SpringBoot启动过程的实现

SpringBoot启动过程分析,首先打开SpringBoot的启用入口Main类:

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

可以看到main方法里面只有一行核心启用类:SpringApplication.run(ApplicationMain.class, args);这个是关键,在改行打上断点,debug模式启动该main类。点击下一步进入SpringApplication的源码对应的run方法:

public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
  return (new SpringApplication(sources)).run(args);
 }

初始化SpringApplication

SpringApplication实例化之前会调用构造方法进行初始化:

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);
 }

而SpringApplication构造方法的核心是:this.initialize(sources);初始化方法,SpringApplication通过调用该方法来初始化。

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

1.deduceWebEnvironment方法是用来判断当前应用的环境,该方法通过获取这两个类来判断当前环境是否是web环境,如果能获得这两个类说明是web环境,否则不是。

javax.servlet.Servlet
org.springframework.web.context.ConfigurableWebApplicationContext

2.getSpringFactoriesInstances方法主要用来从spring.factories文件中找出key为ApplicationContextInitializer的类并实例化,然后调用setInitializers方法设置到SpringApplication的initializers属性中。这个过程就是找出所有的应用程序初始化器。

private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
  ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
  Set<String> names = new LinkedHashSet(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
  List<T> instances = this.createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
  AnnotationAwareOrderComparator.sort(instances);
  return instances;
 }
public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
  String factoryClassName = factoryClass.getName();

  try {
   //从spring.factories文件中找出key为ApplicationContextInitializer的类
   Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
   ArrayList result = new ArrayList();

   while(urls.hasMoreElements()) {
    URL url = (URL)urls.nextElement();
    Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
    String factoryClassNames = properties.getProperty(factoryClassName);
    result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
   }

   return result;
  } catch (IOException var8) {
   throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + "META-INF/spring.factories" + "]", var8);
  }
 }

当前的初始化器有如下几个:


3.同理调用getSpringFactoriesInstances从spring.factories文件中找出key为ApplicationListener的类并实例化,然后调用setListeners方法设置到SpringApplication的listeners属性中。这个过程就是找出所有的应用程序事件监听器。
当前的事件监听器有如下几个:


4.调用deduceMainApplicationClass方法找出main类,就是这里的ApplicationMain类。

运行SpringApplication

初始化SpringApplication完成之后,调用run方法运行:

public ConfigurableApplicationContext run(String... args) {
  //计时器,统计任务的执行时间
  StopWatch stopWatch = new StopWatch();
  //开始执行
  stopWatch.start();
  ConfigurableApplicationContext context = null;
  FailureAnalyzers analyzers = null;
  this.configureHeadlessProperty();
  // 获取SpringApplicationRunListeners启动事件监听器,这里只有一个EventPublishingRunListener
  SpringApplicationRunListeners listeners = this.getRunListeners(args);
  // 封装成SpringApplicationEvent事件然后广播出去给SpringApplication中的listeners所监听
  listeners.starting();

  try {
   // 构造一个应用程序参数持有类
   ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
   // 准备并配置环境
   ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
    // 打印banner图形
   Banner printedBanner = this.printBanner(environment);
   // 创建Spring容器
   context = this.createApplicationContext();
   new FailureAnalyzers(context);
   // 配置Spring容器
   this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
   // 容器上下文刷新
   this.refreshContext(context);
   // 容器创建完成之后调用afterRefresh方法
   this.afterRefresh(context, applicationArguments);
   // 调用监听器,广播Spring启动结束的事件
   listeners.finished(context, (Throwable)null);
   // 停止计时器
   stopWatch.stop();
   if (this.logStartupInfo) {
    (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
   }

   return context;
  } catch (Throwable var9) {
   this.handleRunFailure(context, listeners, (FailureAnalyzers)analyzers, var9);
   throw new IllegalStateException(var9);
  }
 }

SpringApplicationRunListeners

1.获取启动事件监听器,可以看看该方法:

SpringApplicationRunListeners listeners = this.getRunListeners(args);

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

同样的通过调用getSpringFactoriesInstances方法去META-INF/spring.factories文件中拿到SpringApplicationRunListener监听器,当前的SpringApplicationRunListener事件监听器只有一个EventPublishingRunListener广播事件监听器:


SpringApplicationRunListeners内部持有SpringApplicationRunListener集合和1个Log日志类。用于SpringApplicationRunListener监听器的批量执行。

SpringApplicationRunListener用于监听SpringApplication的run方法的执行,它定义了5个步骤:

1.starting:run方法执行的时候立马执行,对应的事件类型是ApplicationStartedEvent
2.environmentPrepared:ApplicationContext创建之前并且环境信息准备好的时候调用,对应的事件类型是ApplicationEnvironmentPreparedEvent
3.contextPrepared: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
4.contextLoaded:ApplicationContext创建并加载之后并在refresh之前调用,对应的事件类型是ApplicationPreparedEvent
5.finished:run方法结束之前调用,对应事件的类型是ApplicationReadyEvent或ApplicationFailedEvent

SpringApplicationRunListener目前只有一个实现类EventPublishingRunListener,详见获取SpringApplicationRunListeners。它把监听的过程封装成了SpringApplicationEvent事件并让内部属性ApplicationEventMulticaster接口的实现类SimpleApplicationEventMulticaster广播出去,广播出去的事件对象会被SpringApplication中的listeners属性进行处理。

所以说SpringApplicationRunListener和ApplicationListener之间的关系是通过ApplicationEventMulticaster广播出去的SpringApplicationEvent所联系起来的

2.启动事件监听器

通过listeners.starting()可以启动事件监听器SpringApplicationRunListener ,SpringApplicationRunListener 是一个启动事件监听器接口:

public interface SpringApplicationRunListener {
 void starting();

 void environmentPrepared(ConfigurableEnvironment var1);

 void contextPrepared(ConfigurableApplicationContext var1);

 void contextLoaded(ConfigurableApplicationContext var1);

 void finished(ConfigurableApplicationContext var1, Throwable var2);
}

SpringApplicationRunListener 接口的具体实现就是EventPublishingRunListener类,我们主要来看一下它的startting方法,该方法会封装成SpringApplicationEvent事件然后广播出去给SpringApplication中的listeners所监听。

public void starting() {
  this.initialMulticaster.multicastEvent(new ApplicationStartedEvent(this.application, this.args));
 }

配置并准备环境

private ConfigurableEnvironment prepareEnvironment(
  SpringApplicationRunListeners listeners,
  ApplicationArguments applicationArguments) {
 // 创建应用程序的环境信息。如果是web程序,创建StandardServletEnvironment;否则,创建StandardEnvironment
 ConfigurableEnvironment environment = getOrCreateEnvironment();
 // 配置环境信息。比如profile,命令行参数
 configureEnvironment(environment, applicationArguments.getSourceArgs());
 // 广播出ApplicationEnvironmentPreparedEvent事件给相应的监听器执行
 listeners.environmentPrepared(environment);
 // 环境信息的校对
 if (!this.webEnvironment) {
  environment = new EnvironmentConverter(getClassLoader())
    .convertToStandardEnvironmentIfNecessary(environment);
 }
 return environment;
}

判断环境,如果是web程序,创建StandardServletEnvironment;否则,创建StandardEnvironment。

private ConfigurableEnvironment getOrCreateEnvironment() {
  if (this.environment != null) {
   return this.environment;
  } else {
   return (ConfigurableEnvironment)(this.webEnvironment ? new StandardServletEnvironment() : new StandardEnvironment());
  }
 }

创建Spring容器上下文

protected ConfigurableApplicationContext createApplicationContext() {
 Class<?> contextClass = this.applicationContextClass;
 if (contextClass == null) {
  try {
   // 判断是否是web应用,
   // 如果是则创建AnnotationConfigEmbeddedWebApplicationContext,否则创建AnnotationConfigApplicationContext
   contextClass = Class.forName(this.webEnvironment
     ? DEFAULT_WEB_CONTEXT_CLASS : DEFAULT_CONTEXT_CLASS);
  }
  catch (ClassNotFoundException ex) {
   throw new IllegalStateException(
     "Unable create a default ApplicationContext, "
       + "please specify an ApplicationContextClass",
     ex);
  }
 }
 return (ConfigurableApplicationContext) BeanUtils.instantiate(contextClass);
}

配置Spring容器上下文

private void prepareContext(ConfigurableApplicationContext context,
  ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
  ApplicationArguments applicationArguments, Banner printedBanner) {
 // 设置Spring容器上下文的环境信息
 context.setEnvironment(environment);
 // Spring容器创建之后做一些额外的事
 postProcessApplicationContext(context);
 // SpringApplication的初始化器开始工作
 applyInitializers(context);
 // 遍历调用SpringApplicationRunListener的contextPrepared方法。目前只是将这个事件广播器注册到Spring容器中
 listeners.contextPrepared(context);
 if (this.logStartupInfo) {
  logStartupInfo(context.getParent() == null);
  logStartupProfileInfo(context);
 }

 // 把应用程序参数持有类注册到Spring容器中,并且是一个单例
 context.getBeanFactory().registerSingleton("springApplicationArguments",
   applicationArguments);
 if (printedBanner != null) {
  context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
 }

 // 加载sources,sources是main方法所在的类
 Set<Object> sources = getSources();
 Assert.notEmpty(sources, "Sources must not be empty");
 // 将sources加载到应用上下文中。最终调用的是AnnotatedBeanDefinitionReader.registerBean方法
 load(context, sources.toArray(new Object[sources.size()]));
 // 广播出ApplicationPreparedEvent事件给相应的监听器执行
 // 执行EventPublishingRunListener.contextLoaded方法
 listeners.contextLoaded(context);
}

Spring容器创建之后回调方法postProcessApplicationContext

protected void postProcessApplicationContext(ConfigurableApplicationContext context) {
 // 如果SpringApplication设置了实例命名生成器,则注册到Spring容器中
 if (this.beanNameGenerator != null) {
  context.getBeanFactory().registerSingleton(
    AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR,
    this.beanNameGenerator);
 }
 // 如果SpringApplication设置了资源加载器,设置到Spring容器中
 if (this.resourceLoader != null) {
  if (context instanceof GenericApplicationContext) {
   ((GenericApplicationContext) context)
     .setResourceLoader(this.resourceLoader);
  }
  if (context instanceof DefaultResourceLoader) {
   ((DefaultResourceLoader) context)
     .setClassLoader(this.resourceLoader.getClassLoader());
  }
 }
}

初始化器开始工作

protected void applyInitializers(ConfigurableApplicationContext context) {
 // 遍历每个初始化器,调用对应的initialize方法
 for (ApplicationContextInitializer initializer : getInitializers()) {
  Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
    initializer.getClass(), ApplicationContextInitializer.class);
  Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
  initializer.initialize(context);
 }
}

Spring容器创建完成之后会调用afterRefresh方法

ApplicationRunner、CommandLineRunner类都是在在afterRefresh方法中调用的,也就是说在Spring容器创建之后执行的。

protected void applyInitializers(ConfigurableApplicationContext context) {
 // 遍历每个初始化器,调用对应的initialize方法
 for (ApplicationContextInitializer initializer : getInitializers()) {
  Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
    initializer.getClass(), ApplicationContextInitializer.class);
  Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
  initializer.initialize(context);
 }
}

参考:https://blog.wangqi.love/articles/Spring/SpringBoot启动过程.html

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