① . view the main startup class as a whole

• ① . main startup classes are as follows:

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

• ② 2. Key analysis
  @SpringBootConfiguration @EnableAutoConfiguration @ComponentScan

@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}
)}
)

②. @SpringBootConfiguration

• ① There is an annotation @ Configuration in SpringBootConfiguration. The class marked with this annotation is a Configuration class

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
    @AliasFor(
        annotation = Configuration.class
    )
    boolean proxyBeanMethods() default true;
}

• ② The @ Configuration annotation is used to define the Configuration class in the way of JavaConfig, which can replace the xml Configuration file. The annotated class contains one or more methods annotated by @ Bean. These methods will be scanned by the annotationconfigapplicationcontext or AnnotationConfig WebApplicationContext class, and used to build the Bean definition and initialize the Spring container. This is also why there is no xml Configuration file in SpringBoot. SpringBoot injects beans into the container of SpringIOC by configuring classes

③. @ComponentScan

• Scan indicates the class or its sub package of @ SpringBootApplication for a scan

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Documented
@Repeatable(ComponentScans.class)
public @interface ComponentScan {

④. @EnableAutoConfiguration

• ① The main codes are as follows

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {

• ② . @ AutoConfigurationPackage: import a component into the container, use the Registrar to import a series of components into the container, and import all components under a specified package (under the package where MainApplication is located)
  
• ③. @Import(AutoConfigurationImportSelector.class)
  (1) . use getautoconfigurationentry (annotation metadata); Batch import some components into the container
  (2) . call list configurations = getcandidate configurations (annotation metadata, attributes) to get all configuration classes that need to be imported into the container
  (3) . use the factory to load map < string, list > loadspringfactories (@ nullable classloader classloader): get all components
  (4) . load a file from the META-INF/spring.factories location.
  Scan all META-INF/spring.factories files in our current system by default
  spring-boot-autoconfigure-2.3.4.RELEASE.jar package also contains META-INF/spring.factories
  Not all beans will be initialized. Use Condition in the configuration class to load qualified beans

   public String[] selectImports(AnnotationMetadata annotationMetadata) {
       if (!this.isEnabled(annotationMetadata)) {
           return NO_IMPORTS;
       } else {
           //(1) . use getautoconfigurationentry (annotation metadata); Batch import some components into the container
           AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry =
           this.getAutoConfigurationEntry(annotationMetadata);
           return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
       }
   }
   protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
      if (!this.isEnabled(annotationMetadata)) {
          return EMPTY_ENTRY;
      } else {
          AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
          //(2) . call list < string > configurations = getcandidateconfigurations (annotationmetadata, attributes)
          //Get all the configuration classes that need to be imported into the container
          List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
          configurations = this.removeDuplicates(configurations);
          Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
          this.checkExcludedClasses(configurations, exclusions);
          configurations.removeAll(exclusions);
          configurations = this.getConfigurationClassFilter().filter(configurations);
          this.fireAutoConfigurationImportEvents(configurations, exclusions);
          return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
      }
  }
   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;
  }
  //(3) . use the factory to load map < string, list < string > > loadspringfactories (@ nullable classloader) 
  //classLoader) to get all the components
  public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
      String factoryTypeName = factoryType.getName();
      return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
  }
  private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
     MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
      if (result != null) {
          return result;
      } else {
          try {
          //(4) . load a file from the META-INF/spring.factories location
          Enumeration<URL> urls = classLoader != null ? classLoader.getResources
          ("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
          LinkedMultiValueMap result = new LinkedMultiValueMap();

# Although all the automatic configurations of our 127 scenarios are loaded by default when they are started. xxxxAutoConfiguration
# According to the conditional assembly rule (@ Conditiona), it will be configured as required
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.LifecycleAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.r2dbc.R2dbcDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.r2dbc.R2dbcRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.r2dbc.R2dbcTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchRestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\
org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.availability.ApplicationAvailabilityAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\
org.springframework.boot.autoconfigure.r2dbc.R2dbcAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketRequesterAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketServerAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketStrategiesAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.rsocket.RSocketSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyAutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

• ④ B. best practices

1. Introduce scenario dependency
2. View which are automatically configured (optional)
   After self analysis, the automatic configuration corresponding to the imported scenario generally takes effect
   debug=true in the configuration file enables the automatic configuration report
   Negative positive
3. What configuration files need to be modified Here, take the diagram of modifying SpringBoot startup as an example

spring:
  banner:
    image:
      location: classpath:bug.png

⑤. Lombok

• ① . lombok annotation (which can then be used on beans)

1. @Data annotation: used in JavaBeans or class JavaBeans. This annotation covers the widest range. It includes getter, setter and NoArgsConstructor annotations, that is, when the current annotation is used, all the included methods will be automatically generated
2. @Getter annotation: used in JavaBean or JavaBean class. Using this annotation will generate corresponding getter methods
3. @Setter annotation: used in JavaBean or JavaBean class. Using this annotation will generate the corresponding setter method
4. @NoArgsConstructor annotation: used in JavaBeans or class JavaBeans. Using this annotation will generate the corresponding parameterless constructor
5. @AllArgsConstructor annotation: used in JavaBeans or class JavaBeans. Using this annotation will generate the corresponding parametric constructor
6. @ToString annotation: used in JavaBeans or JavaBeans. Using this annotation will automatically override the corresponding toString method
7. @EqualsAndHashCode annotation: used in JavaBeans or JavaBeans. Using this annotation will automatically override the corresponding equals method and hashCode method
8. @Slf4j: used in classes that need to print logs. When the slf4j print log framework is used in the project, using this annotation will simplify the log printing process. Just call the info method

• ② . find lombok in Idea
  
• ③ . add lombok dependency to pom.xml

	<dependency>
	 <groupId>org.projectlombok</groupId> 
	 <artifactId>lombok</artifactId> 
	</dependency>

⑥ . best practices - Dev tools

• ① . add dependency

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-devtools</artifactId>
        <optional>true</optional>
    </dependency>
</dependencies>

• ② . in IDEA, after modifying the project or page: Ctrl+F9

⑦ . best practices - Spring Initailizr

• ① . in IDEA, menu bar new - > Project - > spring initailizr
  
• ② . quickly create a SpringBoot project