Spring Boot @EnableAutoConfiguration和 @Configuration的区别
Spring Boot @EnableAutoConfiguration和 @Configuration的区别
Spring Boot @EnableAutoConfiguration和 @Configuration的区别
在Spring Boot中，我们会使用@SpringBootApplication来开启Spring Boot程序。在之前的文章中我们讲到了@SpringBootApplication相当于@EnableAutoConfiguration，@ComponentScan，@Configuration三者的集合。
其中@Configuration用在类上面，表明这个是个配置类，如下所示：
@Configuration
public class MySQLAutoconfiguration {
...
}而@EnableAutoConfiguration则是开启Spring Boot的自动配置功能。什么是自动配置功能呢？简单点说就是Spring Boot根据依赖中的jar包，自动选择实例化某些配置。
接下来我们看一下@EnableAutoConfiguration是怎么工作的。
先看一下@EnableAutoConfiguration的定义：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
/**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
Class<?>[] exclude() default {};
/**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
String[] excludeName() default {};
}注意这一行：@Import(AutoConfigurationImportSelector.class)
AutoConfigurationImportSelector实现了ImportSelector接口，并会在实例化时调用selectImports。下面是其方法：
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());
}这个方法中的getCandidateConfigurations会从类加载器中查找所有的META-INF/spring.factories，并加载其中实现了@EnableAutoConfiguration的类。有兴趣的朋友可以具体研究一下这个方法的实现。
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = cache.get(classLoader);
if (result != null) {
return result;
}
try {
Enumeration<URL> urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
result = new LinkedMultiValueMap<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry<?, ?> entry : properties.entrySet()) {
String factoryTypeName = ((String) entry.getKey()).trim();
for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
result.add(factoryTypeName, factoryImplementationName.trim());
}
}
}
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}我们再看一下spring-boot-autoconfigure-2.2.2.RELEASE.jar中的META-INF/spring.factories。
spring.factories里面的内容是key=value形式的，我们重点关注一下EnableAutoConfiguration：
# 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,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
...这里只列出了一部分内容，根据上面的代码， 所有的EnableAutoConfiguration的实现都会被自动加载。这就是自动加载的原理了。
如果我们仔细去看具体的实现：
@Configuration(proxyBeanMethods = false)
@AutoConfigureAfter(JmxAutoConfiguration.class)
@ConditionalOnProperty(prefix = "spring.application.admin", value = "enabled", havingValue = "true",
matchIfMissing = false)
public class SpringApplicationAdminJmxAutoConfiguration {可以看到里面使用了很多@Conditional*** 的注解，这种注解的作用就是判断该配置类在什么时候能够起作用。
更多教程请参考 flydean的博客