springboot启动流程 (1) 流程概览

本文将通过阅读源码方式分析SpringBoot应用的启动流程，不涉及Spring启动部分(有相应的文章介绍)。

本文不会对各个流程做展开分析，后续会有文章介绍详细流程。

SpringApplication类

应用启动入口

使用以下方式启动一个SpringBoot应用：

@SpringBootApplication
public class SpringBootDemoApplication {

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

run方法

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

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

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);
		// 获取应用env
		ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
		configureIgnoreBeanInfo(environment);
		// 打印banner
		Banner printedBanner = printBanner(environment);
		// 创建ApplicationContext
		context = createApplicationContext();
		exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
				new Class[] { ConfigurableApplicationContext.class }, context);
		// 一些准备工作
		prepareContext(context, environment, listeners, applicationArguments, printedBanner);
		// refresh ApplicationContext
		refreshContext(context);
		afterRefresh(context, applicationArguments);
		stopWatch.stop();
		if (this.logStartupInfo) {
			new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
		}
		// 调用listener
		listeners.started(context);
		// 调用ApplicationRunner和CommandLineRunner
		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;
}

获取应用env

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
		ApplicationArguments applicationArguments) {
	// 创建StandardServletEnvironment, 会初始化四个PropertySource:
	// servletConfigInitParams, servletContextInitParams, systemProperties, systemEnvironment
	// 比如-Dserver.port=8888会在systemProperties中
	ConfigurableEnvironment environment = getOrCreateEnvironment();
	// 添加defaultProperties和命令行配置参数即CommandLinePropertySource
	// 通常都没有这两个配置
	configureEnvironment(environment, applicationArguments.getSourceArgs());
	// 使用ConfigurationPropertySourcesPropertySource封装并暴露所有的PropertySource集
	ConfigurationPropertySources.attach(environment);
	// 添加ApplicationEnvironmentPreparedEvent事件并触发multicastEvent加载应用配置文件
	listeners.environmentPrepared(environment);
	// 将spring.main.xx配置加载到SpringApplication对象
	bindToSpringApplication(environment);
	if (!this.isCustomEnvironment) {
		environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,
				deduceEnvironmentClass());
	}
	ConfigurationPropertySources.attach(environment);
	return environment;
}

加载配置文件的入口在ConfigFileApplicationListener类中：

public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
	addPropertySources(environment, application.getResourceLoader());
}

protected void addPropertySources(ConfigurableEnvironment environment, ResourceLoader resourceLoader) {
	// 添加RandomValuePropertySource
	RandomValuePropertySource.addToEnvironment(environment);
	// 加载配置文件
	new Loader(environment, resourceLoader).load();
}

加载配置文件的源码较多，此处不做记录，简单梳理一下流程：

1. 加载active profile配置文件
   • 如果配置了spring.config.additional-location或spring.config.location参数，会使用它们作为配置文件。如果这两个参数值是目录，则会从这两个目录下查找配置文件
   • 默认从classpath:/,classpath:/config/,file:./,file:./config/*/,file:./config/目录下查找application-xx.properties或application-xx.yml文件
   • 使用PropertiesPropertySourceLoader和YamlPropertySourceLoader解析配置文件
   • 会将配置参数封装成OriginTrackedMapPropertySource类型对象，使用applicationConfig: [classpath:/application-dev.yml]之类的字符串作为PropertySource的名称
2. 加载默认的application.properties或application.yml文件
3. 解析出来的所有PropertySource都会添加到environment的propertySources中，propertySources是一个MutablePropertySources对象，管理着所有的PropertySource集，在这个过程中，添加的先后顺序决定了配置的优先级

创建ApplicationContext

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

prepareContext

private void prepareContext(
		ConfigurableApplicationContext context, ConfigurableEnvironment environment,
		SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments,
		Banner printedBanner) {
	context.setEnvironment(environment);
	postProcessApplicationContext(context);
	// Apply any ApplicationContextInitializers to the context before it is refreshed.
	// ApplicationContextInitializers集是在创建SpringApplication对象的时候初始化的
	applyInitializers(context);
	// 触发contextPrepared事件
	listeners.contextPrepared(context);
	if (this.logStartupInfo) {
		logStartupInfo(context.getParent() == null);
		logStartupProfileInfo(context);
	}
	// 获取BeanFactory并注册必要的Bean
	ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
	// 注册启动参数
	beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
	// 注册banner printer
	if (printedBanner != null) {
		beanFactory.registerSingleton("springBootBanner", printedBanner);
	}
	// 设置是否允许Bean覆盖，使用spring.main.allowBeanDefinitionOverriding参数配置
	if (beanFactory instanceof DefaultListableBeanFactory) {
		((DefaultListableBeanFactory) beanFactory)
				.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
	}
	if (this.lazyInitialization) {
		context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
	}
	// Load the sources
	Set<Object> sources = getAllSources();
	// 将SpringApplication.run(Xxx.class, args)方法传入的Class注册到容器
	// 使用AnnotatedBeanDefinitionReader.register(Class<?>...)方法注册启动类
	load(context, sources.toArray(new Object[0]));
	// 触发contextLoaded事件
	listeners.contextLoaded(context);
}

refreshApplicationContext

protected void refresh(ConfigurableApplicationContext applicationContext) {
	applicationContext.refresh();
}

调用的是ServletWebServerApplicationContext的refresh方法：

public final void refresh() throws BeansException, IllegalStateException {
	try {
		super.refresh();
	} catch (RuntimeException ex) {
		// 关闭web server
		WebServer webServer = this.webServer;
		if (webServer != null) {
			webServer.stop();
		}
		throw ex;
	}
}

绝大多数的refresh逻辑都在AbstractApplicationContext类里面，ServletWebServerApplicationContext中会在onRefresh阶段创建webServer：

protected void onRefresh() {
	super.onRefresh();
	try {
		createWebServer();
	} catch (Throwable ex) {
		throw new ApplicationContextException("Unable to start web server", ex);
	}
}

调用ApplicationRunner和CommandLineRunner

private void callRunners(ApplicationContext context, ApplicationArguments args) {
	List<Object> runners = new ArrayList<>();
	runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
	runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
	AnnotationAwareOrderComparator.sort(runners);
	for (Object runner : new LinkedHashSet<>(runners)) {
		if (runner instanceof ApplicationRunner) {
			callRunner((ApplicationRunner) runner, args);
		}
		if (runner instanceof CommandLineRunner) {
			callRunner((CommandLineRunner) runner, args);
		}
	}
}

SpringBootApplication注解

指示一个配置类，该类声明一个或多个@Bean方法，并触发自动配置和组件扫描。这是一个方便的注解，相当于声明@Configuration、@EnableAutoConfiguration和@ComponentScan注解。

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

	/**
	 * Exclude specific auto-configuration classes such that they will never be applied.
	 */
	@AliasFor(annotation = EnableAutoConfiguration.class)
	Class<?>[] exclude() default {};

	/**
	 * Exclude specific auto-configuration class names such that they will never be
	 * applied.
	 */
	@AliasFor(annotation = EnableAutoConfiguration.class)
	String[] excludeName() default {};

	/**
	 * Base packages to scan for annotated components. Use scanBasePackageClasses
	 * for a type-safe alternative to String-based package names.
	 */
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
	String[] scanBasePackages() default {};

	/**
	 * Type-safe alternative to scanBasePackages for specifying the packages to
	 * scan for annotated components. The package of each class specified will be scanned.
	 */
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
	Class<?>[] scanBasePackageClasses() default {};

	/**
	 * The BeanNameGenerator class to be used for naming detected components
	 * within the Spring container.
	 */
	@AliasFor(annotation = ComponentScan.class, attribute = "nameGenerator")
	Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;

	/**
	 * Specify whether @Bean methods should get proxied in order to enforce
	 * bean lifecycle behavior, e.g. to return shared singleton bean instances even in
	 * case of direct @Bean method calls in user code. This feature requires
	 * method interception, implemented through a runtime-generated CGLIB subclass which
	 * comes with limitations such as the configuration class and its methods not being
	 * allowed to declare final.
	 */
	@AliasFor(annotation = Configuration.class)
	boolean proxyBeanMethods() default true;
}

SpringBootConfiguration注解

指示一个类提供Spring Boot application @Configuration功能。可以替代Spring的标准@Configuration注解，以便可以自动找到配置类。

应用程序应该只标注一个@SpringBootConfiguration，大多数SpringBoot应用程序将从@SpringBootApplication继承它。

@Configuration
public @interface SpringBootConfiguration {

	@AliasFor(annotation = Configuration.class)
	boolean proxyBeanMethods() default true;
}

EnableAutoConfiguration注解

启用SpringBoot自动装配功能，尝试猜测和配置可能需要的组件Bean。

自动装配类通常是根据类路径和定义的Bean来应用的。例如，如果类路径上有tomcat-embedded.jar，那么可能需要一个TomcatServletWebServerFactory(除非已经定义了自己的Servlet WebServerFactory Bean)。

自动装配试图尽可能地智能化，并将随着开发者定义自己的配置而取消自动装配相冲突的配置。开发者可以使用exclude()排除不想使用的配置，也可以通过spring.autoconfig.exclude属性排除这些配置。自动装配总是在用户定义的Bean注册之后应用。

用@EnableAutoConfiguration注解标注的类所在包具有特定的意义，通常用作默认扫描的包。通常建议将@EnableAutoConfiguration(如果没有使用@SpringBootApplication注解)放在根包中，以便可以搜索所有子包和类。

自动装配类是普通的Spring @Configuration类，使用SpringFactoriesLoader机制定位。通常使用@Conditional方式装配，最常用的是@ConditionalOnClass和@ConditionalOnMissingBean注解。

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

	/**
	 * Exclude specific auto-configuration classes such that they will never be applied.
	 */
	Class<?>[] exclude() default {};

	/**
	 * Exclude specific auto-configuration class names such that they will never be
	 * applied.
	 * 当类路径下没有指定的类时，可以使用这个属性指定排除的类
	 */
	String[] excludeName() default {};
}

AutoConfigurationPackage注解

Registers packages with AutoConfigurationPackages. When no base packages or base package classes are specified, the package of the annotated class is registered.

@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

	String[] basePackages() default {};

	Class<?>[] basePackageClasses() default {};
}

AutoConfigurationImportSelector类

DeferredImportSelector接口的实现类，处理自动装配，导出所有需要自动装配的类。

创建WebServer

SpringBoot会在onRefresh阶段创建webServer，首先从spring容器获取ServletWebServerFactory，然后调用getWebServer方法创建webServer。

getWebServer方法需要传入ServletContextInitializer集来初始化ServletContext。

@FunctionalInterface
public interface ServletContextInitializer {

	void onStartup(ServletContext servletContext) throws ServletException;
}

我们开发者如果需要使用ServletContextInitializer来初始化ServletContext的话，也可以编写一个实现类，然后将其注册到spring容器即可。

另外，SpringBoot还会自动装配DispatcherServletAutoConfiguration类，这个类会创建DispatcherServlet和DispatcherServletRegistrationBean。DispatcherServlet是SpringWebMvc的最核心组件，DispatcherServletRegistrationBean实现了ServletContextInitializer接口，可以将DispatcherServlet注册到ServletContext。以TomcatServletWebServerFactory为例，这个类会通过TomcatStarter来调用所有的ServletContextInitializer，TomcatStarter实现了ServletContainerInitializer接口，Tomcat的ServletContext在启动阶段会调用ServletContainerInitializer的onStartup方法来初始化Servlet容器。

SpringBoot启动流程

• 初始化environment应用配置参数：servletConfigInitParams, servletContextInitParams, systemProperties, systemEnvironment及配置文件等
• 创建ApplicationContext对象，SpringBoot应用默认使用的是AnnotationConfigServletWebServerApplicationContext类
• prepareContext阶段：触发一些事件，将启动类注册到Spring容器
• refresh阶段：扫描应用组件，自动装配
• onRefresh阶段：创建并初始化WebServer
• 调用ApplicationRunner和CommandLineRunner