可能是全网最全的SpringBoot启动流程源码分析（最新3.x版本）

忘是亡心i 2023-10-14 20:47 1阅读 0赞

@SpringBootApplication(
        scanBasePackages = {"com.javaedge.base"}
    )
    public class BaseApplication {
        public BaseApplication() {
        }
    
        public static void main(String[] args) {
            SpringApplication.run(BaseApplication.class, args);
        }
    }

## 1 启动入口 ##

静态辅助类，可用于运行使用默认配置（即我们添加的一系列注解）的指定源的 SpringApplication 。

*  primarySource - 要载入的主要源，即指定源，这里为传入的Application.class Class<?> ：泛型决定了任何类都可以传入
 *  args - 应用程序参数（通常从main方法传递）
 *  返回：正在运行的ApplicationContext

java复制代码public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
      return run(new Class<?>[] { primarySource }, args);
    }

java复制代码public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
      return new SpringApplication(primarySources).run(args);
    }

构造一个SpringApplication的实例，然后再调用这里实例的run方法就表示启动SpringBoot。因此，想要分析SpringBoot的启动过程，我们需要熟悉：

*  SpringApplication的构造过程
 *  SpringApplication的run方法执行过程

## 2 SpringApplication的构造过程 ##

创建新 SpringApplication 实例。应用程序上下文将从指定的主要源加载 bean。实例可以在调用 run(String...)之前自定义。

java复制代码	public SpringApplication(Class<?>... primarySources) {
    		this(null, primarySources);
    	}

java复制代码	@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));
        
        // 判断是否是web程序：
        // jakarta.servlet.Servlet
        // org.springframework.web.context.ConfigurableWebApplicationContext
        // 须都在类加载器中存在，并设置到webEnvironment属性
    		this.webApplicationType = WebApplicationType.deduceFromClasspath();
    
        // 从 META-INF/spring.factories 中找K=ApplicationContextInitializer的类并实例化后
        // 设置到SpringApplication的initializers属性。即找出所有的应用程序初始化器
    		this.bootstrapRegistryInitializers = new ArrayList<>(
    				getSpringFactoriesInstances(BootstrapRegistryInitializer.class));
        
    		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    
        // 从spring.factories文件中找出key为ApplicationListener的类并实例化后
        // 设置到SpringApplication的listeners属性。即找出所有的应用程序事件监听器
    		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        // 找出main类，这里是 BaseApplication 类
    		this.mainApplicationClass = deduceMainApplicationClass();
    	}

## ApplicationContextInitializer ##

![format_png][]

应用程序初始化器，做一些初始化的工作。

用于在刷新之前初始化 Spring ConfigurableApplicationContext 的回调接口。 通常用于需要对应用程序上下文进行某种编程初始化的 Web 应用程序中。如针对上下文的环境注册属性源或激活配置文件。

ApplicationContextInitializer 鼓励处理器检测 Spring 的 Ordered 接口是否已实现或 @Order 注解是否存在，并在调用之前对实例进行相应的排序（如果有）

java复制代码@FunctionalInterface
    public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
    
    	/**
    	 * Initialize the given application context.
    	 * @param applicationContext the application to configure
    	 */
    	void initialize(C applicationContext);
    
    }

## ApplicationListener ##

![format_png 1][]

![get_code_OWIwZTZjZmE0ZWNjMGU3YTY0NzkyNGQzZDM4NDFlNzYsMTY5MjE1MjQ0ODY0MQ][]

ApplicationEvent监听器：

java复制代码@FunctionalInterface
    public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
    
    	void onApplicationEvent(E event);
    
    	static <T> ApplicationListener<PayloadApplicationEvent<T>> forPayload(Consumer<T> consumer) {
    		return event -> consumer.accept(event.getPayload());
    	}
    
    }

## SpringApplicationEvent ##

![format_png 2][]

应用程序的：

*  启动事件，ApplicationStartingEvent。一旦一个 SpringApplication 开始，事件就会尽早发布 - 在 or ApplicationContext 可用之前Environment，但在注册之后ApplicationListener。事件的来源是本身SpringApplication，但要注意不要在这个早期阶段过多地使用其内部状态，因为它可能会在生命周期的后期被修改
 *  失败事件，ApplicationFailedEvent
 *  准备事件，ApplicationPreparedEvent 事件发布为 当一个SpringApplication正在启动并且ApplicationContext已完全准备好但未refresh。将加载 Bean 定义，并在此阶段可以使用
 *  ApplicationEnvironmentPreparedEvent
 *  ContextClosedEvent

应用程序事件监听器跟监听事件是绑定的，如：

*  ConfigServerBootstrapApplicationListener只跟ApplicationEnvironmentPreparedEvent事件绑定
 *  LiquibaseServiceLocatorApplicationListener只跟ApplicationStartedEvent事件绑定
 *  LoggingApplicationListener跟所有事件绑定

## 3 SpringApplication的执行 ##

先看一些事件和监听器概念：

*  SpringApplicationRunListeners类
 *  SpringApplicationRunListener类

## 3.1 SpringApplicationRunListeners ##

用于监听SpringApplication的run方法的执行。用于SpringApplicationRunListener监听器的批量执行。

finish：run方法结束之前调用；对应事件的类型是ApplicationReadyEvent或ApplicationFailedEvent

java复制代码class SpringApplicationRunListeners {
    
      // Log日志类
    	private final Log log;
    
      // 持有SpringApplicationRunListener集合
    	private final List<SpringApplicationRunListener> listeners;
    
    	private final ApplicationStartup applicationStartup;
    
      // run方法执行时立马执行；对应事件类型ApplicationStartingEvent
    	void starting(ConfigurableBootstrapContext bootstrapContext, Class<?> mainApplicationClass) {
    		doWithListeners("spring.boot.application.starting", (listener) -> listener.starting(bootstrapContext),
    				(step) -> {
    					if (mainApplicationClass != null) {
    						step.tag("mainApplicationClass", mainApplicationClass.getName());
    					}
    				});
    	}
    
      // ApplicationContext创建之前并且环境信息准备好的时候调用；对应事件类型ApplicationEnvironmentPreparedEvent
    	void environmentPrepared(ConfigurableBootstrapContext bootstrapContext, ConfigurableEnvironment environment) {
    		doWithListeners("spring.boot.application.environment-prepared",
    				(listener) -> listener.environmentPrepared(bootstrapContext, environment));
    	}
    
      // ApplicationContext创建好并且在source加载之前调用一次；没有具体的对应事件
    	void contextPrepared(ConfigurableApplicationContext context) {
    		doWithListeners("spring.boot.application.context-prepared", (listener) -> listener.contextPrepared(context));
    	}
    
      // ApplicationContext创建并加载之后，并在refresh之前调用
      // 对应事件类型ApplicationPreparedEvent
    	void contextLoaded(ConfigurableApplicationContext context) {
    		doWithListeners("spring.boot.application.context-loaded", (listener) -> listener.contextLoaded(context));
    	}
    
    
    	void started(ConfigurableApplicationContext context, Duration timeTaken) {
    		doWithListeners("spring.boot.application.started", (listener) -> listener.started(context, timeTaken));
    	}
    
    	void ready(ConfigurableApplicationContext context, Duration timeTaken) {
    		doWithListeners("spring.boot.application.ready", (listener) -> listener.ready(context, timeTaken));
    	}
    
    	void failed(ConfigurableApplicationContext context, Throwable exception) {
    		doWithListeners("spring.boot.application.failed",
    				(listener) -> callFailedListener(listener, context, exception), (step) -> {
    					step.tag("exception", exception.getClass().toString());
    					step.tag("message", exception.getMessage());
    				});
    	}
    
    	private void callFailedListener(SpringApplicationRunListener listener, ConfigurableApplicationContext context,
    			Throwable exception) {
    		try {
    			listener.failed(context, exception);
    		}
    		catch (Throwable ex) {
    			if (exception == null) {
    				ReflectionUtils.rethrowRuntimeException(ex);
    			}
    			if (this.log.isDebugEnabled()) {
    				this.log.error("Error handling failed", ex);
    			}
    			else {
    				String message = ex.getMessage();
    				message = (message != null) ? message : "no error message";
    				this.log.warn("Error handling failed (" + message + ")");
    			}
    		}
    	}
    
    	private void doWithListeners(String stepName, Consumer<SpringApplicationRunListener> listenerAction) {
    		doWithListeners(stepName, listenerAction, null);
    	}
    
    	private void doWithListeners(String stepName, Consumer<SpringApplicationRunListener> listenerAction,
    			Consumer<StartupStep> stepAction) {
    		StartupStep step = this.applicationStartup.start(stepName);
    		this.listeners.forEach(listenerAction);
    		if (stepAction != null) {
    			stepAction.accept(step);
    		}
    		step.end();
    	}
    
    }

SpringApplicationRunListener目前只有一个实现类EventPublishingRunListener：

![format_png 3][]

![get_code_ODQxYjQ0ZTYzZDI4MDAyNzk1YzM0YjAyOGIyYzFlYzAsMTY5MjE1MjQ0ODY0MQ][]

java复制代码/**
     * SpringApplicationRunListener to publish SpringApplicationEvents.
     * Uses an internal ApplicationEventMulticaster for the events that are fired
     * before the context is actually refreshed.
     */
    class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {
    
    	private final SpringApplication application;
    
    	private final String[] args;
    
    	private final SimpleApplicationEventMulticaster initialMulticaster;
    
    	@Override
    	public void starting(ConfigurableBootstrapContext bootstrapContext) {
    		multicastInitialEvent(new ApplicationStartingEvent(bootstrapContext, this.application, this.args));
    	}
    
    	@Override
    	public void environmentPrepared(ConfigurableBootstrapContext bootstrapContext,
    			ConfigurableEnvironment environment) {
    		multicastInitialEvent(
    				new ApplicationEnvironmentPreparedEvent(bootstrapContext, this.application, this.args, environment));
    	}
    
    	@Override
    	public void contextPrepared(ConfigurableApplicationContext context) {
    		multicastInitialEvent(new ApplicationContextInitializedEvent(this.application, this.args, context));
    	}
    
    	@Override
    	public void contextLoaded(ConfigurableApplicationContext context) {
    		for (ApplicationListener<?> listener : this.application.getListeners()) {
    			if (listener instanceof ApplicationContextAware contextAware) {
    				contextAware.setApplicationContext(context);
    			}
    			context.addApplicationListener(listener);
    		}
    		multicastInitialEvent(new ApplicationPreparedEvent(this.application, this.args, context));
    	}
    
    	@Override
    	public void started(ConfigurableApplicationContext context, Duration timeTaken) {
    		context.publishEvent(new ApplicationStartedEvent(this.application, this.args, context, timeTaken));
    		AvailabilityChangeEvent.publish(context, LivenessState.CORRECT);
    	}
    
    	@Override
    	public void ready(ConfigurableApplicationContext context, Duration timeTaken) {
    		context.publishEvent(new ApplicationReadyEvent(this.application, this.args, context, timeTaken));
    		AvailabilityChangeEvent.publish(context, ReadinessState.ACCEPTING_TRAFFIC);
    	}
    
    	@Override
    	public void failed(ConfigurableApplicationContext context, Throwable exception) {
    		ApplicationFailedEvent event = new ApplicationFailedEvent(this.application, this.args, context, exception);
    		if (context != null && context.isActive()) {
    			// Listeners have been registered to the application context so we should
    			// use it at this point if we can
    			context.publishEvent(event);
    		}
    		else {
    			// An inactive context may not have a multicaster so we use our multicaster to
    			// call all the context's listeners instead
    			if (context instanceof AbstractApplicationContext abstractApplicationContext) {
    				for (ApplicationListener<?> listener : abstractApplicationContext.getApplicationListeners()) {
    					this.initialMulticaster.addApplicationListener(listener);
    				}
    			}
    			this.initialMulticaster.setErrorHandler(new LoggingErrorHandler());
    			this.initialMulticaster.multicastEvent(event);
    		}
    	}
    
      // 广播事件出去
    	private void multicastInitialEvent(ApplicationEvent event) {
    		refreshApplicationListeners();
    		this.initialMulticaster.multicastEvent(event);
    	}
    
    	private void refreshApplicationListeners() {
    		this.application.getListeners().forEach(this.initialMulticaster::addApplicationListener);
    	}
    
    	private static class LoggingErrorHandler implements ErrorHandler {
    
    		private static final Log logger = LogFactory.getLog(EventPublishingRunListener.class);
    
    		@Override
    		public void handleError(Throwable throwable) {
    			logger.warn("Error calling ApplicationEventListener", throwable);
    		}
    
    	}
    
    }

它把监听过程封装成SpringApplicationEvent事件，并让内部属性initialMulticaster，ApplicationEventMulticaster接口的实现类SimpleApplicationEventMulticaster广播出去：

![format_png 4][]

![get_code_MDk1YzIyY2MxNDA4OGFlNWQ3MzZkODJjOWIwZWVlOWYsMTY5MjE1MjQ0ODY0MQ][]

广播出去的事件对象会被SpringApplication中的listeners属性进行处理。

![format_png 5][]

![get_code_ZTQ4Y2FlYmQzMDY4ZTEwODk3ZWUwYmM0Nzk5NTkwYjIsMTY5MjE1MjQ0ODY0MQ][]

所以SpringApplicationRunListener和ApplicationListener之间的关系是通过ApplicationEventMulticaster广播出去的SpringApplicationEvent所联系。

![img][]

![get_code_ZTc0MGIzN2JkOWRkNGYyNGVjYjQxMjc4ODU4MzNlYmIsMTY5MjE1MjQ0ODY0MQ][]

## 创建所有 Spring 运行监听器并发布应用启动事件 ##

调用 getSpringFactoriesInstances 获取配置的监听器名称，并实例化所有的类。

SpringApplicationRunListener 所有监听器配置在 pring.factories ：

![format_png 6][]

![get_code_ZDFlNjhhMzViMjgwOTZmZDEzM2NiZTcyNWVkNGEzYzMsMTY5MjE1MjQ0ODY0MQ][]

## 3.2 run ##

java复制代码	public ConfigurableApplicationContext run(String... args) {
        // 开始执行，记录开始时间
    		long startTime = System.nanoTime();
    		DefaultBootstrapContext bootstrapContext = createBootstrapContext();
    		ConfigurableApplicationContext context = null;
        // 设置系统属性 java.awt.headless 的值，默认为true
    		configureHeadlessProperty();
        
        // 获取SpringApplicationRunListeners，内部只有一个EventPublishingRunListener
        // 创建所有 Spring 运行监听器并发布应用启动事件
    		SpringApplicationRunListeners listeners = getRunListeners(args);
        
        // 会封装成SpringApplicationEvent事件然后广播出去给SpringApplication中的listeners所监听
        // 这里接受ApplicationStartedEvent事件的listener会执行相应操作
    		listeners.starting(bootstrapContext, this.mainApplicationClass);
    		try {
          // 初始化默认应用参数类
    			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
          
          // 根据运行监听器和应用参数来准备 Spring 环境
    			ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
    			Banner printedBanner = printBanner(environment);
          
          // 创建Spring容器，即创建应用上下文
    			context = createApplicationContext();
    			context.setApplicationStartup(this.applicationStartup);
         // 准备应用上下文
    			prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
    			refreshContext(context);
          
          // 容器创建完成之后执行额外一些操作
    			afterRefresh(context, applicationArguments);
    			Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);
    			if (this.logStartupInfo) {
    				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);
    			}
          // 广播出ApplicationReadyEvent事件给相应的监听器执行
          // 发布应用上下文启动完成事件
    			listeners.started(context, timeTakenToStartup);
    			callRunners(context, applicationArguments);
    		}
    		catch (Throwable ex) {
    			if (ex instanceof AbandonedRunException) {
    				throw ex;
    			}
    			handleRunFailure(context, ex, listeners);
    			throw new IllegalStateException(ex);
    		}
    		try {
    			if (context.isRunning()) {
    				Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);
            // 发布应用上下文就绪事件
    				listeners.ready(context, timeTakenToReady);
    			}
    		}
    		catch (Throwable ex) {
    			if (ex instanceof AbandonedRunException) {
    				throw ex;
    			}
          // 过程报错的话会执行一些异常操作
          // 然后广播出ApplicationFailedEvent事件给相应的监听器执行
    			handleRunFailure(context, ex, null);
    			throw new IllegalStateException(ex);
    		}
        // 返回Spring容器
    		return context;
    	}

## 设置系统属性 java.awt.headless 的值 ##

java复制代码private void configureHeadlessProperty() {
      System.setProperty(SYSTEM_PROPERTY_JAVA_AWT_HEADLESS,
          System.getProperty(SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, Boolean.toString(this.headless)));
    }

> 对于一个 Java 服务器来说经常要处理一些图形元素，例如地图的创建或者图形和图表等。这些API基本上总是需要运行一个X-server以便能使用AWT（Abstract Window Toolkit，抽象窗口工具集）。然而运行一个不必要的 X-server 并不是一种好的管理方式。有时你甚至不能运行 X-server,因此最好的方案是运行 headless 服务器，来进行简单的图像处理。

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