通过代码实例了解SpringBoot启动原理

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

@Target({ElementType.TYPE})  //定义其使用时机
@Retention(RetentionPolicy.RUNTIME) //编译程序将Annotation储存于class档中，可由VM使用反射机制的代码所读取和使用。
@Documented //这个注解应该被 javadoc工具记录
@Inherited //被注解的类会自动继承. 更具体地说,如果定义注解时使用了 @Inherited 标记,然后用定义的注解来标注另一个父类, 父类又有一个子类(subclass),则父类的所有属性将被继承到它的子类中.
@SpringBootConfiguration //@SpringBootConfiguration就相当于@Configuration。JavaConfig配置形式
@EnableAutoConfiguration
@ComponentScan(
  excludeFilters = {@Filter(
  type = FilterType.CUSTOM,
  classes = {TypeExcludeFilter.class}
), @Filter(
  type = FilterType.CUSTOM,
  classes = {AutoConfigurationExcludeFilter.class}
)} //自动扫描并加载符合条件的组件。以通过basePackages等属性来细粒度的定制@ComponentScan自动扫描的范围，如果不指定，则默认Spring框架实现会从声明@ComponentScan所在类的package进行扫描。
注：所以SpringBoot的启动类最好是放在root package下，因为默认不指定basePackages。
)
public @interface SpringBootApplication {
  @AliasFor(
    annotation = EnableAutoConfiguration.class
  )
  Class<?>[] exclude() default {};

  @AliasFor(
    annotation = EnableAutoConfiguration.class
  )
  String[] excludeName() default {};

  @AliasFor(
    annotation = ComponentScan.class,
    attribute = "basePackages"
  )
  String[] scanBasePackages() default {};

  @AliasFor(
    annotation = ComponentScan.class,
    attribute = "basePackageClasses"
  )
  Class<?>[] scanBasePackageClasses() default {};
}

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
  String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

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

  String[] excludeName() default {};
}

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
  private static final String[] NO_IMPORTS = new String[0];
  private static final Log logger = LogFactory.getLog(AutoConfigurationImportSelector.class);
  private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";
  private ConfigurableListableBeanFactory beanFactory;
  private Environment environment;
  private ClassLoader beanClassLoader;
  private ResourceLoader resourceLoader;
  public AutoConfigurationImportSelector() {
  }

  public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
      return NO_IMPORTS;
    } else {
      // 从配置文件中加载 AutoConfigurationMetadata
      AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
      AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
      // 获取所有候选配置类EnableAutoConfiguration
      // 使用了内部工具使用SpringFactoriesLoader，查找classpath上所有jar包中的
      // META-INF\spring.factories，找出其中key为
      // org.springframework.boot.autoconfigure.EnableAutoConfiguration 
      // 的属性定义的工厂类名称。
      // 虽然参数有annotationMetadata,attributes,但在 AutoConfigurationImportSelector 的
      // 实现 getCandidateConfigurations()中，这两个参数并未使用
      List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
      //去重
      configurations = this.removeDuplicates(configurations);
      Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
      // 应用 exclusion 属性
      this.checkExcludedClasses(configurations, exclusions);
      configurations.removeAll(exclusions);
      // 应用过滤器AutoConfigurationImportFilter，
      // 对于 spring boot autoconfigure，定义了一个需要被应用的过滤器 ：
      // org.springframework.boot.autoconfigure.condition.OnClassCondition，
      // 此过滤器检查候选配置类上的注解@ConditionalOnClass，如果要求的类在classpath
      // 中不存在，则这个候选配置类会被排除掉
      configurations = this.filter(configurations, autoConfigurationMetadata);
     // 现在已经找到所有需要被应用的候选配置类
     // 广播事件 AutoConfigurationImportEvent
      this.fireAutoConfigurationImportEvents(configurations, exclusions);
      return StringUtils.toStringArray(configurations);
    }
  }

  protected AnnotationAttributes getAttributes(AnnotationMetadata metadata) {
    String name = this.getAnnotationClass().getName();
    AnnotationAttributes attributes = AnnotationAttributes.fromMap(metadata.getAnnotationAttributes(name, true));
    Assert.notNull(attributes, () -> {
      return "No auto-configuration attributes found. Is " + metadata.getClassName() + " annotated with " + ClassUtils.getShortName(name) + "?";
    });
    return attributes;
  }

  protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());
    Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.");
    return configurations;
  }


public abstract class SpringFactoriesLoader {
  public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
  private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);
  private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap();

  public SpringFactoriesLoader() {
  }
    public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
  }
}

  private List<String> filter(List<String> configurations, AutoConfigurationMetadata autoConfigurationMetadata) {
    long startTime = System.nanoTime();
    String[] candidates = StringUtils.toStringArray(configurations);
    // 记录候选配置类是否需要被排除,skip为true表示需要被排除,全部初始化为false,不需要被排除
    boolean[] skip = new boolean[candidates.length];
    // 记录候选配置类中是否有任何一个候选配置类被忽略，初始化为false
    boolean skipped = false;
    Iterator var8 = this.getAutoConfigurationImportFilters().iterator();
    // 获取AutoConfigurationImportFilter并逐个应用过滤
    while(var8.hasNext()) {
      AutoConfigurationImportFilter filter = (AutoConfigurationImportFilter)var8.next();
      // 对过滤器注入其需要Aware的信息
      this.invokeAwareMethods(filter);
      // 使用此过滤器检查候选配置类跟autoConfigurationMetadata的匹配情况
      boolean[] match = filter.match(candidates, autoConfigurationMetadata);

      for(int i = 0; i < match.length; ++i) {
        if (!match[i]) {
         // 如果有某个候选配置类不符合当前过滤器，将其标记为需要被排除，
    // 并且将 skipped设置为true，表示发现了某个候选配置类需要被排除
          skip[i] = true;
          skipped = true;
        }
      }
    }

    if (!skipped) {
    // 如果所有的候选配置类都不需要被排除，则直接返回外部参数提供的候选配置类集合
      return configurations;
    } else {
    // 逻辑走到这里因为skipped为true，表明上面的的过滤器应用逻辑中发现了某些候选配置类
    // 需要被排除，这里排除那些需要被排除的候选配置类，将那些不需要被排除的候选配置类组成
    // 一个新的集合返回给调用者
      List<String> result = new ArrayList(candidates.length);

      int numberFiltered;
      for(numberFiltered = 0; numberFiltered < candidates.length; ++numberFiltered) {
        if (!skip[numberFiltered]) {
          result.add(candidates[numberFiltered]);
        }
      }

      if (logger.isTraceEnabled()) {
        numberFiltered = configurations.size() - result.size();
        logger.trace("Filtered " + numberFiltered + " auto configuration class in " + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime) + " ms");
      }

      return new ArrayList(result);
    }
  }

  /**
   * 使用内部工具 SpringFactoriesLoader，查找classpath上所有jar包中的
   * META-INF\spring.factories，找出其中key为
   * org.springframework.boot.autoconfigure.AutoConfigurationImportFilter 
   * 的属性定义的过滤器类并实例化。
   * AutoConfigurationImportFilter过滤器可以被注册到 spring.factories用于对自动配置类
   * 做一些限制，在这些自动配置类的字节码被读取之前做快速排除处理。
   * spring boot autoconfigure 缺省注册了一个 AutoConfigurationImportFilter :
   * org.springframework.boot.autoconfigure.condition.OnClassCondition
  **/
  protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
    return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class, this.beanClassLoader);
  }

public ConfigurableApplicationContext run(String... args) {
  StopWatch stopWatch = new StopWatch();
  stopWatch.start();
  ConfigurableApplicationContext context = null;
  Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
  this.configureHeadlessProperty();
  //从类路径下META-INF/spring.factories获取 
  SpringApplicationRunListeners listeners = getRunListeners(args);
  //回调所有的获取SpringApplicationRunListener.starting()方法
  listeners.starting();
  try {
    //封装命令行参数
    ApplicationArguments applicationArguments = new DefaultApplicationArguments(
          args);
    //准备环境 
    ConfigurableEnvironment environment = prepareEnvironment(listeners,
          applicationArguments);
    //创建环境完成后回调 
    SpringApplicationRunListener.environmentPrepared()；表示环境准备完成
    this.configureIgnoreBeanInfo(environment);
    //打印Banner图
    Banner printedBanner = printBanner(environment);
    //创建ApplicationContext,决定创建web的ioc还是普通的ioc 
    context = createApplicationContext();
    //异常分析报告
    exceptionReporters = getSpringFactoriesInstances(
          SpringBootExceptionReporter.class,
          new Class[] { ConfigurableApplicationContext.class }, context);
    //准备上下文环境，将environment保存到ioc中
    //applyInitializers()：回调之前保存的所有的ApplicationContextInitializer的initialize方法 
    //listeners.contextPrepared(context) 
    //prepareContext运行完成以后回调所有的SpringApplicationRunListener的contextLoaded（）
    this.prepareContext(context, environment, listeners, applicationArguments,
          printedBanner);
    //刷新容器,ioc容器初始化（如果是web应用还会创建嵌入式的Tomcat）
    //扫描，创建，加载所有组件的地方,（配置类，组件，自动配置）
    this.refreshContext(context);
    this.afterRefresh(context, applicationArguments);
    stopWatch.stop();
    if (this.logStartupInfo) {
      new StartupInfoLogger(this.mainApplicationClass)
              .logStarted(getApplicationLog(), stopWatch);
    }
    //所有的SpringApplicationRunListener回调started方法
    listeners.started(context);
    //从ioc容器中获取所有的ApplicationRunner和CommandLineRunner进行回调，
    //ApplicationRunner先回调，CommandLineRunner再回调
    this.callRunners(context, applicationArguments);
  }
  catch (Throwable ex) {
    this.handleRunFailure(context, ex, exceptionReporters, listeners);
    throw new IllegalStateException(ex);
  }
  try {
    //所有的SpringApplicationRunListener回调running方法
    listeners.running(context);
  }
  catch (Throwable ex) {
    this.handleRunFailure(context, ex, exceptionReporters, null);
    throw new IllegalStateException(ex);
  }
  //整个SpringBoot应用启动完成以后返回启动的ioc容器
  return context;
}