SpringBoot能流行起来我觉得有个重要的原因就是它的自动装配，想想使用SpringBoot与Spring开发项目的区别，第一反应就是减少了很多配置。
其实SpringBoot是基于Spring基础之上的，通过MAVEN引入SpringBoot也会发现，它传递依赖了Spring。所以配置并没有减少，只是它帮我们自动配置了，这些自动配置的类都在spring-boot-autoconfigure-版本.jar包中。
以一个最简单的SpringBoot应用为例，通常需要写这样一个启动类：
@SpringBootApplication
public class Application extends SpringBootServletInitializer {
public static void main(String[] args) {
SpringApplication springApplication = new SpringApplication(Application.class);
springApplication.setBannerMode(Banner.Mode.LOG);
springApplication.run(args);
}
}
进入@SpringBootApplication注解内部，可以看到与自动装配有关的@EnableAutoConfiguration：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
// ...
}
@EnableAutoConfiguration通过@import方式引入了EnableAutoConfigurationImportSelector：
@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
// ...
}
EnableAutoConfigurationImportSelector是一个ImportSelector，我们查看其selectImports方法，其内部调用getCandidateConfigurations方法，读取META-INF/spring.factories里配置的Configuration类。
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
try {
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
AnnotationAttributes attributes = getAttributes(annotationMetadata);
List configurations = getCandidateConfigurations(annotationMetadata, attributes); // META-INF/spring.factories
configurations = removeDuplicates(configurations);
configurations = sort(configurations, autoConfigurationMetadata);
Set 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);
}
}
在SpringBoot的spring-boot-autoconfigure-版本.jar包中，spring.factories文件里预定义了很多Configuration类：
# 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 Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener
# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnClassCondition
# 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,\
......
到这里自动装配的主流程就结束了，然后具体的装配逻辑就在各自的Configuration类中。
以常见的HttpEncodingAutoConfiguration为例，这些Configuration类都添加了@Configuration注解表名都是配置类，然后通过例如：@ConditionalXXX等判断通过代码方式创建需要用到的Bean。
@Configuration
@EnableConfigurationProperties(HttpEncodingProperties.class)
@ConditionalOnWebApplication
@ConditionalOnClass(CharacterEncodingFilter.class)
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)
public class HttpEncodingAutoConfiguration {
private final HttpEncodingProperties properties;
public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) {
this.properties = properties;
}
@Bean
@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;
}
@Bean
public LocaleCharsetMappingsCustomizer localeCharsetMappingsCustomizer() {
return new LocaleCharsetMappingsCustomizer(this.properties);
}
private static class LocaleCharsetMappingsCustomizer implements EmbeddedServletContainerCustomizer, Ordered {
private final HttpEncodingProperties properties;
LocaleCharsetMappingsCustomizer(HttpEncodingProperties properties) {
this.properties = properties;
}
@Override
public void customize(ConfigurableEmbeddedServletContainer container) {
if (this.properties.getMapping() != null) {
container.setLocaleCharsetMappings(this.properties.getMapping());
}
}
@Override
public int getOrder() {
return 0;
}
}
}
在此我向大家推荐一个架构学习交流圈。点击加入交流圈 里面资深架构师会分享一些整理好的录制视频录像和BATJ面试题：有Spring，MyBatis，Netty源码分析，高并发、高性能、分布式、微服务架构的原理，JVM性能优化、分布式架构等这些成为架构师必备的知识体系。还能领取免费的学习资源，目前受益良多。
注：
作者：笨鸟飞不停
出处：http://www.imooc.com/article/291648