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/ -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。

QQ截图20190828173328.jpg

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(Annotation data var1);

}
QQ截图20190828174002.jpg
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(Annotation data annotation data) {

if (!isEnabled(annotation data)) {    return NO_IMPORTS;}AutoConfiguration data autoConfiguration data = AutoConfiguration dataLoader.load data(this.beanClassLoader);AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfiguration data,annotation data);return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());

}

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfiguration data autoConfiguration data,Annotation data annotation data) {

if (!isEnabled(annotation data)) {    return EMPTY_ENTRY;}AnnotationAttributes attributes = getAttributes(annotation data);List<String> configurations = getCandidateConfigurations(annotation data, attributes);configurations = removeDuplicates(configurations);Set<String> exclusions = getExclusions(annotation data, attributes);checkExcludedClasses(configurations, exclusions);configurations.removeAll(exclusions);configurations = filter(configurations, autoConfiguration data);fireAutoConfigurationImportEvents(configurations, exclusions);return new AutoConfigurationEntry(configurations, exclusions);

}

protected List getCandidateConfigurations(Annotation data data, 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 -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 loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {

String factoryClassName = factoryClass.getName();return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());

}

private static Map> 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(" -INF/spring.factories") : ClassLoader.getSystemResources(" -INF/spring.factories");         edMultiValueMap result = new  edMultiValueMap();        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 [ -INF/spring.factories]", var13);    }}

}
自动配置都有哪些内容呢？

Auto Configure

org.spring work.boot.autoconfigure.EnableAutoConfiguration=
org.spring work.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,
org.spring work.boot.autoconfigure.aop.AopAutoConfiguration,
org.spring work.boot.autoconfigure.amqp.RabbitAutoConfiguration,
org.spring work.boot.autoconfigure.batch.BatchAutoConfiguration,
org.spring work.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< ,  > redisTemplate(RedisConnectionFactory redisConnectionFactory)        throws UnknownHostException {    RedisTemplate< ,  > template = new RedisTemplate<>();    template.setConnectionFactory(redisConnectionFactory);    return template;}@Bean@ConditionalOnMissingBeanpublic 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 data  = 0;private String url;private String host = "localhost";private String password;private int port = 6379;private boolean ssl;

}
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