淺析spring boot自動配置原理

spring boot 源碼刨析：

一、spring boot 啓動類

@SpringBootApplication
public class MgmtApplication extends SpringBootServletInitializer {
    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder.sources(MgmtApplication.class);
    }

    public static void main(String[] args) {
        SpringApplication.run(MgmtApplication.class, args);
    }
}

咱們看到，MgmtApplication做爲入口類，入口類中有一個main方法，這個方法其實就是一個標準的Java應用的入口方法，通常在main方法中使用SpringApplication.run()來啓動整個應用。值得注意的是，這個入口類要使用@SpringBootApplication註解聲明，它是SpringBoot的核心註解。

二、@SpringBootApplication 源碼

@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 {
   //... 
}

這個註解裏面，最主要的就是@EnableAutoConfiguration

三、@EnableAutoConfiguration的源碼

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

能夠看到，在@EnableAutoConfiguration註解內使用到了@import註解來完成導入配置的功能，而EnableAutoConfigurationImportSelector內部則是使用了SpringFactoriesLoader.loadFactoryNames方法進行掃描具備META-INF/spring.factories文件的jar包。下面是2.1.8.RELEASE實現源碼：

public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    } else {
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
        AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }
}

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    } else {
        AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
　　　   //掃描具備META-INF/spring.factories文件的jar包　
        List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
        configurations = this.removeDuplicates(configurations);
        Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
        this.checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = this.filter(configurations, autoConfigurationMetadata);
        this.fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
    }
}

//加載spring.factories實現

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { 
　　List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader()); 
   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."); 
　　return configurations;
}

任何一個springboot應用，都會引入spring-boot-autoconfigure，而spring.factories文件就在該包下面。spring.factories文件是Key=Value形式，多個Value時使用,隔開，該文件中定義了關於初始化，監聽器等信息，而真正使自動配置生效的key是org.springframework.boot.autoconfigure.EnableAutoConfiguration，以下所示：

# 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,\
...

上面的EnableAutoConfiguration配置了多個類，這些都是Spring Boot中的自動配置相關類；在啓動過程當中會解析對應類配置信息。每一個Configuation類都定義了相關bean的實例化配置。都說明了哪些bean能夠被自動配置，什麼條件下能夠自動配置，並把這些bean實例化出來。