上一篇文章 Spring Boot 手摸手系列-hello word(二)要点分析 ,提到过 @EnableAutoConfiguration 表示开启自动配置功能。 下面就做下详细分析:
我们的配置文件 application.yml 或者 application.properties, 为什么很多属性配置之后就能立即生效呢?
这个就得益于springboot的自动配置,springboot会将很多配置类自动为我们注入到spring容器中,
1、 springboot主程序启动时开启自动配置 @EnableAutoConfiguration
@SpringBootApplication
public class StartApplication {
public static void main(String[] args) {
// springboot 应用启动
SpringApplication.run(StartApplication.class, args);
}
}
@SpringBootApplication
是个复合注解或派生注解,查看源码就是下方显示
@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 {
2、 @EnableAutoConfiguration 通过 AutoConfigurationImportSelector 导入( @Import )组件
AutoConfigurationImportSelector 是一个组件选择器;它筛选出一些组件,然后注入到ioc容器中;那么它如何筛选?
直接看它源码:
@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());
}
/**
* Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
* of the importing {@link Configuration @Configuration} class.
* @param autoConfigurationMetadata the auto-configuration metadata
* @param annotationMetadata the annotation metadata of the configuration class
* @return the auto-configurations that should be imported
*/
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);
}
我们可以看到 selectImports 返回了一个sting[] ,这个字符串数组就是要注入的类全类名。然而实际真正获取全类名数组的方法是 :
List configurations = getCandidateConfigurations(annotationMetadata, attributes);
getCandidateConfigurations
方法是去获取候选的配置
List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),getBeanClassLoader());
查看 SpringFactoriesLoader.loadFactoryNames()
源码,我们发现如下代码;它是扫描所有jar包类路径下META-INF/spring.factories
;
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
String factoryTypeName = factoryType.getName();
return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}
loadSpringFactories(classLoader)
看它源码如下:
Enumeration<URL> urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
FACTORIES_RESOURCE_LOCATION
点过去一看其实就是 META-INF/spring.factories
/**
* The location to look for factories.
* <p>Can be present in multiple JAR files.
*/
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
同时看源码我们得以见得,通过Enumeration urls
我们得到了Properties properties
;这个过程其实就是将扫描到的内容转成 Properties对象。
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
而获取的所有 Properties
对象
loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
获取得到一个 叫 factoryTypeName 的组件;
factoryTypeName 咋来的呢,上面提到的 getSpringFactoriesLoaderFactoryClass()
我们打开一看其实就是EnableAutoConfiguration.class
所以这里简单的概括其实就是 从 Properties 对象中 获取 EnableAutoConfiguration.class
类名对应的值,然后把它们添加到容器中。
3、
EnableAutoConfiguration.class
在 spring-boot-autoconfigur jar 包下的 META-INF/spring.factories 可以找到对应的值。如下图:
而在将 类路径下 META-INF/spring.factories 中含有 EnableAutoConfiguration.class
的值加入到容器中,具体操作是什么呢?
请看下文 Spring Boot 手摸手系列-自动配置原理(二)
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