SpringBoot自動裝配原理分析

SpringBoot能流行起來我覺得有個重要的原因就是它的自動裝配，想想使用SpringBoot與Spring開發專案的區別，第一反應就是減少了很多配置。

其實SpringBoot是基於Spring基礎之上的，透過MAVEN引入SpringBoot也會發現，它傳遞依賴了Spring。所以配置並沒有減少，只是它幫我們自動配置了，這些自動配置的類都在spring-boot-autoconfigure-版本.jar包中。

以一個最簡單的SpringBoot應用為例，通常需要寫這樣一個啟動類：

@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);
    }
    
}

進入@SpringBootApplication註解內部，可以看到與自動裝配有關的@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透過@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是一個ImportSelector，我們檢視其selectImports方法，其內部呼叫getCandidateConfigurations方法，讀取META-INF/spring.factories裡配置的Configuration類。

@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檔案裡預定義了很多Configuration類：

# 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,
......

到這裡自動裝配的主流程就結束了，然後具體的裝配邏輯就在各自的Configuration類中。

以常見的HttpEncodingAutoConfiguration為例，這些Configuration類都新增了@Configuration註解表名都是配置類，然後透過例如：@ConditionalXXX等判斷透過程式碼方式建立需要用到的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;
        }

    }
    
}