SpringBoot | 是如何實現自動配置的？

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配置文件能寫什麼？

相信接觸過 SpringBoot 的朋友都知道 SpringBoot 有各類 starter 依賴，想要什麼直接勾選加進來就能夠了。想要自定義的時候就直接在配置文件寫本身的配置就好。但大家有沒有困惑，爲何 SpringBoot 如此智能，到底配置文件裏面能寫什麼呢？

帶着這個疑問，我翻了下 SpringBoot 官網看到這麼一些配置樣例：

發現 SpringBoot 可配置的東西很是多，上圖只是節選。有興趣的查看這個網址：
https://docs.spring.io/spring...

自動配置原理

這裏我拿以前建立過的 SpringBoot 來舉例講解 SpringBoot 的自動配置原理，首先看這麼一段代碼：

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

毫無疑問這裏只有 @SpringBootApplication 值得研究，進入 @SpringBootApplication 的源碼：

SpringBoot 啓動的時候加載主配置類，開啓了自動配置功能 @EnableAutoConfiguration，再進入 @EnableAutoConfiguration 源碼：

發現最重要的就是 @Import(AutoConfigurationImportSelector.class) 這個註解，其中的 AutoConfigurationImportSelector 類的做用就是往 Spring 容器中導入組件，咱們再進入這個類的源碼，發現有這幾個方法：

/**
* 方法用於給容器中導入組件
**/
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    }
    AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
        .loadMetadata(this.beanClassLoader);
    AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
        autoConfigurationMetadata, annotationMetadata);  // 獲取自動配置項
    return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}

// 獲取自動配置項
protected AutoConfigurationEntry getAutoConfigurationEntry(
    AutoConfigurationMetadata autoConfigurationMetadata,
    AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    }
    AnnotationAttributes attributes = getAttributes(annotationMetadata);
    List < String > configurations = getCandidateConfigurations(annotationMetadata,
        attributes);  //  獲取一個自動配置 List ，這個 List 就包含了全部自動配置的類名
    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);
}

//   獲取一個自動配置 List ，這個 List 就包含了全部的自動配置的類名
protected List < String > getCandidateConfigurations(AnnotationMetadata metadata,
    AnnotationAttributes attributes) {
    // 經過 getSpringFactoriesLoaderFactoryClass 獲取默認的 EnableAutoConfiguration.class 類名，傳入 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;
}

// 默認的 EnableAutoConfiguration.class 類名
protected Class<?> getSpringFactoriesLoaderFactoryClass() {
    return EnableAutoConfiguration.class;
}

代碼註釋很清楚：

1. 首先注意到 selectImports 方法，其實從方法名就能看出，這個方法用於給容器中導入組件，而後跳到 getAutoConfigurationEntry 方法就是用於獲取自動配置項的。
2. 再來進入 getCandidateConfigurations 方法就是 獲取一個自動配置 List ，這個 List 就包含了全部的自動配置的類名 。
3. 再進入 SpringFactoriesLoader 類的 loadFactoryNames 方法，跳轉到 loadSpringFactories 方法發現 ClassLoader 類加載器指定了一個 FACTORIES_RESOURCE_LOCATION 常量。
4. 而後利用 PropertiesLoaderUtils 把 ClassLoader 掃描到的這些文件的內容包裝成 properties 對象，從 properties 中獲取到 EnableAutoConfiguration.class 類（類名）對應的值，而後把他們添加在容器中。

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 {
        // 掃描全部 jar 包類路徑下  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);
            // 把掃描到的這些文件的內容包裝成 properties 對象
            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())) {
                    // 從 properties 中獲取到 EnableAutoConfiguration.class 類（類名）對應的值，而後把他們添加在容器中
                    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);
    }
}

點擊 FACTORIES_RESOURCE_LOCATION 常量，我發現它指定的是 jar 包類路徑下 META-INF/spring.factories 文件：

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

將類路徑下 META-INF/spring.factories 裏面配置的全部 EnableAutoConfiguration 的值加入到了容器中，全部的 EnableAutoConfiguration 以下所示：注意到 EnableAutoConfiguration 有一個 = 號，= 號後面那一串就是這個項目須要用到的自動配置類。

# 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

每個這樣的 xxxAutoConfiguration 類都是容器中的一個組件，都加入到容器中，用他們來作自動配置。上述的每個自動配置類都有自動配置功能，也可在配置文件中自定義配置。

舉例說明 Http 編碼自動配置原理

@Configuration 
// 表示這是一個配置類，之前編寫的配置文件同樣，也能夠給容器中添加組件

@EnableConfigurationProperties(HttpEncodingProperties.class)  
// 啓動指定類的 ConfigurationProperties 功能；將配置文件中對應的值和 HttpEncodingProperties 綁定起來；並把 HttpEncodingProperties 加入到 ioc 容器中

@ConditionalOnWebApplication 
// Spring 底層 @Conditional 註解，根據不一樣的條件，若是知足指定的條件，整個配置類裏面的配置就會生效；判斷當前應用是不是 web 應用，若是是，當前配置類生效

@ConditionalOnClass(CharacterEncodingFilter.class) 
// 判斷當前項目有沒有這個類 CharacterEncodingFilter；SpringMVC 中進行亂碼解決的過濾器；

@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true) 
// 判斷配置文件中是否存在某個配置  spring.http.encoding.enabled；若是不存在，判斷也是成立的
// 即便咱們配置文件中不配置 pring.http.encoding.enabled=true，也是默認生效的；
public class HttpEncodingAutoConfiguration {

    // 已經和 SpringBoot 的配置文件創建映射關係了
    private final HttpEncodingProperties properties;

    //只有一個有參構造器的狀況下，參數的值就會從容器中拿
    public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) {
        this.properties = properties;
    }

    @Bean 
    // 給容器中添加一個組件，這個組件的某些值須要從 properties 中獲取
    @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;
    }

自動配置類作了什麼不在這裏贅述，請見上面代碼註釋。全部在配置文件中能配置的屬性都是在 xxxxProperties 類中封裝的；配置文件能配置什麼就能夠參照某個功能對應的這個屬性類，例如上述提到的 @EnableConfigurationProperties(HttpProperties.class) ，咱們打開 HttpProperties 文件源碼節選：

@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;

}

在上面能夠發現裏面的屬性 charset 、force 等，都是咱們能夠在配置文件中指定的，它的前綴就是 spring.http.encoding 如：

另外，若是配置文件中有配該屬性就取配置文件的，若無就使用 XxxxProperties.class 文件的默認值，好比上述代碼的 Charset 屬性，若是不配那就使用 UTF-8 默認值。

總結

1. SpringBoot 啓動會加載大量的自動配置類
2. 咱們看咱們須要的功能有沒有 SpringBoot 默認寫好的自動配置類；
3. 咱們再來看這個自動配置類中到底配置了哪些組件 ( 只要咱們要用的組件有，咱們就不須要再來配置，若沒有，咱們可能就要考慮本身寫一個配置類讓 SpringBoot 掃描了)
4. 給容器中自動配置類添加組件的時候，會從 properties 類中獲取某些屬性。咱們就能夠在配置文件中指定這些屬性的值；

xxxxAutoConfigurartion 自動配置類的做用就是給容器中添加組件

xxxxProperties 的做用就是封裝配置文件中相關屬性

至此，總算弄明白了 SpringBoot 的自動配置原理。我水平優先，若有不當之處，敬請指出，相互交流學習，但願對大家有幫助。

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