主入口代码

@SpringBootApplication
public class HelloWorldMainApplication {

    public static void main(String[] args) {

        // 让Spring应用启动起来，这个原理就是启动spring的主入口
        SpringApplication.run(HelloWorldMainApplication.class,args);
    }
}

1@SpringBootApplication:

设置：这个类是SpringBoot的主配置类（HelloWorldMainApplication ），SpringBoot就应该运行这个类的main方法来启动SpringBoot应用；

SpringApplication.run(主配置类的类对象，main方法的参数);这个功能就像很久以前Java的启动类，args就是启动类参数。

@SpringBootApplication类

1.1@SpringBootConfiguration

容器初始化的时候会读取配置类信息到容器中，相当于配置web.xml读取配置文件

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

Spring Boot的配置类，标注在某个类上，表示这是一个Spring Boot的配置类；

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration//配置类上来标注这个注解，初始化配置类的信息到容器中
public @interface SpringBootConfiguration {
}

1.11@Configuration

用于定义配置类，可替换xml配置文件，被注解的类内部包含有一个或多个被@Bean注解的方法，这些方法将会被AnnotationConfigApplicationContext或AnnotationConfigWebApplicationContext类进行扫描，并用于构建bean定义，初始化Spring容器。

换句话说：使用这个注解将将某个包下的配置类初始化到容器中。

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
    String value() default "";
}

1.12@EnableAutoConfiguration:

开启自动配置功能,打个比方说，项目中集成redis，那么一定要有redis.xml的配置。当我们开启这个注解后，它会去自动配置redis的配置信息。

@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

1.121@AutoConfigurationPackage

自动配置包，相当于spring扫描包的功能（

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)//将主配置类（@SpringBootApplication标注的类）的所在包及下面所有子包里面的所有组件扫描到Spring容器；
public @interface AutoConfigurationPackage {

}

@Import(AutoConfigurationPackages.Registrar.class)//将主配置类（@SpringBootApplication标注的类）的所在包及下面所有子包里面的所有组件扫描到Spring容器；

@Order(Ordered.HIGHEST_PRECEDENCE)
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

   @Override
   public void registerBeanDefinitions(AnnotationMetadata metadata,
         BeanDefinitionRegistry registry) {
//注册主入口类的包名
      register(registry, new PackageImport(metadata).getPackageName());
   }

1.122@Import(EnableAutoConfigurationImportSelector.class)

给容器中导入选择器中的组件。
EnableAutoConfigurationImportSelector：导入的组件选择器；
将所有需要导入的组件以全类名的方式返回；这些组件就会被添加到容器中；
会给容器中导入非常多的自动配置类（xxxAutoConfiguration）；就是给容器中导入这个场景需要的所有组件，并配置好这些组件。

换句话说：当我们需要redis组件的时候，使用这个注解，redis的所有配置就会导入进来。

@Deprecated
public class EnableAutoConfigurationImportSelector
      extends AutoConfigurationImportSelector {

   @Override
   protected boolean isEnabled(AnnotationMetadata metadata) {
      if (getClass().equals(EnableAutoConfigurationImportSelector.class)) {
         return getEnvironment().getProperty(
               EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
               true);
      }
      return true;
   }
}

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
      AnnotationAttributes attributes) {
   List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
         getSpringFactoriesLoaderFactoryClass(), 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 static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();

    try {
        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)));
        }

        retur

META-INF/spring.factories

这些值作为自动配置类导入到容器中，自动配置类就生效，帮我们进行自动配置工作；

比如说：这里面有redis配置。

@Configuration
@ConditionalOnClass({ JedisConnection.class, RedisOperations.class, Jedis.class })
@EnableConfigurationProperties(RedisProperties.class)
public class RedisAutoConfiguration {

   /**
    * Redis connection configuration.
    */
   @Configuration
   @ConditionalOnClass(GenericObjectPool.class)
   protected static class RedisConnectionConfiguration {

      private final RedisProperties properties;

      private final RedisSentinelConfiguration sentinelConfiguration;

      private final RedisClusterConfiguration clusterConfiguration;

      public RedisConnectionConfiguration(RedisProperties properties,
            ObjectProvider<RedisSentinelConfiguration> sentinelConfiguration,
            ObjectProvider<RedisClusterConfiguration> clusterConfiguration) {
         this.properties = properties;
         this.sentinelConfiguration = sentinelConfiguration.getIfAvailable();
         this.clusterConfiguration = clusterConfiguration.getIfAvailable();
      }

      @Bean
      @ConditionalOnMissingBean(RedisConnectionFactory.class)
      public JedisConnectionFactory redisConnectionFactory()
            throws UnknownHostException {
         return applyProperties(createJedisConnectionFactory());
      }

RedisProperties.class

@ConfigurationProperties(prefix = "spring.redis")
public class RedisProperties {

   /**
    * Database index used by the connection factory.
    */
   private int database = 0;

   /**
    * Redis url, which will overrule host, port and password if set.
    */
   private String url;

   /**
    * Redis server host.
    */
   private String host = "localhost";

   /**
    * Login password of the redis server.
    */
   private String password;

SpringBoot启动的主入口注解SpringBootApplication分析

1@SpringBootApplication:

1.1@SpringBootConfiguration

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