@EnableAutoConfiguration 的作用

[Bpazy]

@EnableAutoConfiguration 是一个加载 Starter 目录包之外的需要 Spring 自动生成 bean 对象（是否需要的依据是 META-INF/spring.factories 中 org.springframework.boot.autoconfigure.EnableAutoConfiguration 后面是有能找到那个 bean）的带有@Configuration 注解的类。一般就是对各种引入的 spring-boot-starter 依赖包指定的（spring.factories）类进行实例化。

整体流程：

1. 我们启动类中会加上 @SpringBootApplication
2. @SpringBootApplication 包含了 @EnableAutoConfiguration，告知应用启动的时候需要扫描依赖包中需要实例化的类
3. Springboot 启动的时候会去扫描 META-INF/spring.factories，查看具体是哪些类需要实例化
4. 扫描哪些需要实例化的类，看下是否有 @Configuration 注解，如果有，则实例化
5. 实例化的时候可能需要一些配置属性，一般这些类都会加上 @EnableConfigurationProperties(RocketMQProperties.class)
6. RocketMQProperties 会将属性映射为 bean 类

[Bpazy]

在Spring boot中，我们可以通过 @Configuration, @Service, @Component 等注解生成对象到 IOC 容器中。

未必所有实例化都是通过 @EnableAutoConfiguration, @Configuration, @Service, @Component 等注解，也可以通过其他方式（说白了怎么加载类的问题，因为实例化最终就是找到类，然后通过反射生成对象而已）。

其他实例化方式其实可以从 META-INF/spring.factories 中看出来，里面不仅有org.springframework.boot.autoconfigure.EnableAutoConfiguration，还有org.springframework.context.ApplicationContextInitializer（初始化的时候实例化），org.springframework.context.ApplicationListener（初始化监听的时候实例化），org.springframework.boot.diagnostics.FailureAnalyzer（应用启动失败分析类）等等。

但是对于这些实例化方式有个共同点，都需要在 META-INF/spring.factories 中配置。

[Bpazy]

默认自动装配的类

@EnableAutoConfiguration 所属的 jar 包 spring-boot-autoconfigure 下的 spring.factories 里有所有:

# 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.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
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.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
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.ldap.LdapDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
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.redis.RedisAutoConfiguration,\
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.jest.JestAutoConfiguration,\
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.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.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.kafka.KafkaAutoConfiguration,\
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.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.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.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.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

[Bpazy]

加载流程解析

@EnableAutoConfiguration 有注解 @Import(EnableAutoConfigurationImportSelector.class)，其中的 EnableAutoConfigurationImportSelector 是关键实现，关键方法:

# org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#selectImports
    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        try {
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                    .loadMetadata(this.beanClassLoader);
            AnnotationAttributes attributes = getAttributes(annotationMetadata);
            List<String> configurations = getCandidateConfigurations(annotationMetadata,
                    attributes);
            configurations = removeDuplicates(configurations);
            configurations = sort(configurations, autoConfigurationMetadata);
            Set<String> exclusions = getExclusions(annotationMetadata, attributes);
            checkExcludedClasses(configurations, exclusions);
            configurations.removeAll(exclusions);
            configurations = filter(configurations, autoConfigurationMetadata);
            fireAutoConfigurationImportEvents(configurations, exclusions);
            return configurations.toArray(new String[configurations.size()]);
        }
        catch (IOException ex) {
            throw new IllegalStateException(ex);
        }
    }

getCandidateConfigurations 方法:

    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;
    }

核心就在于 org.springframework.core.io.support.SpringFactoriesLoader#loadFactoryNames：

    public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        try {
            Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            List<String> result = new ArrayList<String>();
            while (urls.hasMoreElements()) {
                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 ex) {
            throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                    "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

看代码可得出流程:

1. 扫描 classpath 下的所有 META-INF/spring.factories 文件
2. 获取 META-INF/spring.factories 文件中 key 为 org.springframework.boot.autoconfigure.EnableAutoConfiguration 的所有 value，就是这些: https://github.com/Bpazy/blog/issues/306#issuecomment-1752489787
3. 因为 spring.factories 可以有很多，所以要去重
4. 排序：先按 class name 字母正序排序、再按 @Order 排序、最后处理 @AutoConfigureBefore @AutoConfigureAfter 注解排序
5. 排除被 @EnableAutoConfiguration 排除掉的类（exclude, excludeName）
6. 过滤，获取所有 AutoConfigurationImportFilter 实例指定的需要被过滤的过滤 class，具体逻辑点此：https://github.com/Bpazy/blog/issues/306#issuecomment-1753065428

[Bpazy]

AutoConfigurationImportFilter

提供给开发者的扩展接口，用于过滤 spring.factories 中自动配置类。

核心逻辑在 org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#filter 中:

    private List<String> filter(List<String> configurations,
            AutoConfigurationMetadata autoConfigurationMetadata) {
        long startTime = System.nanoTime();
        String[] candidates = configurations.toArray(new String[configurations.size()]);
        boolean[] skip = new boolean[candidates.length];
        boolean skipped = false;
        for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {
            invokeAwareMethods(filter);
            boolean[] match = filter.match(candidates, autoConfigurationMetadata);
            for (int i = 0; i < match.length; i++) {
                if (!match[i]) {
                    skip[i] = true;
                    skipped = true;
                }
            }
        }
        if (!skipped) {
            return configurations;
        }
        List<String> result = new ArrayList<String>(candidates.length);
        for (int i = 0; i < candidates.length; i++) {
            if (!skip[i]) {
                result.add(candidates[i]);
            }
        }
        if (logger.isTraceEnabled()) {
            int numberFiltered = configurations.size() - result.size();
            logger.trace("Filtered " + numberFiltered + " auto configuration class in "
                    + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime)
                    + " ms");
        }
        return new ArrayList<String>(result);
    }

    protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class,
                this.beanClassLoader);
    }

翻译一下，就是从 spring.factories 中获取 AutoConfigurationImportFilter 实例列表，然后依据具体实现类决定某个 EnableAutoConfiguration 对应的类是否应该被过滤掉。

应用场景

该能力一般用于用配置文件控制某些类是否会被自动初始化，比如:

public class MyConfigurationImportFilter implements AutoConfigurationImportFilter, EnvironmentAware {

    private Environment environment;

    @Override
    public boolean[] match(String[] autoConfigurationClasses, AutoConfigurationMetadata autoConfigurationMetadata) {
        boolean[] matches = new boolean[autoConfigurationClasses.length];
        for (int i = 0; i < matches.length; i++) {
            // 当读取到 MyAutoConfiguration 自动配置类时, 如果配置文件中 my-auto-configuration.enabled 为 true, 则加载该自动配置类
            matches[i] = MyAutoConfiguration.class.getName().equals(autoConfigurationClasses[i]) ? environment.getProperty("my-auto-configuration.enabled", Boolean.class, true) : true;
        }
        return matches;
    }

    @Override
    public void setEnvironment(Environment environment) {
        this.environment = environment;
    }
}

@Configuration
public class MyAutoConfiguration {
    @Bean
    public User user() {
        return new User();
    }
}

public class User {
    public String getName() {
        return "hello world";
    }
}

@EnableAutoConfiguration
public class Application {
    public static void main(String[] args) {
        ConfigurableApplicationContext applicationContext = SpringApplication.run(Test.class, args);
        System.out.println(applicationContext.getBean(User.class).getName());
    }
}

如果配置文件中 my-auto-configuration.enabled 值为 true 时,则装载 MyAutoConfiguration 自动配置类。

然后在 resources/META-INF 下建立 spring.factories 文件:

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
com.github.bpazy.example.autoconfigure.MyConfigurationImportFilter

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.github.bpazy.example.autoconfigure.MyAutoConfiguration

然后在 application.properties 中指定配置:

my-auto-configuration.enabled=true

运行 Application.main() 输出：

hello world

Refer: https://blog.csdn.net/qq_38644901/article/details/123555895