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:
@Configurationpublic 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。