApplication.run(); 这个方法，下面主要分析一下这个方法。

一、实例化 SpringApplication

  SpringApplication 初始化时主要做三件事情：

• 1. 根据 classpath 下是否存在 (ConfigurableWebApplicationContext) 判断是否要启动一个 web applicationContext
• 2.SpringFactoriesInstances 加载 classpath 下所有可用的 ApplicationContextInitializer
• 3.SpringFactoriesInstances 加载 classpath 下所有可用的 ApplicationListener

/**
 * Create a new {@link SpringApplication} instance. The application context will load
 * beans from the specified primary sources (see {@link SpringApplication class-level}
 * documentation for details. The instance can be customized before calling
 * {@link #run(String...)}.
 * @param resourceLoader the resource loader to use
 * @param primarySources the primary bean sources
 * @see #run(Class, String[])
 * @see #setSources(Set)
 */
@SuppressWarnings({"unchecked", "rawtypes"})
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    //1. 根据 classpath 下是否存在 (ConfigurableWebApplicationContext) 判断是否要启动一个 web applicationContext
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    //2.SpringFactoriesInstances 加载 classpath 下所有可用的 ApplicationContextInitializer
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    //3.SpringFactoriesInstances 加载 classpath 下所有可用的 ApplicationListener
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    this.mainApplicationClass = deduceMainApplicationClass();}

二、实例化完成后调用 run()方法

  调用 run()方法执行的过程主要分为以下几步：

• 1. 遍历 SpringApplication 初始化过程中加载的 SpringApplicationRunListeners
• 2. 调用 Starting()监听 SpringApplication 的启动
• 3. 加载 SpringBoot 配置环境(ConfigurableEnvironment)
• 4. 设置 banner 属性
• 5. 创建 ConfigurableApplicationContext（应用配置上下文）
• 6. 将 listeners、environment、applicationArguments、bannner 等重要组件与上下文对象关联
• 7.bean 的实力化完成

/**
 * Run the Spring application, creating and refreshing a new
 * {@link ApplicationContext}.
 * @param args the application arguments (usually passed from a Java main method)
 * @return a running {@link ApplicationContext}
 */
public ConfigurableApplicationContext run(String... args) {StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    //1. 遍历 SpringApplication 初始化过程中加载的 SpringApplicationRunListeners
    SpringApplicationRunListeners listeners = getRunListeners(args);
    //2. 调用 starting()监听 SpringApplication 的启动
    listeners.starting();
    try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        //3. 加载 SpringBoot 配置环境
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        configureIgnoreBeanInfo(environment);
        //4. 设置 banner 属性
        Banner printedBanner = printBanner(environment);
        //5. 创建 ConfigurableApplicationContext(应用配置上下文)
        context = createApplicationContext();
        exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class}, context);
        //6. 将 listeners、environment、applicationArguments、banner 等重要组件与上下文对象关联
        prepareContext(context, environment, listeners, applicationArguments, printedBanner);
        //7. 实例化 bean
        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, ex, exceptionReporters, listeners);
        throw new IllegalStateException(ex);
    }

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

1. 遍历 SpringApplication 初始化过程中加载的 SpringApplicationRunListeners

private SpringApplicationRunListeners getRunListeners(String[] args) {Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
    return new SpringApplicationRunListeners(logger,
            getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
}

2. 调用 Starting()监听 SpringApplication 的启动

public void starting() {// 遍历所有的 SpringApplicationRunListener，调用 starting()方法监听 SpringApplication 的启动
    for (SpringApplicationRunListener listener : this.listeners) {listener.starting();
    }
}

3. 加载 SpringBoot 配置环境(ConfigurableEnvironment)

  加载 SpringBoot 配置环境（configurableEnvironment），如果是通过 web 容器发布，会加载 StandardEnvironment。将配置文件 (Environment) 加入到监听器对象中(SpringApplicationRunListeners)

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
        ApplicationArguments applicationArguments) {
    // Create and configure the environment
    // 如果 environment 不为空直接返回 || 如果是 web 环境则直接实例化 StandardServletEnvironment 类 || 如果不是 web 环境则直接实例化 StandardEnvironment 类
    ConfigurableEnvironment environment = getOrCreateEnvironment();
    // 配置环境信息
    configureEnvironment(environment, applicationArguments.getSourceArgs());
    // 通知所有的监听者，环境已经准备好了
    listeners.environmentPrepared(environment);
    bindToSpringApplication(environment);
    if (!this.isCustomEnvironment) {environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,
                deduceEnvironmentClass());
    }
    ConfigurationPropertySources.attach(environment);
    return environment;
}

4. 设置 banner 属性

private Banner printBanner(ConfigurableEnvironment environment) {
    // 如果未开启 banner 打印直接返回
    if (this.bannerMode == Banner.Mode.OFF) {return null;}
    // 创建 ResourceLoader 对象
    ResourceLoader resourceLoader = (this.resourceLoader != null) ? this.resourceLoader
            : new DefaultResourceLoader(getClassLoader());
    // 创建 SpringApplicationBannerPrinter，该对象用来打印 banner
    SpringApplicationBannerPrinter bannerPrinter = new SpringApplicationBannerPrinter(resourceLoader, this.banner);
    // 如果 bannerMode 模式为 LOG，则将 bannner 打印到 log 文件中
    if (this.bannerMode == Mode.LOG) {return bannerPrinter.print(environment, this.mainApplicationClass, logger);
    }
    // 打印 banner 到控制台
    return bannerPrinter.print(environment, this.mainApplicationClass, System.out);
}

5. 初始化 ConfigurableApplicationContext（应用配置上下文）

  在 SpringBoot 中，应用类型分为三类

public enum WebApplicationType {
    /**
     * The application should not run as a web application and should not start an
     * embedded web server.
     */
    // 应用程序不是 web 应用，也不应该用 web 服务器去启动
    NONE,
    /**
     * The application should run as a servlet-based web application and should start an
     * embedded servlet web server.
     */
    // 应用程序应作为基于 servlet 的 web 应用程序运行，并应启动嵌入式 servlet web（tomcat）服务器
    SERVLET,
    /**
     * The application should run as a reactive web application and should start an
     * embedded reactive web server.
     */
    // 应用程序应作为 reactive web 应用程序运行，并应启动嵌入式 reactive web 服务器。REACTIVE;
}

  根据 webEnvironment 是否是 web 环境创建默认的 contextClass，AnnotationConfigEnbeddedWebApplicationContext(通过扫描所有注解类来加载 bean)和 ConfigurableWebApplicationContext), 最后通过 BeanUtils 实例化上下文对象，并返回。

/**
 * Strategy method used to create the {@link ApplicationContext}. By default this
 * method will respect any explicitly set application context or application context
 * class before falling back to a suitable default.
 * @return the application context (not yet refreshed)
 * @see #setApplicationContextClass(Class)
 */
protected ConfigurableApplicationContext createApplicationContext() {
    // 根据 webEnvironment 是否是 web 环境创建默认的 contextClass
    Class<?> contextClass = this.applicationContextClass;
    if (contextClass == null) {
        try {switch (this.webApplicationType) {
            case SERVLET:
                //AnnotationConfigServletWebServerApplicationContext
                contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
                break;
            case REACTIVE:
                //AnnotationConfigReactiveWebServerApplicationContext
                contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
                break;
            default:
                //AnnotationConfigApplicationContext
                contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
            }
        }
        catch (ClassNotFoundException ex) {
            throw new IllegalStateException(
                    "Unable create a default ApplicationContext," + "please specify an ApplicationContextClass",
                    ex);
        }
    }
    //BeanUtils 实例化上下文对象
    return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

6. 将 listeners、environment、applicationArguments、banner 等重要组件与上下文对象关联

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,
        SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
    // 设置上下文的 environment
    context.setEnvironment(environment);
    // 应用上下文后处理
    postProcessApplicationContext(context);
    // 在 context refresh 之前，对其应用 ApplicationContextInitializer
    applyInitializers(context);
    // 上下文准备
    listeners.contextPrepared(context);
    // 打印启动日志和启动应用的 profile
    if (this.logStartupInfo) {logStartupInfo(context.getParent() == null);
        logStartupProfileInfo(context);
    }
    // Add boot specific singleton beans
    ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
    // 向 beanFactory 注册单例 bean：命令行参数 bean
    beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
    if (printedBanner != null) {
        // 向 beanFactory 注册单例 bean：banner bean
        beanFactory.registerSingleton("springBootBanner", printedBanner);
    }
    if (beanFactory instanceof DefaultListableBeanFactory) {((DefaultListableBeanFactory) beanFactory)
                .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
    }
    // Load the sources
    // 获取 SpringApplication 的 primarySources 属性
    Set<Object> sources = getAllSources();
    Assert.notEmpty(sources, "Sources must not be empty");
    // 将 bean 加载到应用上下文
    load(context, sources.toArray(new Object[0]));
    // 向上下文添加 ApplicationListener，并广播 ApplicationPreparedEvent 事件
    listeners.contextLoaded(context);
}

7.bean 的实例化完成，刷新应用上下文