浅析spring boot自动配置原理

spring boot 源码刨析：

1、spring boot 启动类

@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);
    }
}

我们看到，MgmtApplication作为入口类，入口类中有一个main方法，这个方法其实就是一个标准的Java应用的入口方法，一般在main方法中使用SpringApplication.run()来启动整个应用。值得注意的是，这个入口类要使用@SpringBootApplication注解声明，它是SpringBoot的核心注解。

2、@SpringBootApplication 源码

@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 {
   //... 
}

这个注解里面，最主要的就是@EnableAutoConfiguration

3、@EnableAutoConfiguration的源码

@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 {};
}

可以看到，在@EnableAutoConfiguration注解内使用到了@import注解来完成导入配置的功能，而EnableAutoConfigurationImportSelector内部则是使用了SpringFactoriesLoader.loadFactoryNames方法进行扫描具有META-INF/spring.factories文件的jar包。下面是2.1.8.RELEASE实现源码：

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);
　　　   //扫描具有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);
    }
}

//加载spring.factories实现

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;
}

任何一个springboot应用，都会引入spring-boot-autoconfigure，而spring.factories文件就在该包下面。spring.factories文件是Key=Value形式，多个Value时使用,隔开，该文件中定义了关于初始化，监听器等信息，而真正使自动配置生效的key是org.springframework.boot.autoconfigure.EnableAutoConfiguration，如下所示：

# 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配置了多个类，这些都是Spring Boot中的自动配置相关类；在启动过程中会解析对应类配置信息。每个Configuation类都定义了相关bean的实例化配置。都说明了哪些bean可以被自动配置，什么条件下可以自动配置，并把这些bean实例化出来。