springboot自动配置原理 - 好文

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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