spring Boot是一个偏执的开源框架，它可用于创建可执行的spring应用程序，采用了习惯优于配置的方法。
此框架的神奇之处在于@EnableAutoConfiguration注释，此注释自动载入应用程序所需的所有Bean——这依赖于Spring Boot在类路径中的查找。
一、@Enable*注释
@Enable*注释并不是新发明的注释，早在Spring 3框架就引入了这些注释，用这些注释替代XML配置文件。
很多Spring开发者都知道@EnableTransactionManagement注释，它能够声明事务管理；@EnableWebMvc注释，它能启用Spring MVC；以及@EnableScheduling注释，它可以初始化一个调度器。
这些注释事实上都是简单的配置，通过@Import注释导入。
**@Import类似于spring配置文件中的include，这样spring加载配置文件时就不用加载多个**
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import({ EnableAutoConfigurationImportSelector.class,
AutoConfigurationPackages.Registrar.class })
public @interface EnableAutoConfiguration {
/**
* Exclude specific auto-configuration classes such that they will never be applied.
*/
Class<?>[] exclude() default {};
}EnableAutoConfigurationImportSelector类使用了Spring Core包的SpringFactoriesLoader类的loadFactoryNamesof()方法。
SpringFactoriesLoader会查询META-INF/spring.factories文件中包含的JAR文件。
当找到spring.factories文件后，SpringFactoriesLoader将查询配置文件命名的属性。在例子中，是org.springframework.boot.autoconfigure.EnableAutoConfiguration
让我们来看看spring-boot-autoconfigure JAR文件，它真的包含了一个spring.factories文件，内容如下：
# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
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.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.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
# Failure analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer
# 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.web.JspTemplateAvailabilityProvider在这个文件中，可以看到一系列Spring Boot自动配置的列表。下面我们来看这些配置的细节，以MongoAutoConfiguration为例：
@Configuration
@ConditionalOnClass(Mongo.class)
@EnableConfigurationProperties(MongoProperties.class)
public class MongoAutoConfiguration {
@Autowired
private MongoProperties properties;
private Mongo mongo;
@PreDestroy
public void close() throws UnknownHostException {
if (this.mongo != null) {
this.mongo.close();
}
}
@Bean
@ConditionalOnMissingBean
public Mongo mongo() throws UnknownHostException {
this.mongo = this.properties.createMongoClient();
return this.mongo;
}
}
如果加上
@ConditionalOnMissingBean(type = “org.springframework.data.mongodb.MongoDbFactory”)
Mongo的AutoConfiguration将会在用户引入Mongo相关包时,并且没有自定义MongoDbFactory时被激活,同时配置文件(application.properties之类的)将注入到MongoProperties中.MongoProperties类由@ConfigurationProperties
可以看到MongoClient最终由MongoAutoConfiguration调用MongoProperties的createMongoClient()方法创建.通过标注@Bean将MongoClient发布到Spring容器中.
如果用户已经用@Bean自定义了一个MongoClient,那么Mongo AutoConfig就不会做去初始化MongoClient,配置文件中的配置也就不生效了.
这个类进行了简单的Spring配置，声明了MongoDB所需典型Bean。
这个类跟其它很多类一样，重度依赖于Spring Boot注释：
1）@ConditionOnClass激活一个配置，在类路径中只能存在一到几个这样的类。
2）@EnableConfigurationProperties自动映射一个POJO到Spring Boot配置文件（默认是application.properties文件）的属性集。
3）@ConditionalOnMissingBean启用一个Bean定义，但必须是这个Bean之前未定义过才有效。
还可以使用@ AutoConfigureBefore注释、@AutoConfigureAfter注释来定义这些配置类的载入顺序。
二、属性映射
下面看MongoProperties类，它是一个Spring Boot属性映射（映射到application.properties）的例子：
@ConfigurationProperties(prefix = “spring.data.mongodb”)
public class MongoProperties {
private String host;
private int port = DBPort.PORT;
private String uri = "mongodb://localhost/test";
private String database;
// ... getters/ setters omitted
}
@ConfigurationProperties注释将POJO关联到application.properties指定前缀的每一个属性。例如，spring.data.mongodb.port属性将映射到这个类的端口属性。
强烈建议Spring Boot开发者使用这种方式来删除与配置属性相关的瓶颈代码。
三、@Conditional注释
Spring Boot的强大之处在于使用了Spring 4框架的新特性：@Conditional注释，此注释使得只有在特定条件满足时才启用一些配置。
在Spring Boot的org.springframework.boot.autoconfigure.condition包中说明了使用@Conditional注释能给我们带来什么，下面对这些注释做一个概述：
@ConditionalOnBean
@ConditionalOnClass
@ConditionalOnExpression
@ConditionalOnMissingBean
@ConditionalOnMissingClass
@ConditionalOnNotWebApplication
@ConditionalOnResource
@ConditionalOnWebApplication
以@ConditionalOnExpression注释为例，它允许在Spring的EL表达式中写一个条件。
@Conditional(OnExpressionCondition.class)
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
public @interface ConditionalOnExpression {
/**
* The SpEL expression to evaluate. Expression should return {@code true} if the
* condition passes or {@code false} if it fails.
*/
String value() default "true";
}
在这个类中，我们想利用@Conditional注释，条件在OnExpressionCondition类中定义：
public class OnExpressionCondition extends SpringBootCondition {
@Override
public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
// ...
// we first get a handle on the EL context via the ConditionContext
boolean result = (Boolean) resolver.evaluate(expression, expressionContext);
// ...
// here we create a message the user will see when debugging
return new ConditionOutcome(result, message.toString());
}
}
在最后，@Conditional通过简单的布尔表达式（即ConditionOutcome方法）来决定。
四、应用程序上下文初始化器
spring.factories还提供了第二种可能性，即定义应用程序的初始化。这使得我们可以在应用程序载入前操纵Spring的应用程序上下文ApplicationContext。
特别是，可以在上下文创建监听器，使用ConfigurableApplicationContext类的addApplicationListener()方法。
AutoConfigurationReportLoggingInitializer监听到系统事件时，比如上下文刷新或应用程序启动故障之类的事件，Spring Boot可以执行一些工作。这有助于我们以调试模式启动应用程序时创建自动配置的报告。
要以调试模式启动应用程序，可以使用-Ddebug标识，或者在application.properties文件这添加属性debug= true。
五、调试Spring Boot自动配置
Spring Boot的官方文档（http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#howto-troubleshoot-auto-configuration）有助于理解自动配置期间发生了什么。当以调试模式运行时，运行时，加上参数–debug ，Spring Boot会产生一个报告，如下：
Positive matches:
MessageSourceAutoConfiguration
- @ConditionalOnMissingBean (types: org.springframework.context.MessageSource; SearchStrategy: all) found no beans (OnBeanCondition)
JmxAutoConfiguration
- @ConditionalOnClass classes found: org.springframework.jmx.export.MBeanExporter (OnClassCondition)
- SpEL expression on org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration: ${spring.jmx.enabled:true} (OnExpressionCondition)
- @ConditionalOnMissingBean (types: org.springframework.jmx.export.MBeanExporter; SearchStrategy: all) found no beans (OnBeanCondition)
DispatcherServletAutoConfiguration
- found web application StandardServletEnvironment (OnWebApplicationCondition)
- @ConditionalOnClass classes found: org.springframework.web.servlet.DispatcherServlet (OnClassCondition)
Negative matches:
DataSourceAutoConfiguration
- required @ConditionalOnClass classes not found: org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType (OnClassCondition)
DataSourceTransactionManagerAutoConfiguration
- required @ConditionalOnClass classes not found: org.springframework.jdbc.core.JdbcTemplate,org.springframework.transaction.PlatformTransactionManager (OnClassCondition)
MongoAutoConfiguration
- required @ConditionalOnClass classes not found: com.mongodb.Mongo (OnClassCondition)
FallbackWebSecurityAutoConfiguration
- SpEL expression on org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration: !${security.basic.enabled:true} (OnExpressionCondition)
SecurityAutoConfiguration
- required @ConditionalOnClass classes not found: org.springframework.security.authentication.AuthenticationManager (OnClassCondition)
EmbeddedServletContainerAutoConfiguration.EmbeddedJetty
- required @ConditionalOnClass classes not found: org.eclipse.jetty.server.Server,org.eclipse.jetty.util.Loader (OnClassCondition)
WebMvcAutoConfiguration.WebMvcAutoConfigurationAdapter#localeResolver
- @ConditionalOnMissingBean (types: org.springframework.web.servlet.LocaleResolver; SearchStrategy: all) found no beans (OnBeanCondition)
- SpEL expression: ‘${spring.mvc.locale:}’ != ” (OnExpressionCondition)
WebSocketAutoConfiguration
- required @ConditionalOnClass classes not found: org.springframework.web.socket.WebSocketHandler,org.apache.tomcat.websocket.server.WsSci (OnClassCondition)
对于每个自动配置，可以看到它启动或失败的原因。
六、结论
Spring Boot很好地利用了Spring 4框架的功能，可以创建一个自动配置的可执行JAR。
不要忘记，可以使用自己的配置来替代自动配置，见： http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#using-boot-replacing-auto-configuration
over!
作者：glowd
原文：https://blog.csdn.net/zengqiang1/article/details/54603483
版权声明：本文为博主原创文章，转载请附上博文链接！
