今天聊一下SpringApplication 深入探索

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

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@SpringBootApplication
public class SpringbotApplication {  
	public static void main(String[] args) {
		SpringApplication app = new SpringApplication(SpringbotApplication.class);
 		// 自定义应用程序的配置
 		//app.setXxx()
 		app.run(args)
	}
}

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public static void main(String[] args) {
//		SpringApplication.run(Springboot2Application.class, args);
  SpringApplication application = new SpringApplication(Springboot2Application.class);
  application.setBanner((environment, sourceClass, out) -> {
    //这里打印一个logo
    System.out.println("      _      _       _\n" +
                       "     | |    (_)     | |\n" +
                       " ____| |__   _  ___ | |__    ___  _ __    __ _\n" +
                       "|_  /| '_ \\ | |/ __|| '_ \\  / _ \\| '_ \\  / _` |\n" +
                       " / / | | | || |\\__ \\| | | ||  __/| | | || (_| |\n" +
                       "/___||_| |_||_||___/|_| |_| \\___||_| |_| \\__, |\n" +
                       "                                          __/ |\n" +
                       "                                         |___/\n");
  });
  application.setBannerMode(Banner.Mode.CONSOLE);
  //你还可以干其他的定制化初始设置
  application.run(args);
}

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//静态方法，可用于使用默认配置运行 SpringApplication
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);
}

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public SpringApplication(Class<?>... primarySources) {
  this(null, primarySources);
}
//创建一个 SpringApplication 实例，应用上下文会根据指定的主要资源加载 beans ，实例在调用 run 方法之前可以定制化
@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));
  this.webApplicationType = deduceWebApplicationType();
  setInitializers((Collection) getSpringFactoriesInstances(
    ApplicationContextInitializer.class));
  setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
  this.mainApplicationClass = deduceMainApplicationClass();
}

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private static final String REACTIVE_WEB_ENVIRONMENT_CLASS = "org.springframework.web.reactive.DispatcherHandler";
private static final String MVC_WEB_ENVIRONMENT_CLASS = "org.springframework.web.servlet.DispatcherServlet";
private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",
			"org.springframework.web.context.ConfigurableWebApplicationContext" };

private WebApplicationType deduceWebApplicationType() {
  if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null)
      && !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)) {
    return WebApplicationType.REACTIVE;	//该程序是 REACTIVE 程序
  }
  for (String className : WEB_ENVIRONMENT_CLASSES) {
    if (!ClassUtils.isPresent(className, null)) {
      return WebApplicationType.NONE;	//该程序为 NONE
    }
  }
  return WebApplicationType.SERVLET;	//默认返回是 SERVLET 程序
}

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private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
  return getSpringFactoriesInstances(type, new Class<?>[] {});
}
//获取所有的 Spring 工厂实例
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
Class<?>[] parameterTypes, Object... args) {
  ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
  // Use names and ensure unique to protect against duplicates
  Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader)); //获取所有 Spring Factories 的名字
  List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
                                                     classLoader, args, names);
  AnnotationAwareOrderComparator.sort(instances); //Spring 工厂实例排序
  return instances;
}
//根据读取到的名字创建对象（Spring 工厂实例）
private <T> List<T> createSpringFactoriesInstances(Class<T> type,
 Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args, Set<String> names) {
  List<T> instances = new ArrayList<>(names.size());
  for (String name : names) {
    try {
      Class<?> instanceClass = ClassUtils.forName(name, classLoader);
      Assert.isAssignable(type, instanceClass);
      Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);
      T instance = (T) BeanUtils.instantiateClass(constructor, args);
      instances.add(instance);
    }
    catch (Throwable ex) {
      throw new IllegalArgumentException(
        "Cannot instantiate " + type + " : " + name, ex);
    }
  }
  return instances;
}

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public void setInitializers(
			Collection<? extends ApplicationContextInitializer<?>> initializers) {
  this.initializers = new ArrayList<>();
  this.initializers.addAll(initializers);
}

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public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {
  this.listeners = new ArrayList<>();
  this.listeners.addAll(listeners);
}

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private Class<?> deduceMainApplicationClass() {
  try {
    StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
    for (StackTraceElement stackTraceElement : stackTrace) {
      if ("main".equals(stackTraceElement.getMethodName())) {
        return Class.forName(stackTraceElement.getClassName());
      }
    }
  }
  catch (ClassNotFoundException ex) {
    // Swallow and continue
  }
  return null;
}

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//运行 Spring 应用程序，创建并刷新一个新的 ApplicationContext
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);
			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;
	}

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private static final String SYSTEM_PROPERTY_JAVA_AWT_HEADLESS = "java.awt.headless";
private boolean headless = true;

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

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 //TODO:  xxx
SpringApplicationRunListeners listeners = getRunListeners(args);//初始化监听器
listeners.starting();
try {
  prepareContext(context, environment, listeners, applicationArguments, printedBanner);
  refreshContext(context);
  afterRefresh(context, applicationArguments);
  listeners.started(context);
  callRunners(context, applicationArguments);
}
try {
  listeners.running(context);
}

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

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public DefaultApplicationArguments(String[] args) {
  Assert.notNull(args, "Args must not be null");
  this.source = new Source(args);
  this.args = args;
}

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private ConfigurableEnvironment prepareEnvironment(
  SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments) {
  // Create and configure the environment
  ConfigurableEnvironment environment = getOrCreateEnvironment();
  configureEnvironment(environment, applicationArguments.getSourceArgs());
  listeners.environmentPrepared(environment);
  bindToSpringApplication(environment);
  if (this.webApplicationType == WebApplicationType.NONE) {
    environment = new EnvironmentConverter(getClassLoader())
      .convertToStandardEnvironmentIfNecessary(environment);
  }
  ConfigurationPropertySources.attach(environment);
  return environment;
}
//如果 environment 不为空，直接 get 到，否则创建
private ConfigurableEnvironment getOrCreateEnvironment() {
  if (this.environment != null) {
    return this.environment;
  }
  if (this.webApplicationType == WebApplicationType.SERVLET) {
    return new StandardServletEnvironment();
  }
  return new StandardEnvironment();
}
//配置环境
protected void configureEnvironment(ConfigurableEnvironment environment,String[] args) {
  configurePropertySources(environment, args);//配置要使用的PropertySources
  configureProfiles(environment, args);//配置要使用的Profiles
}
//将环境绑定到 SpringApplication
protected void bindToSpringApplication(ConfigurableEnvironment environment) {
  try {
    Binder.get(environment).bind("spring.main", Bindable.ofInstance(this));
  }
  catch (Exception ex) {
    throw new IllegalStateException("Cannot bind to SpringApplication", ex);
  }
}

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public static final String IGNORE_BEANINFO_PROPERTY_NAME = "spring.beaninfo.ignore";

private void configureIgnoreBeanInfo(ConfigurableEnvironment environment) {
  if (System.getProperty(
    CachedIntrospectionResults.IGNORE_BEANINFO_PROPERTY_NAME) == null) {
    Boolean ignore = environment.getProperty("spring.beaninfo.ignore",
                                             Boolean.class, Boolean.TRUE);
    System.setProperty(CachedIntrospectionResults.IGNORE_BEANINFO_PROPERTY_NAME,
                       ignore.toString());
  }
}

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private Banner printBanner(ConfigurableEnvironment environment) {
  if (this.bannerMode == Banner.Mode.OFF) {	//如果设置为 off，不打印 Banner
    return null;
  }
  ResourceLoader resourceLoader = this.resourceLoader != null ? this.resourceLoader
    : new DefaultResourceLoader(getClassLoader());
  SpringApplicationBannerPrinter bannerPrinter = new SpringApplicationBannerPrinter(
    resourceLoader, this.banner);
  if (this.bannerMode == Mode.LOG) {
    return bannerPrinter.print(environment, this.mainApplicationClass, logger);
  }
  return bannerPrinter.print(environment, this.mainApplicationClass, System.out);
}

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public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext";
public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";
public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context.annotation.AnnotationConfigApplicationContext";

protected ConfigurableApplicationContext createApplicationContext() {
  Class<?> contextClass = this.applicationContextClass;
  if (contextClass == null) {
    try {
      switch (this.webApplicationType) {	//根据应用程序的类型来初始化容器
        case SERVLET:	//servlet 应用程序
          contextClass = Class.forName(DEFAULT_WEB_CONTEXT_CLASS);
          break;
        case REACTIVE:	//reactive 应用程序
          contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
          break;
        default:		//默认
          contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
      }
    } catch (ClassNotFoundException ex) {
  throw new IllegalStateException(
    "Unable create a default ApplicationContext,please specify an 		    ApplicationContextClass",ex);
    }
  }
  //最后通过Spring的工具类 BeanUtils 初始化容器类 bean
  return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

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private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
            Class<?>[] parameterTypes, Object... args) {
  ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
  // Use names and ensure unique to protect against duplicates
  Set<String> names = new LinkedHashSet<>(
    SpringFactoriesLoader.loadFactoryNames(type, classLoader));
  List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
       classLoader, args, names);//根据类型 key 为 SpringBootExceptionReporter 去加载
  AnnotationAwareOrderComparator.sort(instances);//对实例排序
  return instances;
}

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//装配 Context
private void prepareContext(ConfigurableApplicationContext context,
   ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
   ApplicationArguments applicationArguments, Banner printedBanner) {
  //将之前准备好的 environment 设置给创建好的 ApplicationContext 使用
  context.setEnvironment(environment);
  //1、
  postProcessApplicationContext(context);
  //2、
  applyInitializers(context);
  listeners.contextPrepared(context);
  if (this.logStartupInfo) {//启动日志
    logStartupInfo(context.getParent() == null);
    logStartupProfileInfo(context);
  }
  // Add boot specific singleton beans
  context.getBeanFactory().registerSingleton("springApplicationArguments",
                                             applicationArguments);
  if (printedBanner != null) {
    context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
  }
  // Load the sources
  Set<Object> sources = getAllSources();
  Assert.notEmpty(sources, "Sources must not be empty");
  //3、
  load(context, sources.toArray(new Object[0]));
  listeners.contextLoaded(context);
}

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public static final String CONFIGURATION_BEAN_NAME_GENERATOR = "org.springframework.context.annotation.internalConfigurationBeanNameGenerator";

protected void postProcessApplicationContext(ConfigurableApplicationContext context) {
  if (this.beanNameGenerator != null) {
    context.getBeanFactory().registerSingleton(
      AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR,
      this.beanNameGenerator);
  }
  if (this.resourceLoader != null) {
    if (context instanceof GenericApplicationContext) {
      ((GenericApplicationContext) context)
      .setResourceLoader(this.resourceLoader);
    }
    if (context instanceof DefaultResourceLoader) {
      ((DefaultResourceLoader) context)
      .setClassLoader(this.resourceLoader.getClassLoader());
    }
  }
}

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protected void applyInitializers(ConfigurableApplicationContext context) {
  for (ApplicationContextInitializer initializer : getInitializers()) {
    Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
      initializer.getClass(), ApplicationContextInitializer.class);
    Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
    initializer.initialize(context);
  }
}

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protected void load(ApplicationContext context, Object[] sources) {
  BeanDefinitionLoader loader = createBeanDefinitionLoader( //2
    getBeanDefinitionRegistry(context), sources);// 1
  if (this.beanNameGenerator != null) {
    loader.setBeanNameGenerator(this.beanNameGenerator);
  }
  if (this.resourceLoader != null) {
    loader.setResourceLoader(this.resourceLoader);
  }
  if (this.environment != null) {
    loader.setEnvironment(this.environment);
  }
  loader.load();//3
}

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private BeanDefinitionRegistry getBeanDefinitionRegistry(ApplicationContext context) {
  if (context instanceof BeanDefinitionRegistry) {
    return (BeanDefinitionRegistry) context;
  }
  if (context instanceof AbstractApplicationContext) {
    return (BeanDefinitionRegistry) ((AbstractApplicationContext) context)
      .getBeanFactory();
  }
  throw new IllegalStateException("Could not locate BeanDefinitionRegistry");
}

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private void refreshContext(ConfigurableApplicationContext context) {
  refresh(context);//1
  if (this.registerShutdownHook) {
    try {
      context.registerShutdownHook();
    }
    catch (AccessControlException ex) {
      // Not allowed in some environments.
    }
  }
}
//刷新底层的 ApplicationContext
protected void refresh(ApplicationContext applicationContext) {
  Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
  ((AbstractApplicationContext) applicationContext).refresh();
}

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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); //1
      // Last step: publish corresponding event.
      finishRefresh();
    } catch (BeansException 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();
    }
  }
}

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public void preInstantiateSingletons() throws BeansException {
  // Iterate over a copy to allow for init methods which in turn register new bean definitions.
  // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
  List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

  // Trigger initialization of all non-lazy singleton beans...
  for (String beanName : beanNames) {
    RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
    if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
      if (isFactoryBean(beanName)) {
        Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
        if (bean instanceof FactoryBean) {
          final FactoryBean<?> factory = (FactoryBean<?>) bean;
          boolean isEagerInit;
         if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean){
            isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
             ((SmartFactoryBean<?>) factory)::isEagerInit, getAccessControlContext());
          } else {
            isEagerInit = (factory instanceof SmartFactoryBean &&
                           ((SmartFactoryBean<?>) factory).isEagerInit());
          }
          if (isEagerInit) {
            getBean(beanName);
          }
        }
      } else {
        getBean(beanName);
      }
    }
  }

  // Trigger post-initialization callback for all applicable beans...
  for (String beanName : beanNames) {
    Object singletonInstance = getSingleton(beanName);
    if (singletonInstance instanceof SmartInitializingSingleton) {
      final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
      if (System.getSecurityManager() != null) {
        AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
          smartSingleton.afterSingletonsInstantiated();
          return null;
        }, getAccessControlContext());
      } else {
        smartSingleton.afterSingletonsInstantiated();
      }
    }
  }
}

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