Eureka作为服务注册中心，主要的功能是服务注册和服务发现，是微服务框架的基础功能和核心功能。
Eureka的使用可参考：
Eureka服务端：Spring Cloud Eureka Server使用（注册中心），
Eureka客户端：Eureka Client的使用，
Eureka服务端：Eureka的高可用
Eureka分为客户端和服务端，这里主要介绍客户端源码
1、Eureka客户端主要使用EnableDiscoveryClient注解
1）@EnableDiscoveryClient源码如下
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(EnableDiscoveryClientImportSelector.class) //主要是这个注解
public @interface EnableDiscoveryClient {
/**
* If true, the ServiceRegistry will automatically register the local server.
* @return - {@code true} if you want to automatically register.
*/
boolean autoRegister() default true;
}
2)、EnableDiscoveryClientImportSelector
@Order(Ordered.LOWEST_PRECEDENCE - 100)
public class EnableDiscoveryClientImportSelector
extends SpringFactoryImportSelector<EnableDiscoveryClient> {
@Override
public String[] selectImports(AnnotationMetadata metadata) {
// 1.获取需要注册到Spring的类
String[] imports = super.selectImports(metadata);
AnnotationAttributes attributes = AnnotationAttributes.fromMap(
metadata.getAnnotationAttributes(getAnnotationClass().getName(), true));
boolean autoRegister = attributes.getBoolean("autoRegister");
// 2.autoRegister默认为true，同时则注册AutoServiceRegistrationConfiguration类到Spring中
if (autoRegister) {
List<String> importsList = new ArrayList<>(Arrays.asList(imports));
importsList.add("org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationConfiguration");
imports = importsList.toArray(new String[0]);
}
return imports;
}
...
}
3)、 super.selectImports(metadata);
SpringFactoryImportSelector的selectImports方法
public abstract class SpringFactoryImportSelector<T>
implements DeferredImportSelector, BeanClassLoaderAware, EnvironmentAware {
@Override
public String[] selectImports(AnnotationMetadata metadata) {
//1、默认isEnabled()为true
if (!isEnabled()) {
return new String[0];
}
AnnotationAttributes attributes = AnnotationAttributes.fromMap(
metadata.getAnnotationAttributes(this.annotationClass.getName(), true));
Assert.notNull(attributes, "No " + getSimpleName() + " attributes found. Is "
+ metadata.getClassName() + " annotated with @" + getSimpleName() + "?");
//主要是这里 Find all possible auto configuration classes, filtering duplicates
List<String> factories = new ArrayList<>(new LinkedHashSet<>(SpringFactoriesLoader
.loadFactoryNames(this.annotationClass, this.beanClassLoader)));
if (factories.isEmpty() && !hasDefaultFactory()) {
throw new IllegalStateException("Annotation @" + getSimpleName()
+ " found, but there are no implementations. Did you forget to include a starter?");
}
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()]);
}
}
//SpringFactoriesLoader.loadFactoryNames
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
//factoryTypeName 值为org.springframework.cloud.client.discovery.EnableDiscoveryClient
String factoryTypeName = factoryType.getName();
return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}
//SpringFactoriesLoader.loadSpringFactories
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
if (result != null) {
return result;
} else {
try {
//1、获取所有META-INF/spring.factories文件
Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
LinkedMultiValueMap result = new LinkedMultiValueMap();
//遍历所有spring.factories 文件
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 factoryTypeName = ((String)entry.getKey()).trim();
String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
int var10 = var9.length;
for(int var11 = 0; var11 < var10; ++var11) {
String factoryImplementationName = var9[var11];
//3、获取properties中key为EnableDiscoveryClient对应的value值列表。
result.add(factoryTypeName, factoryImplementationName.trim());
}
}
}
cache.put(classLoader, result);
return result;
} catch (IOException var13) {
throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
}
}
}
获取properties中key为EnableDiscoveryClient对应的value值列表。对应的值可以在spring-cloud-netflix-eureka-client-x.x.x.RELEASE.jar中META-INF的spring.factories。
值为：org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration
spring.factories的详情如下：
org.springframework.boot.autoconfigure.EnableAutoConfiguration= org.springframework.cloud.netflix.eureka.config.EurekaClientConfigServerAutoConfiguration, org.springframework.cloud.netflix.eureka.config.EurekaDiscoveryClientConfigServiceAutoConfiguration, org.springframework.cloud.netflix.eureka.EurekaClientAutoConfiguration, org.springframework.cloud.netflix.ribbon.eureka.RibbonEurekaAutoConfiguration, org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration, org.springframework.cloud.netflix.eureka.reactive.EurekaReactiveDiscoveryClientConfiguration, org.springframework.cloud.netflix.eureka.loadbalancer.LoadBalancerEurekaAutoConfiguration org.springframework.cloud.bootstrap.BootstrapConfiguration= org.springframework.cloud.netflix.eureka.config.EurekaDiscoveryClientConfigServiceBootstrapConfiguration
其实，spring.factories中EnableAutoConfiguration对应的value值列表的类会在SpringBoot项目启动的时候注册到Spring容器中，EurekaClient的关键功能就在EurekaClientAutoConfiguration
2、EurekaClientConfigServerAutoConfiguration功能解析
@Configuration( proxyBeanMethods = false)
@EnableConfigurationProperties
@ConditionalOnClass({EurekaInstanceConfigBean.class, EurekaClient.class, ConfigServerProperties.class})
public class EurekaClientConfigServerAutoConfiguration {
@Autowired( required = false)
private EurekaInstanceConfig instance;
@Autowired( required = false)
private ConfigServerProperties server;
public EurekaClientConfigServerAutoConfiguration() {
}
@PostConstruct
public void init() {
if (this.instance != null && this.server != null) {
String prefix = this.server.getPrefix();
if (StringUtils.hasText(prefix) && !StringUtils.hasText((String)this.instance.getMetadataMap().get("configPath"))) {
this.instance.getMetadataMap().put("configPath", prefix);
}
}
}
}
通过注解@ConditionalOnClass可知， EurekaClientConfigServerAutoConfiguration 类的产生需要EurekaInstanceConfigBean.class, EurekaClient.class, ConfigServerProperties.class 这三个类先产生。
我们重点关注EurekaClient ，源码如下
@ImplementedBy(DiscoveryClient.class)
public interface EurekaClient extends LookupService {
就是一个接口，并定义了默认实现类DiscoveryClient，该接口定义了Eureka客户端主要功能，包括获取服务URL、注册当前服务等功能。
3、DiscoveryClient（com.netflix.discovery.DiscoveryClient）
Eureka主要功能有： 服务注册、服务续约、服务下线、服务调用
1) 服务注册（发送注册请求到注册中心）
boolean register() throws Throwable {
EurekaHttpResponse httpResponse;
try {
//主要的注册功能在这里，真正的实现在AbstractJerseyEurekaHttpClient.register()
httpResponse = this.eurekaTransport.registrationClient.register(this.instanceInfo);
} catch (Exception var3) {
logger.warn("DiscoveryClient_{} - registration failed {}", new Object[]{this.appPathIdentifier, var3.getMessage(), var3});
throw var3;
}
...
return httpResponse.getStatusCode() == Status.NO_CONTENT.getStatusCode();
}
//AbstractJerseyEurekaHttpClient.register()
@Override
public EurekaHttpResponse<Void> register(InstanceInfo info) {
String urlPath = "apps/" + info.getAppName();
ClientResponse response = null;
try {
//1、构造一个HTTP请求
Builder resourceBuilder = jerseyClient.resource(serviceUrl).path(urlPath).getRequestBuilder();
addExtraHeaders(resourceBuilder);
//2、发送post请求到serviceUrl，serviceUrl即我们在配置文件中配置的defaultZone
response = resourceBuilder
.header("Accept-Encoding", "gzip")
.type(MediaType.APPLICATION_JSON_TYPE)
.accept(MediaType.APPLICATION_JSON)
.post(ClientResponse.class, info);
//3、返回响应状态
return anEurekaHttpResponse(response.getStatus()).headers(headersOf(response)).build();
} finally {
if (logger.isDebugEnabled()) {
logger.debug("Jersey HTTP POST {}/{} with instance {}; statusCode={}", serviceUrl, urlPath, info.getId(),
response == null ? "N/A" : response.getStatus());
}
if (response != null) {
response.close();
}
}
}
2）服务续约
本质就是发送当前应用的心跳请求
boolean renew() {
EurekaHttpResponse<InstanceInfo> httpResponse;
try {
//1、本质就是发送心跳请求
//2、真正实现为AbstractJerseyEurekaHttpClient.sendHeartBeat()
httpResponse = eurekaTransport.registrationClient.sendHeartBeat(instanceInfo.getAppName(), instanceInfo.getId(), instanceInfo, null);
logger.debug(PREFIX + "{} - Heartbeat status: {}", appPathIdentifier, httpResponse.getStatusCode());
//3、如果请求失败，则调用注册服务请求
if (httpResponse.getStatusCode() == Status.NOT_FOUND.getStatusCode()) {
REREGISTER_COUNTER.increment();
logger.info(PREFIX + "{} - Re-registering apps/{}", appPathIdentifier, instanceInfo.getAppName());
long timestamp = instanceInfo.setIsDirtyWithTime();
boolean success = register();
if (success) {
instanceInfo.unsetIsDirty(timestamp);
}
return success;
}
return httpResponse.getStatusCode() == Status.OK.getStatusCode();
} catch (Throwable e) {
logger.error(PREFIX + "{} - was unable to send heartbeat!", appPathIdentifier, e);
return false;
}
}
//AbstractJerseyEurekaHttpClient.sendHeartBeat()
@Override
public EurekaHttpResponse<InstanceInfo> sendHeartBeat(String appName, String id, InstanceInfo info, InstanceStatus overriddenStatus) {
String urlPath = "apps/" + appName + '/' + id;
ClientResponse response = null;
try {
//将当前实例的元信息（InstanceInfo）以及状态(UP)通过HTTP请求发送到serviceUrl
WebResource webResource = jerseyClient.resource(serviceUrl)
.path(urlPath)
.queryParam("status", info.getStatus().toString())
.queryParam("lastDirtyTimestamp", info.getLastDirtyTimestamp().toString());
if (overriddenStatus != null) {
webResource = webResource.queryParam("overriddenstatus", overriddenStatus.name());
}
Builder requestBuilder = webResource.getRequestBuilder();
addExtraHeaders(requestBuilder);
response = requestBuilder.put(ClientResponse.class);
EurekaHttpResponseBuilder<InstanceInfo> eurekaResponseBuilder = anEurekaHttpResponse(response.getStatus(), InstanceInfo.class).headers(headersOf(response));
if (response.hasEntity() &&
!HTML.equals(response.getType().getSubtype())) { //don't try and deserialize random html errors from the server
eurekaResponseBuilder.entity(response.getEntity(InstanceInfo.class));
}
return eurekaResponseBuilder.build();
} finally {
if (logger.isDebugEnabled()) {
logger.debug("Jersey HTTP PUT {}/{}; statusCode={}", serviceUrl, urlPath, response == null ? "N/A" : response.getStatus());
}
if (response != null) {
response.close();
}
}
}
3)服务调用
本质就是获取调用服务名所对应的服务提供者实例信息，包括IP、port等
@Override
public List<InstanceInfo> getInstancesByVipAddress(String vipAddress, boolean secure) {
return getInstancesByVipAddress(vipAddress, secure, instanceRegionChecker.getLocalRegion());
}
@Override
public List<InstanceInfo> getInstancesByVipAddress(String vipAddress, boolean secure,
@Nullable String region) {
if (vipAddress == null) {
throw new IllegalArgumentException(
"Supplied VIP Address cannot be null");
}
Applications applications;
//1、判断服务提供方是否是当前region，若是的话直接从localRegionApp中获取
if (instanceRegionChecker.isLocalRegion(region)) {
applications = this.localRegionApps.get();
}
//2、否则的话从远程region获取
else {
applications = remoteRegionVsApps.get(region);
if (null == applications) {
logger.debug("No applications are defined for region {}, so returning an empty instance list for vip "
+ "address {}.", region, vipAddress);
return Collections.emptyList();
}
}
//从applications中获取服务名称对应的实例名称列表
if (!secure) {
return applications.getInstancesByVirtualHostName(vipAddress);
} else {
return applications.getInstancesBySecureVirtualHostName(vipAddress);
}
}
4)服务下线
void unregister() {
// It can be null if shouldRegisterWithEureka == false
if(eurekaTransport != null && eurekaTransport.registrationClient != null) {
try {
logger.info("Unregistering ...");
EurekaHttpResponse<Void> httpResponse = eurekaTransport.registrationClient.cancel(instanceInfo.getAppName(), instanceInfo.getId());
logger.info(PREFIX + "{} - deregister status: {}", appPathIdentifier, httpResponse.getStatusCode());
} catch (Exception e) {
logger.error(PREFIX + "{} - de-registration failed{}", appPathIdentifier, e.getMessage(), e);
}
}
}
//AbstractJerseyEurekaHttpClient.cancel()
@Override
public EurekaHttpResponse<Void> cancel(String appName, String id) {
String urlPath = "apps/" + appName + '/' + id;
ClientResponse response = null;
try {
Builder resourceBuilder = jerseyClient.resource(serviceUrl).path(urlPath).getRequestBuilder();
addExtraHeaders(resourceBuilder);
//本质就是发送delete请求到注册中心
response = resourceBuilder.delete(ClientResponse.class);
return anEurekaHttpResponse(response.getStatus()).headers(headersOf(response)).build();
} finally {
if (logger.isDebugEnabled()) {
logger.debug("Jersey HTTP DELETE {}/{}; statusCode={}", serviceUrl, urlPath, response == null ? "N/A" : response.getStatus());
}
if (response != null) {
response.close();
}
}
}
通过上面的分析可知，服务注册、续约、下线、调用等都是通过HTTP请求来实现的。那这些是什么时候被调用呢？