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;

}