Springboot series: @SpringBootApplication annotation

Posted on 2022-03-25 by Admin

Forwarding reason: This article is a more comprehensive SpringbootApplication annotation description I have seen.

When using the Springboot framework for development, we usually add the @SpringBootApplication annotation to the main function. Today, I will analyze @SpringBootApplication for everyone. If there is anything wrong, please criticize and correct it.

@SpringBootApplication

The @SpringBootApplication source code is as follows:

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan
public @interface SpringBootApplication {
    Class<?>[] exclude() default {};

    String[] excludeName() default {};

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

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

It is known from the source code that the @SpringBootApplication 被 @SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScanannotation is modified, in other words, Springboot provides a unified annotation to replace the above three annotations to simplify the configuration of the program. The function of each annotation is explained below.

1.@SpringBootConfiguration

The spring documentation states:

SpringBootConfiguration is a class-level annotation indicating that an object is a source of bean definitions. SpringBootConfiguration classes declare beans via public @Bean annotated methods.

SpringBootConfiguration is a class-level annotation that indicates that the object is a source of bean definitions. The SpringBootConfiguration class declares beans via public methods annotated with @bean.

The @Bean annotation is used to indicate that a method instantiates, configures and initializes a new object to be managed by the Spring IoC container.

The @Bean annotation is used to denote a method instantiation, configuration and initialization of a new object managed by the Spring IoC container.

Generally speaking, SpringBootConfiguration is generally used in conjunction with the @Bean annotation. Annotating classes with SpringBootConfiguration is equivalent to configuring beans in XML, and using @Bean annotation methods is equivalent to configuring beans in XML. for example:

The XML configuration code is as follows:

<beans>
    <bean id = "userService" class="com.user.UserService">
        <property name="userDAO" ref = "userDAO"></property>
    </bean>
    <bean id = "userDAO" class="com.user.UserDAO"></bean>
</beans>

Equivalent to

package org.spring.com.user;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

SpringBootConfiguration
public class Config {
    @Bean
    public UserService getUserService(){
        UserService userService = new UserService();
        userService.setUserDAO(null);
        return userService;
    }

    @Bean
    public UserDAO getUserDAO(){
        return new UserDAO();
    }
}

package org.spring.com.user;

public class UserService {
    private UserDAO userDAO;

    public UserDAO getUserDAO() {
        return userDAO;
    }

    public void setUserDAO(UserDAO userDAO) {
        this.userDAO = userDAO;
    }
}

package org.spring.com.user;

public class UserDAO {
}

Notice:

Use public to decorate the method of @Bean annotation;
UserService and UserDAO classes do not need to be declared as @Component, @Service, @Repository, @Controller; after
reading the above Java code, some people may ask, why not inject UserDAO directly with @Autowired annotation? I think so @Configuration, @Bean pay more attention to the content of the configuration, for example, we can set the special information of the UserDAO class object in the @Bean method, such as specifying the transactionManager and so on.

2.@EnableAutoConfiguration

The spring documentation states:

Enable auto-configuration of the spring Application Context, attempting to guess and configure beans that you are likely to need. Auto-configuration classes are usually applied based on your classpath and what beans you have defined.

Enables auto-configuration of the Spring application context, trying to guess and configure beans you might need. Auto-configured classes are usually applied based on your classpath and already defined beans objects.

The package of the class that is annotated with @EnableAutoConfiguration has specific significance and is often used as a ‘default’. For example, it will be used when scanning for@Entity classes. It is generally recommended that you place@EnableAutoConfiguration in a root package so that all sub-packages and classes can be searched.

The package in which the class annotated with @EnableAutoConfiguration resides has a specific meaning and is used as the default configuration. For example, it will be used when scanning for @Entity classes. It is generally recommended to configure @EnableAutoConfiguration under the root package, so that all subpackages and classes can be found.

Auto-configuration classes are regular Spring Configuration beans. They are located using the SpringFactoriesLoader mechanism (keyed against this class).Generally auto-configuration beans are @Conditional beans (most often using @ConditionalOnClass and @ConditionalOnMissingBean annotations).

Auto-configuration classes are regular Spring configuration beans. They use the SpringFactoriesLoader mechanism (keyed on the EnableAutoConfiguration classpath). Usually auto-configuration beans are @Conditional beans (used in most cases with @ConditionalOnClass and @ConditionalOnMissingBean annotations).

SpringFactoriesLoader mechanism:
SpringFactoriesLoader will query the JAR containing the META-INF/spring.factories file. When the spring.factories file is found, SpringFactoriesLoader will query the properties named in the configuration file. The key value of EnableAutoConfiguration is org.springframework.boot.autoconfigure.EnableAutoConfiguration. Perform spring configuration according to the value corresponding to this key. In the spring-boot-autoconfigure.jar file, there is a spring.factories file with the following contents:

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# 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.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
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.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
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.jms.hornetq.HornetQAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\
org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\
org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\
org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.velocity.VelocityAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration

# Template availability providers
org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.velocity.VelocityTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.JspTemplateAvailabilityProvider

3.@ComponentScan

Use annotation configuration to achieve automatic scanning. By default, the current package and all sub-packages will be scanned, which is the same as the automatic scanning effect of xml configuration.

The spring documentation states:

Configures component scanning directives for use with @Configuration classes. Provides support parallel with Spring XML’s element.

Configure component scan directives for @Configuration annotated classes. Also provides parallel support for Spring XML's elements.

Either basePackageClasses() or basePackages() (or its alias value()) may be specified to define specific packages to scan. If specific packages are not defined, scanning will occur from the package of the class that declares this annotation.

Either basePackageClasses() or basePackages() (or its alias value) can define specific packages to scan. If the specified package is not defined, it will be scanned from the package where the class in which the annotation is declared is located.

Note that the element has an annotation-config attribute; however, this annotation does not. This is because in almost all cases when using @ComponentScan, default annotation config processing (e.g. processing @Autowired and friends) is assumed. Furthermore, when using AnnotationConfigApplicationContext, annotation config processors are always registered, meaning that any attempt to disable them at the @ComponentScan level would be ignored.

Note that the element has an annotation-config attribute (details: http://www.cnblogs.com/exe19/p/5391712.html ), but @ComponentScan does not. This is because in almost all cases where the @ComponentScan annotation is used, the default annotation configuration processing is assumed. Additionally, when using AnnotationConfigApplicationContext, the annotation configuration handler is always registered, meaning that any attempt to invalidate the scan at the @ComponentScan level will be ignored.

In layman's terms, the @ComponentScan annotation will automatically scan all the classes marked with the @Component annotation under the specified package, and register them as beans, of course, including the sub-annotations @Service, @Repository, @Controller under @Component. @ComponentScan annotation.

ProgrammerSought