浅谈springboot自动配置原理

从main函数说起

一切的开始要从SpringbootApplication注解说起。

@SpringBootApplication
public class MyBootApplication {
  public static void main(String[] args) {
    SpringApplication.run(MyBootApplication.class);
  }
}

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan
public @interface SpringBootApplication {

}

其中最重要的就是EnableAutoConfiguration注解，开启自动配置。

@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 {};
}

通过Import注解导入AutoConfigurationImportSelector。在这个类中加载/META-INF/spring.factories文件的信息，然后筛选出以EnableAutoConfiguration为key的数据，加载到IOC容器中，实现自动配置功能。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

}

从表面看就是自动配置包，主要使用了Import注解，导入了Registrar类。这里Registrar类的registerBeanDefinitions方法导包，也就是导入当前main函数所在路径的包地址，我这里是com.zhangfei。

怎么自动装配其他N个类

Import({AutoConfigurationImportSelector.class})该注解给当前配置类导入另外N个自动配置类。

这里既然导入N个自动配置类，那么都导入哪些类呢？

//AutoConfigurationImportSelector实现DeferredImportSelector接口，而DeferredImportSelector接口又继承了ImportSelector
public interface ImportSelector {
  String[] selectImports(AnnotationMetadata var1);
}

AutoConfigurationImportSelector通过实现接口ImportSelector的selectImports方法返回需要导入的组件，selectImports方法返回一个全类名字符串数组。

主角上场

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

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,AnnotationMetadata annotationMetadata) {
  if (!isEnabled(annotationMetadata)) {
    return EMPTY_ENTRY;
  }
  AnnotationAttributes attributes = getAttributes(annotationMetadata);
  List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
  configurations = removeDuplicates(configurations);
  Set<String> exclusions = getExclusions(annotationMetadata, attributes);
  checkExcludedClasses(configurations, exclusions);
  configurations.removeAll(exclusions);
  configurations = filter(configurations, autoConfigurationMetadata);
  fireAutoConfigurationImportEvents(configurations, exclusions);
  return new AutoConfigurationEntry(configurations, exclusions);
}

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

这里又开始调用SpringFactoriesLoader.loadFactoryNames。
SpringFactoriesLoader.loadFactoryNames方法中关键的三步：
(1)从当前项目的类路径中获取所有 META-INF/spring.factories 这个文件下的信息.
(2)将上面获取到的信息封装成一个 Map 返回，EnableAutoConfiguration为key。
(3)从返回的Map中通过刚才传入的 EnableAutoConfiguration.class参数，获取该 key 下的所有值。

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
  String factoryClassName = factoryClass.getName();
  return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
  MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
  if (result != null) {
    return result;
  } else {
    try {
      Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
      LinkedMultiValueMap result = new LinkedMultiValueMap();

      while(urls.hasMoreElements()) {
        URL url = (URL)urls.nextElement();
        UrlResource resource = new UrlResource(url);
        Properties properties = PropertiesLoaderUtils.loadProperties(resource);
        Iterator var6 = properties.entrySet().iterator();

        while(var6.hasNext()) {
          Entry<?, ?> entry = (Entry)var6.next();
          String factoryClassName = ((String)entry.getKey()).trim();
          String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
          int var10 = var9.length;

          for(int var11 = 0; var11 < var10; ++var11) {
            String factoryName = var9[var11];
            result.add(factoryClassName, factoryName.trim());
          }
        }
      }

      cache.put(classLoader, result);
      return result;
    } catch (IOException var13) {
      throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
    }
  }
}

自动配置都有哪些内容呢？

# 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,\
...其他省略

XXXAutoConfiguration和XXProperties

在spring.factories文件中看到的都是自动配置类，那么自动配置用到的属性值在那里呢？我们拿出redis为例

@Configuration
@ConditionalOnClass(RedisOperations.class) //判断当前项目有没有这个类RedisOperations.class
@EnableConfigurationProperties(RedisProperties.class) //启用配置属性，这里看到了熟悉的XXXProperties
@Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class }) //导入这两个类
public class RedisAutoConfiguration {

  @Bean
  @ConditionalOnMissingBean(name = "redisTemplate")
  public RedisTemplate<Object, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory)
      throws UnknownHostException {
    RedisTemplate<Object, Object> template = new RedisTemplate<>();
    template.setConnectionFactory(redisConnectionFactory);
    return template;
  }

  @Bean
  @ConditionalOnMissingBean
  public StringRedisTemplate stringRedisTemplate(RedisConnectionFactory redisConnectionFactory)
      throws UnknownHostException {
    StringRedisTemplate template = new StringRedisTemplate();
    template.setConnectionFactory(redisConnectionFactory);
    return template;
  }
}

//这里则保存redis初始化时的属性
@ConfigurationProperties(prefix = "spring.redis")
public class RedisProperties {

  private int database = 0;

  private String url;

  private String host = "localhost";

  private String password;

  private int port = 6379;

  private boolean ssl;

}

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