死磕源码系列【springcloud之@EnableDiscoveryClient More than one implementation及加不加@EnableDiscoveryClient注解都可

╰+哭是因爲堅強的太久メ 2022-12-27 07:26 103阅读 0赞

最近做consul作为注册中心主方法main上添加与不添加@EnableDiscoveryClient启用服务发现注解都可以正常的使用服务发现及注册功能，这点上有些疑惑，所以就扒拉扒拉源码，看看实现原理到底是什么；

##### 1.看下注解@EnableDiscoveryClient源码 #####

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @Import(EnableDiscoveryClientImportSelector.class)
    public @interface EnableDiscoveryClient { 
    
    	/** * 如果为true,服务注册将自动注册本地服务 */
    	boolean autoRegister() default true;
    
    }

此注解通过@Import注解引入了一个EnableDiscoveryClientImportSelector类，具体核心自动化配置在此类中完成；

##### 2.EnableDiscoveryClientImportSelector类源码分析 #####

//此类实现了SpringFactoryImportSelector抽象类，而SpringFactoryImportSelector类又实现了DeferredImportSelector接口，看到DeferredImportSelector接口如果你了解springboot自动化配置就应该很清楚在哪里会调用了，此处不再详解，不了解的可以翻看我之前的文章
    @Order(Ordered.LOWEST_PRECEDENCE - 100)
    public class EnableDiscoveryClientImportSelector
    		extends SpringFactoryImportSelector<EnableDiscoveryClient> { 
    
    	@Override
    	public String[] selectImports(AnnotationMetadata metadata) { 
        //此处调用父类SpringFactoryImportSelector的selectImports方法，此方法从spring.factories配置文件中读取key为org.springframework.cloud.client.discovery.EnableDiscoveryClient的自动化配置类
    		String[] imports = super.selectImports(metadata);
    		//获取@EnableDiscoveryClient注解的属性配置
    		AnnotationAttributes attributes = AnnotationAttributes.fromMap(
    				metadata.getAnnotationAttributes(getAnnotationClass().getName(), true));
        //获取属性注解autoRegister的属性值
    		boolean autoRegister = attributes.getBoolean("autoRegister");
        //如果为true，则开启服务自动化注册
    		if (autoRegister) { 
    			List<String> importsList = new ArrayList<>(Arrays.asList(imports));
          //将AutoServiceRegistrationConfiguration类加入到自动注册IOC容器集合
    			importsList.add(
    					"org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationConfiguration");
    			imports = importsList.toArray(new String[0]);
    		}
    		else { 
          //将spring.cloud.service-registry.auto-registration.enabled配置为false的结果加入到环境中
    			Environment env = getEnvironment();
    			if (ConfigurableEnvironment.class.isInstance(env)) { 
    				ConfigurableEnvironment configEnv = (ConfigurableEnvironment) env;
    				LinkedHashMap<String, Object> map = new LinkedHashMap<>();
    				map.put("spring.cloud.service-registry.auto-registration.enabled", false);
    				MapPropertySource propertySource = new MapPropertySource(
    						"springCloudDiscoveryClient", map);
    				configEnv.getPropertySources().addLast(propertySource);
    			}
    
    		}
    
    		return imports;
    	}
    	//是否开启了springcloud服务发现功能
    	@Override
    	protected boolean isEnabled() { 
    		return getEnvironment().getProperty("spring.cloud.discovery.enabled",
    				Boolean.class, Boolean.TRUE);
    	}
    	//是否有默认工厂，默认：true
    	@Override
    	protected boolean hasDefaultFactory() { 
    		return true;
    	}
    
    }

看下父类SpringFactoryImportSelector的实现

public abstract class SpringFactoryImportSelector<T>
    		implements DeferredImportSelector, BeanClassLoaderAware, EnvironmentAware { 
    
    	private final Log log = LogFactory.getLog(SpringFactoryImportSelector.class);
    
    	private ClassLoader beanClassLoader;
    
    	private Class<T> annotationClass;
    
    	private Environment environment;
    
    	@SuppressWarnings("unchecked")
    	protected SpringFactoryImportSelector() { 
    		this.annotationClass = (Class<T>) GenericTypeResolver
    				.resolveTypeArgument(this.getClass(), SpringFactoryImportSelector.class);
    	}
    
    	@Override
    	public String[] selectImports(AnnotationMetadata metadata) { 
        //判定是否开启了自动化注册服务，如果false,直接返回空字符串数组
    		if (!isEnabled()) { 
    			return new String[0];
    		}
        //获取注解@EnableDiscoveryClient的属性
    		AnnotationAttributes attributes = AnnotationAttributes.fromMap(
    				metadata.getAnnotationAttributes(this.annotationClass.getName(), true));
    
    		Assert.notNull(attributes, "No " + getSimpleName() + " attributes found. Is "
    				+ metadata.getClassName() + " annotated with @" + getSimpleName() + "?");
    
    		// 获取所有的自动化配置类，其配置文件中key为org.springframework.cloud.client.discovery.EnableDiscoveryClient，其中consul作为配置中心是配置文件中没有任何值，只有enurke时才有
    		List<String> factories = new ArrayList<>(new LinkedHashSet<>(SpringFactoriesLoader
    				.loadFactoryNames(this.annotationClass, this.beanClassLoader)));
        //如果配置类集合为空，并且没有默认的工厂方法，则抛出异常
    		if (factories.isEmpty() && !hasDefaultFactory()) { 
    			throw new IllegalStateException("Annotation @" + getSimpleName()
    					+ " found, but there are no implementations. Did you forget to include a starter?");
    		}
    		//如果上述自动化配置类集合中的配置类多与1个将会打印一个警告信息
    		if (factories.size() > 1) { 
    			// there should only ever be one DiscoveryClient, but there might be more than
    			// one factory
    			this.log.warn("More than one implementation " + "of @" + getSimpleName()
    					+ " (now relying on @Conditionals to pick one): " + factories);
    		}
    
    		return factories.toArray(new String[factories.size()]);
    	}
    	...
    }

看过EnableDiscoveryClientImportSelector及其父类SpringFactoryImportSelector的源码你应该会发现此处的自动化配置是通过key为org.springframework.cloud.client.discovery.EnableDiscoveryClient的配置来实现的，这是你会想到做微服务相关的自动化配置是可以不使用org.springframework.boot.autoconfigure.EnableAutoConfiguration作为key来配置了，这样做是不是很美好；然后扒拉扒拉的把微服务相关的自动化配置改成org.springframework.cloud.client.discovery.EnableDiscoveryClient做为key，然后启动服务，发现可以正常启用及加载配置类，perfect；但是在仔细看启动日志发现有一个警告warn日志（More than one implementation of @EnableDiscoveryClient (now relying on @Conditionals to pick one): ），瞬间感觉心情不美丽了，虽然程序正常启动没问题，但是总感觉有点啥问题，然后又开始了使劲的查找，在网上找到一篇文章[What’s the difference between EnableEurekaClient and EnableDiscoveryClient?][What_s the difference between EnableEurekaClient and EnableDiscoveryClient]

[Ask Question][What_s the difference between EnableEurekaClient and EnableDiscoveryClient],上面有这样一段解释There are multiple implementations of “Discovery Service” (eureka, [consul][], [zookeeper][]). `@EnableDiscoveryClient` lives in [spring-cloud-commons][] and picks the implementation on the classpath. `@EnableEurekaClient` lives in [spring-cloud-netflix][] and only works for eureka. If eureka is on your classpath, they are effectively the same.

意思是这样的，服务发现有多种实现方式（consul、zookeeper、eureka）,其中客户端注解@EnableDiscoveryClient是基于spring-cloud-commons实现，@EnableEurekaClient是基于spring-cloud-netflix实现，如果是基于consul或zookeeper实现则使用@EnableDiscoveryClient注解启用服务（不建议使用org.springframework.cloud.client.discovery.EnableDiscoveryClient作为key进行自动化配置，否则会有一条警告信息），如果是使用eureka则使用@EnableEurekaClient启用服务（可以通过org.springframework.cloud.client.discovery.EnableDiscoveryClient=org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration）；

--------------------

##### consul做服务注册及发现主方法加上@EnableDiscoveryClient和去掉一样起作用原因分析 #####

> 上述源码我们已经看到了EnableDiscoveryClientImportSelector类的作用是将org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationConfiguration配置类注入到IOC容器之中，那为啥将@EnableDiscoveryClient注解去掉之后服务注册及发现功能一样其作用呢？接下来我们就对这做一个详细的分析；

##### AutoServiceRegistrationConfiguration源码 #####

@Configuration(proxyBeanMethods = false)
    @EnableConfigurationProperties(AutoServiceRegistrationProperties.class)
    @ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled",
    		matchIfMissing = true)
    public class AutoServiceRegistrationConfiguration { 
    
    }

> 配置类有两个作用，第一将AutoServiceRegistrationProperties属性配置类注入IOC容器，其二只有在spring.cloud.service-registry.auto-registration.enabled为true时才将配置类注入IOC容器，默认就为true;

分析：既然@EnableDiscoveryClient注解不配置AutoServiceRegistrationConfiguration一样可以注入到IOC容器，那么说明肯定是有其他的地方有注入，看下spring.factories配置文件中的自动化配置有一个AutoServiceRegistrationAutoConfiguration自动化配置类，再看下其源码：

@Configuration(proxyBeanMethods = false)
    @Import(AutoServiceRegistrationConfiguration.class)
    @ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled",
    		matchIfMissing = true)
    public class AutoServiceRegistrationAutoConfiguration { 
    
    	@Autowired(required = false)
    	private AutoServiceRegistration autoServiceRegistration;
    
    	@Autowired
    	private AutoServiceRegistrationProperties properties;
    
    	@PostConstruct
    	protected void init() { 
    		if (this.autoServiceRegistration == null && this.properties.isFailFast()) { 
    			throw new IllegalStateException("Auto Service Registration has "
    					+ "been requested, but there is no AutoServiceRegistration bean");
    		}
    	}
    
    }

看到上面有一个@Import将AutoServiceRegistrationConfiguration配置类注入IOC容器，这里就说明了如果EnableDiscoveryClientImportSelector类没有将配置类注入IOC容器，那么此处就会将其注入，问题已经找到，可以愉快的玩转微服务了。。。

GitHub地址：[https://github.com/mingyang66/spring-parent][https_github.com_mingyang66_spring-parent]

[What_s the difference between EnableEurekaClient and EnableDiscoveryClient]: https://stackoverflow.com/questions/31976236/whats-the-difference-between-enableeurekaclient-and-enablediscoveryclient
[consul]: https://github.com/spring-cloud/spring-cloud-consul
[zookeeper]: https://github.com/spring-cloud/spring-cloud-zookeeper
[spring-cloud-commons]: https://github.com/spring-cloud/spring-cloud-commons
[spring-cloud-netflix]: https://github.com/spring-cloud/spring-cloud-netflix/
[https_github.com_mingyang66_spring-parent]: https://github.com/mingyang66/spring-parent