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SpringBoot | 自动配置原理

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前言

这个月过去两天了，这篇文章才跟大家见面，最近比较累，大家见谅下。下班后闲着无聊看了下 SpringBoot 中的自动配置，把我的理解跟大家说下。

配置文件能写什么？

相信接触过 SpringBoot 的朋友都知道 SpringBoot 有各种 starter 依赖，想要什么直接勾选加进来就可以了。想要自定义的时候就直接在配置文件写自己的配置就好。但你们有没有困惑，为什么 SpringBoot 如此智能，到底配置文件里面能写什么呢？

带着这个疑问，我翻了下 SpringBoot 官网看到这么一些配置样例：

SpringBoot | 自动配置原理

发现 SpringBoot 可配置的东西非常多，上图只是节选。有兴趣的查看这个网址：

https://docs.spring.io/spring-boot/docs/2.1.3.RELEASE/reference/htmlsingle/#boot-features-external-config-yaml

自动配置原理

这里我拿之前创建过的 SpringBoot 来举例讲解 SpringBoot 的自动配置原理，首先看这么一段代码：

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

毫无疑问这里只有 @SpringBootApplication 值得研究，进入 @SpringBootApplication 的源码：

SpringBoot | 自动配置原理

SpringBoot 启动的时候加载主配置类，开启了自动配置功能 @EnableAutoConfiguration，再进入 @EnableAutoConfiguration 源码：

SpringBoot | 自动配置原理

发现最重要的就是 @Import(AutoConfigurationImportSelector.class) 这个注解，其中的 AutoConfigurationImportSelector 类的作用就是往 Spring 容器中导入组件，我们再进入这个类的源码，发现有这几个方法：

/**
* 方法用于给容器中导入组件
**/
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    }
    AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
        .loadMetadata(this.beanClassLoader);
    AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
        autoConfigurationMetadata, annotationMetadata);  // 获取自动配置项
    return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}

// 获取自动配置项
protected AutoConfigurationEntry getAutoConfigurationEntry(
    AutoConfigurationMetadata autoConfigurationMetadata,
    AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    }
    AnnotationAttributes attributes = getAttributes(annotationMetadata);
    List < String > configurations = getCandidateConfigurations(annotationMetadata,
        attributes);  //  获取一个自动配置 List ，这个 List 就包含了所有自动配置的类名
    configurations = removeDuplicates(configurations);
    Set < String > exclusions = getExclusions(annotationMetadata, attributes);
    checkExcludedClasses(configurations, exclusions);
    configurations.removeAll(exclusions);
    configurations = filter(configurations, autoConfigurationMetadata);
    fireAutoConfigurationImportEvents(configurations, exclusions);
    return new AutoConfigurationEntry(configurations, exclusions);
}

//   获取一个自动配置 List ，这个 List 就包含了所有的自动配置的类名
protected List < String > getCandidateConfigurations(AnnotationMetadata metadata,
    AnnotationAttributes attributes) {
    // 通过 getSpringFactoriesLoaderFactoryClass 获取默认的 EnableAutoConfiguration.class 类名，传入 loadFactoryNames 方法
    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;
}

// 默认的 EnableAutoConfiguration.class 类名
protected Class<?> getSpringFactoriesLoaderFactoryClass() {
    return EnableAutoConfiguration.class;
}

代码注释很清楚：

首先注意到 selectImports 方法，其实从方法名就能看出，这个方法用于给容器中导入组件，然后跳到 getAutoConfigurationEntry 方法就是用于获取自动配置项的。
再来进入 getCandidateConfigurations 方法就是获取一个自动配置 List ，这个 List 就包含了所有的自动配置的类名。
再进入 SpringFactoriesLoader 类的 loadFactoryNames 方法，跳转到 loadSpringFactories 方法发现 ClassLoader 类加载器指定了一个 FACTORIES_RESOURCE_LOCATION 常量。
然后利用PropertiesLoaderUtils 把 ClassLoader 扫描到的这些文件的内容包装成 properties 对象，从 properties 中获取到 EnableAutoConfiguration.class 类（类名）对应的值，然后把他们添加在容器中。

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

private static Map < String, List < String >> loadSpringFactories(@Nullable ClassLoader classLoader) {
    MultiValueMap < String, String > result = cache.get(classLoader);
    if (result != null) {
        return result;
    }

    try {
        // 扫描所有 jar 包类路径下  META-INF/spring.factories
        Enumeration < URL > urls = (classLoader != null ?
            classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
            ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
        result = new LinkedMultiValueMap < > ();
        while (urls.hasMoreElements()) {
            URL url = urls.nextElement();
            UrlResource resource = new UrlResource(url);
            // 把扫描到的这些文件的内容包装成 properties 对象
            Properties properties = PropertiesLoaderUtils.loadProperties(resource);
            for (Map.Entry < ? , ? > entry : properties.entrySet()) {
                String factoryClassName = ((String) entry.getKey()).trim();
                for (String factoryName: StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                    // 从 properties 中获取到 EnableAutoConfiguration.class 类（类名）对应的值，然后把他们添加在容器中
                    result.add(factoryClassName, factoryName.trim());
                }
            }
        }
        cache.put(classLoader, result);
        return result;
    } catch (IOException ex) {
        throw new IllegalArgumentException("Unable to load factories from location [" +
            FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
}

点击 FACTORIES_RESOURCE_LOCATION 常量，我发现它指定的是 jar 包类路径下 META-INF/spring.factories 文件：

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

将类路径下 META-INF/spring.factories 里面配置的所有 EnableAutoConfiguration 的值加入到了容器中，所有的 EnableAutoConfiguration 如下所示：注意到 EnableAutoConfiguration 有一个 = 号，= 号后面那一串就是这个项目需要用到的自动配置类。

# 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.CloudServiceConnectorsAutoConfiguration,/
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.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.ElasticsearchAutoConfiguration,/
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,/
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,/
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.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.jest.JestAutoConfiguration,/
org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,/
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,/
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.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.groovy.template.GroovyTemplateAutoConfiguration,/
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.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.reactor.core.ReactorCoreAutoConfiguration,/
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,/
org.springframework.boot.autoconfigure.security.servlet.SecurityRequestMatcherProviderAutoConfiguration,/
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.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

每一个这样的 xxxAutoConfiguration 类都是容器中的一个组件，都加入到容器中，用他们来做自动配置。上述的每一个自动配置类都有自动配置功能，也可在配置文件中自定义配置。

举例说明 Http 编码自动配置原理

@Configuration 
// 表示这是一个配置类，以前编写的配置文件一样，也可以给容器中添加组件

@EnableConfigurationProperties(HttpEncodingProperties.class)  
// 启动指定类的 ConfigurationProperties 功能；将配置文件中对应的值和 HttpEncodingProperties 绑定起来；并把 HttpEncodingProperties 加入到 ioc 容器中

@ConditionalOnWebApplication 
// Spring 底层 @Conditional 注解，根据不同的条件，如果满足指定的条件，整个配置类里面的配置就会生效；判断当前应用是否是 web 应用，如果是，当前配置类生效

@ConditionalOnClass(CharacterEncodingFilter.class) 
// 判断当前项目有没有这个类 CharacterEncodingFilter；SpringMVC 中进行乱码解决的过滤器；

@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true) 
// 判断配置文件中是否存在某个配置  spring.http.encoding.enabled；如果不存在，判断也是成立的
// 即使我们配置文件中不配置 pring.http.encoding.enabled=true，也是默认生效的；
public class HttpEncodingAutoConfiguration {

    // 已经和 SpringBoot 的配置文件建立映射关系了
    private final HttpEncodingProperties properties;

    //只有一个有参构造器的情况下，参数的值就会从容器中拿
    public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) {
        this.properties = properties;
    }

    @Bean 
    // 给容器中添加一个组件，这个组件的某些值需要从 properties 中获取
    @ConditionalOnMissingBean(CharacterEncodingFilter.class) 
    public CharacterEncodingFilter characterEncodingFilter() {
        CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
        filter.setEncoding(this.properties.getCharset().name());
        filter.setForceRequestEncoding(this.properties.shouldForce(Type.REQUEST));
        filter.setForceResponseEncoding(this.properties.shouldForce(Type.RESPONSE));
        return filter;
    }

自动配置类做了什么不在这里赘述，请见上面代码。所有在配置文件中能配置的属性都是在 xxxxProperties 类中封装的；配置文件能配置什么就可以参照某个功能对应的这个属性类，例如上述提到的 @EnableConfigurationProperties(HttpProperties.class) ，我们打开 HttpProperties 文件源码节选：

@ConfigurationProperties(prefix = "spring.http")
public class HttpProperties {

    public static class Encoding {

        public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

        /**
         * Charset of HTTP requests and responses. Added to the "Content-Type" header if
         * not set explicitly.
         */
        private Charset charset = DEFAULT_CHARSET;

        /**
         * Whether to force the encoding to the configured charset on HTTP requests and
         * responses.
         */
        private Boolean force;

}

在上面可以发现里面的属性 charset 、force 等，都是我们可以在配置文件中指定的，它的前缀就是 spring.http.encoding 如：

SpringBoot | 自动配置原理