SpringBoot自动装配原理分析

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<String> configurations = getCandidateConfigurations(annotationMetadata, attributes); // META-INF/spring.factories
        configurations = removeDuplicates(configurations);
        configurations = sort(configurations, autoConfigurationMetadata);
        Set<String> 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注解表明都是配置类，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;
        }

    }
    
}