Spring-boot自动配置原理

首先利用Spring Initializr方式创建springboot项目，查看springboot主配置类：

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1@SpringBootApplication

2public class SpringBootAutoconfigurationYuanliApplication {

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点击进入SpringBootApplication注解

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1@SpringBootConfiguration

2@EnableAutoConfiguration

3public @interface SpringBootApplication {}

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其中的@SpringBootConfiguration标注在某个类上，表示这是一个Spring Boot的配置类， 点击注解查看源码

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1@Configuration

2public @interface SpringBootConfiguration {

3}

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发现其依赖注解@Configuration
@Configuration配置在该注解类上来标注这个注解是一个配置类。
返回到@EnableAutoConfiguration，点击进入其源码

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1@AutoConfigurationPackage

2@Import({AutoConfigurationImportSelector.class})

3public @interface EnableAutoConfiguration {

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存在着@AutoConfigurationPackage和一个底层注解@Import，先讨论@AutoConfigurationPackage，点击进入其源码

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1@Import({Registrar.class})

2public @interface AutoConfigurationPackage {

3}

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同样使用了@Import，并且参数为Registrar.class再次点击Registrar.class发现如下方法

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1public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {

2   AutoConfigurationPackages.register(registry, (new AutoConfigurationPackages.PackageImport(metadata)).getPackageName());

3}

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以DeBug形式运行分析new AutoConfigurationPackages.PackageImport(metadata).getPackageName()
得到结果是扫描主配置类所在包的所有组件

回到@Import({AutoConfigurationImportSelector.class})
点击进入AutoConfigurationImportSelector.class自动配置选择器的源码中，发现其中有两个方法：

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1public String[] selectImports(AnnotationMetadata annotationMetadata) {

2        if (!this.isEnabled(annotationMetadata)) {

3            return NO_IMPORTS;

4        } else {

5            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);

6            AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);

7            return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());

8        }

9    }

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11    protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) {

12        if (!this.isEnabled(annotationMetadata)) {

13            return EMPTY_ENTRY;

14        } else {

15            AnnotationAttributes attributes = this.getAttributes(annotationMetadata);

16            List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);

17            configurations = this.removeDuplicates(configurations);

18            Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);

19            this.checkExcludedClasses(configurations, exclusions);

20            configurations.removeAll(exclusions);

21            configurations = this.filter(configurations, autoConfigurationMetadata);

22            this.fireAutoConfigurationImportEvents(configurations, exclusions);

23            return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);

24        }

25    }

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第一个方法是导入注解元信息选择器，方法中调用了类中的getAutoConfigurationEntry()方法，也就是第二个方法，第二个方法中返回的信息主要有configurations，和exclusions，在此主要分析List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);点击getCandidateConfigurations方法：

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1protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {

2        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());

3        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.");

4        return configurations;

5    }

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继续点击loadFactoryNames方法：

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1public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {

2        String factoryClassName = factoryClass.getName();

3        return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());

4    }

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6private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {

7    MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);

8    if (result != null) {

9        return result;

10    } else {

11        try {

12            Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");

13            LinkedMultiValueMap result = new LinkedMultiValueMap();

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15            while(urls.hasMoreElements()) {

16                URL url = (URL)urls.nextElement();

17                UrlResource resource = new UrlResource(url);

18                Properties properties = PropertiesLoaderUtils.loadProperties(resource);

19                Iterator var6 = properties.entrySet().iterator();

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21                while(var6.hasNext()) {

22                    Entry<?, ?> entry = (Entry)var6.next();

23                    String factoryClassName = ((String)entry.getKey()).trim();

24                    String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());

25                    int var10 = var9.length;

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27                    for(int var11 = 0; var11 < var10; ++var11) {

28                        String factoryName = var9[var11];

29                        result.add(factoryClassName, factoryName.trim());

30                    }

31                }

32            }

33

34            cache.put(classLoader, result);

35            return result;

36        } catch (IOException var13) {

37            throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);

38        }

39    }

40}

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loadFactoryNames方法返回loadSpringFactories方法的结果，
其中先获取ClassLoader.getSystemResources(“META-INF/spring.factories”)文件中的url，即是扫描jar包类路径下的META-INF/spring.factories文件中的内容，接着，我们查看扫描的是哪一个jar包下的文件，点击 getCandidateConfigurations中的this.getSpringFactoriesLoaderFactoryClass()，

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1 protected Class<?> getSpringFactoriesLoaderFactoryClass() {

2        return EnableAutoConfiguration.class;

3    }

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EnableAutoConfiguration.class是允许自动配置类

文件内容：

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1# Auto Configure

2org.springframework.boot.autoconfigure.EnableAutoConfiguration=\

3org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\

4org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\

5org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\

6org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\

7org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\

8org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\

9org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\

10org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\

11org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\

12org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\

13org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\

14org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\

15org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\

16org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\

17org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\

18org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\

19org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\

20org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\

21org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\

22org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\

23org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\

24org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\

25org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\

26org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\

27org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\

28org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\

29org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\

30org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\

31org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\

32org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\

33org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\

34org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\

35org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\

36org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\

37org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\

38org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\

39org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\

40org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\

41org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\

42org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,\

43org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\

44org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\

45org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\

46org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\

47org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\

48org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\

49org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\

50org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\

51org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\

52org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\

53org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\

54org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\

55org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\

56org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\

57org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\

58org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\

59org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\

60org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\

61org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\

62org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\

63org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\

64org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\

65org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\

66org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\

67org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\

68org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\

69org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\

70org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\

71org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\

72org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\

73org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\

74org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\

75org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\

76org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\

77org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\

78org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\

79org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\

80org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\

81org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\

82org.springframework.boot.autoconfigure.reactor.core.ReactorCoreAutoConfiguration,\

83org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\

84org.springframework.boot.autoconfigure.security.servlet.SecurityRequestMatcherProviderAutoConfiguration,\

85org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\

86org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\

87org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\

88org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\

89org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\

90org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\

91org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\

92org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\

93org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\

94org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\

95org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\

96org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\

97org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\

98org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\

99org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\

100org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\

101org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\

102org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\

103org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\

104org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\

105org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\

106org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\

107org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\

108org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\

109org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\

110org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\

111org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\

112org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\

113org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\

114org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\

115org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\

116org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\

117org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\

118org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\

119org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\

120org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

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将遍历结果包装成properties对象。
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。
以 HttpEncodingAutoConfiguration（Http编码自动配置） 为例解释自动配置原理

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1@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

2@EnableConfigurationProperties({HttpProperties.class}) //启动指定类的ConfigurationProperties功能；

3//将自己编写的application.properties配置文件中对应的值和HttpEncodingProperties类绑定起来；并把HttpEncodingProperties加入到ioc容器中

4@ConditionalOnWebApplication(type = Type.SERVLET)

5@ConditionalOnClass({CharacterEncodingFilter.class})

6@ConditionalOnProperty(

7    prefix = "spring.http.encoding",

8    value = {"enabled"},

9    matchIfMissing = true

10)

11public class HttpEncodingAutoConfiguration {

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点击HttpProperties.class进入此类发现此类中有注解@ConfigurationProperties(prefix = “spring.http”)也就指明了此类配置的编写方式都是以spring.http为前缀，后面属性都是该类中的属性。