Eureka-Server源码核心代码入口

1、服务端

 org.springframework.cloud spring-cloud-starter-netflix-eureka-server 

spring-cloud-netflix-eureka-server中spring.factories中EurekaServerAutoConfiguration

/**
	 * Register the Jersey filter.
	 * @param eurekaJerseyApp an {@link Application} for the filter to be registered
	 * @return a jersey {@link FilterRegistrationBean}
	 */
	@Bean
	public FilterRegistrationBean jerseyFilterRegistration(
			javax.ws.rs.core.Application eurekaJerseyApp) {FilterRegistrationBean bean = new FilterRegistrationBean();
		bean.setFilter(new ServletContainer(eurekaJerseyApp));
		bean.setOrder(Ordered.LOWEST_PRECEDENCE);
		bean.setUrlPatterns(
				Collections.singletonList(EurekaConstants.DEFAULT_PREFIX + "/*"));
		return bean;
	}

1.1、服务注册

ApplicationResource类的addInstance方法接收请求，在对实例的信息进行验证后，向服务注册中心添加实例。

/注意是POST方法

 @POST 
    @Consumes({"application/json", "application/xml"})
    public Response addInstance(InstanceInfo info,
                                @HeaderParam(PeerEurekaNode.HEADER_REPLICATION) String isReplication) { logger.debug("Registering instance {} (replication={})", info.getId(), isReplication);
        // validate that the instanceinfo contains all the necessary required fields
        if (isBlank(info.getId())) { return Response.status(400).entity("Missing instanceId").build();
        } else if (isBlank(info.getHostName())) { return Response.status(400).entity("Missing hostname").build();
        } else if (isBlank(info.getIPAddr())) { return Response.status(400).entity("Missing ip address").build();
        } else if (isBlank(info.getAppName())) { return Response.status(400).entity("Missing appName").build();
        } else if (!appName.equals(info.getAppName())) { return Response.status(400).entity("Mismatched appName, expecting " + appName + " but was " + info.getAppName()).build();
        } else if (info.getDataCenterInfo() == null) { return Response.status(400).entity("Missing dataCenterInfo").build();
        } else if (info.getDataCenterInfo().getName() == null) { return Response.status(400).entity("Missing dataCenterInfo Name").build();
        }
        // handle cases where clients may be registering with bad DataCenterInfo with missing data
        DataCenterInfo dataCenterInfo = info.getDataCenterInfo();
        if (dataCenterInfo instanceof UniqueIdentifier) { String dataCenterInfoId = ((UniqueIdentifier) dataCenterInfo).getId();
            if (isBlank(dataCenterInfoId)) { boolean experimental = "true".equalsIgnoreCase(serverConfig.getExperimental("registration.validation.dataCenterInfoId"));
                if (experimental) { String entity = "DataCenterInfo of type " + dataCenterInfo.getClass() + " must contain a valid id";
                    return Response.status(400).entity(entity).build();
                } else if (dataCenterInfo instanceof AmazonInfo) { AmazonInfo amazonInfo = (AmazonInfo) dataCenterInfo;
                    String effectiveId = amazonInfo.get(AmazonInfo.MetaDataKey.instanceId);
                    if (effectiveId == null) { amazonInfo.getMetadata().put(AmazonInfo.MetaDataKey.instanceId.getName(), info.getId());
                    }
                } else { logger.warn("Registering DataCenterInfo of type {} without an appropriate id", dataCenterInfo.getClass());
                }
            }
        }
		//进入InstanceRegistry的register
        registry.register(info, "true".equals(isReplication));
        return Response.status(204).build();  // 204 to be backwards compatible
    }

进入InstanceRegistry的register在这里做了两个功能

@Override
	public void register(final InstanceInfo info, final boolean isReplication) {//在方法中使用publishEvent发布了监听事件 。Spring支持事件驱动，可以监听者模式进行事件的监听，这里广播给所有监听者，收到一个服务注册的请求。至于监听器，可以由我们自己手写实现，参数中的事件类型spring会帮我们直接注入
		handleRegistration(info, resolveInstanceLeaseDuration(info), isReplication);
		//调用父类PeerAwareInstanceRegistryImpl的register方法
		super.register(info, isReplication);
	}
 
@Component
public class EurekaRegisterListener { @EventListener
  public void registe(EurekaInstanceRegisteredEvent event){ System.out.println(event.getInstanceInfo().getAppName());
  }
}

父类PeerAwareInstanceRegistryImpl的register方法

 public void register(final InstanceInfo info, final boolean isReplication) { //拿到微服务的过期时间，并进行更新
        int leaseDuration = Lease.DEFAULT_DURATION_IN_SECS;
        if (info.getLeaseInfo() != null && info.getLeaseInfo().getDurationInSecs() > 0) { leaseDuration = info.getLeaseInfo().getDurationInSecs();
        }
        //将服务注册交给父类完成 父类AbstractInstanceRegistry的register方法，在这开始真正开始做服务注册
        super.register(info, leaseDuration, isReplication);
        //完成集群信息同步
        replicateToPeers(Action.Register, info.getAppName(), info.getId(), info, null, isReplication);
    }

AbstractInstanceRegistry的register方法

AbstractInstanceRegistry这个类中定义的Eureka-server的服务注册列表的结构
ConcurrentHashMap中外层的String表示服务名称；
Map中的String表示服务节点的id （也就是实例的instanceid）；
Lease是一个心跳续约的对象，InstanceInfo表示实例信息。   
 private final ConcurrentHashMap>> registry
            = new ConcurrentHashMap>>(); 

1、将新的实例写入注册表的数据map中

2、清理缓存

3、主动失效读写缓存里的数据

public void register(InstanceInfo registrant, int leaseDuration, boolean isReplication) { try { read.lock();
            Map> gMap = registry.get(registrant.getAppName());
            REGISTER.increment(isReplication);
            //首先，注册表根据微服务的名称或取Map，如果不存在就新建，使用putIfAbsent。
            if (gMap == null) { //1、将新的实例写入注册表的数据map中
                final ConcurrentHashMap> gNewMap = new ConcurrentHashMap>();
                gMap = registry.putIfAbsent(registrant.getAppName(), gNewMap);
                if (gMap == null) { gMap = gNewMap;
                }
            }
            Lease existingLease = gMap.get(registrant.getId());
            // Retain the last dirty timestamp without overwriting it, if there is already a lease
            //然后，从gMap（gMap就是该服务的实例列表）获取一次服务实例，判断这个微服务的节点是否存在，第一次注册的情况下一般是不存在的。当然，也有可能会发生注册信息冲突时，这时Eureka会根据最后活跃时间来判断到底覆盖哪一个：
            if (existingLease != null && (existingLease.getHolder() != null)) { Long existingLastDirtyTimestamp = existingLease.getHolder().getLastDirtyTimestamp();
                Long registrationLastDirtyTimestamp = registrant.getLastDirtyTimestamp();
                logger.debug("Existing lease found (existing={}, provided={}", existingLastDirtyTimestamp, registrationLastDirtyTimestamp);
                // this is a > instead of a >= because if the timestamps are equal, we still take the remote transmitted
                // InstanceInfo instead of the server local copy.
                if (existingLastDirtyTimestamp > registrationLastDirtyTimestamp) { logger.warn("There is an existing lease and the existing lease's dirty timestamp {} is greater" +
                            " than the one that is being registered {}", existingLastDirtyTimestamp, registrationLastDirtyTimestamp);
                    logger.warn("Using the existing instanceInfo instead of the new instanceInfo as the registrant");
                    registrant = existingLease.getHolder();
                }
            } else { // The lease does not exist and hence it is a new registration
                synchronized (lock) { if (this.expectedNumberOfClientsSendingRenews > 0) { // Since the client wants to register it, increase the number of clients sending renews
                        this.expectedNumberOfClientsSendingRenews = this.expectedNumberOfClientsSendingRenews + 1;
                        updateRenewsPerMinThreshold();
                    }
                }
                logger.debug("No previous lease information found; it is new registration");
            }
			//这段代码中，Eureka拿到存在节点的最后活跃时间，和当前注册节点的发起注册时间，进行对比。当存在的节点的最后活跃时间大于当前注册节点的时间，就说明之前存在的节点更活跃，就替换当前节点。
这里有一个思想，就是如果Eureka缓存的老节点更活跃，就说明它能够使用，而新来的服务我并不知道是否能用，那么Eureka就保守的使用了可用的老节点，从这一点也保证了可用性。
在拿到服务实例后对其进行封装：
            Lease lease = new Lease(registrant, leaseDuration);
            if (existingLease != null) { lease.setServiceUpTimestamp(existingLease.getServiceUpTimestamp());
            }
            //Lease是一个心跳续约的包装类，里面存放了注册信息，最后操作时间，注册时间，过期时间，剔除时间等信息。在这里把注册实例及过期时间放到这个心跳续约对象中，再把心跳续约对象放到gmap注册表中去。之后进行改变服务状态，系统数据统计，至此一个服务注册的流程就完成了。注册完成后，查看一下registry中的服务实例，发现我们启动的Eureka-client都已经放在里面了：
            gMap.put(registrant.getId(), lease);
            recentRegisteredQueue.add(new Pair(
                    System.currentTimeMillis(),
                    registrant.getAppName() + "(" + registrant.getId() + ")"));
            // This is where the initial state transfer of overridden status happens
            if (!InstanceStatus.UNKNOWN.equals(registrant.getOverriddenStatus())) { logger.debug("Found overridden status {} for instance {}. Checking to see if needs to be add to the "
                                + "overrides", registrant.getOverriddenStatus(), registrant.getId());
                if (!overriddenInstanceStatusMap.containsKey(registrant.getId())) { logger.info("Not found overridden id {} and hence adding it", registrant.getId());
                    overriddenInstanceStatusMap.put(registrant.getId(), registrant.getOverriddenStatus());
                }
            }
            InstanceStatus overriddenStatusFromMap = overriddenInstanceStatusMap.get(registrant.getId());
            if (overriddenStatusFromMap != null) { logger.info("Storing overridden status {} from map", overriddenStatusFromMap);
                registrant.setOverriddenStatus(overriddenStatusFromMap);
            }
            // Set the status based on the overridden status rules
            InstanceStatus overriddenInstanceStatus = getOverriddenInstanceStatus(registrant, existingLease, isReplication);
            registrant.setStatusWithoutDirty(overriddenInstanceStatus);
            // If the lease is registered with UP status, set lease service up timestamp
            if (InstanceStatus.UP.equals(registrant.getStatus())) { lease.serviceUp();
            }
            registrant.setActionType(ActionType.ADDED);
            recentlyChangedQueue.add(new RecentlyChangedItem(lease));
            registrant.setLastUpdatedTimestamp();
            //2、清理缓存
            invalidateCache(registrant.getAppName(), registrant.getVIPAddress(), registrant.getSecureVipAddress());
            logger.info("Registered instance {}/{} with status {} (replication={})",
                    registrant.getAppName(), registrant.getId(), registrant.getStatus(), isReplication);
        } finally { read.unlock();
        }
    }

com.netflix.eureka.registry.AbstractInstanceRegistry#invalidateCache

 private void invalidateCache(String appName, @Nullable String vipAddress, @Nullable String secureVipAddress) { // invalidate cache
        responseCache.invalidate(appName, vipAddress, secureVipAddress);
    }

com.netflix.eureka.registry.ResponseCacheImpl#invalidate

 @Override
    public void invalidate(String appName, @Nullable String vipAddress, @Nullable String secureVipAddress) { for (Key.KeyType type : Key.KeyType.values()) { for (Version v : Version.values()) { invalidate(
                        new Key(Key.EntityType.Application, appName, type, v, EurekaAccept.full),
                        new Key(Key.EntityType.Application, appName, type, v, EurekaAccept.compact),
                        new Key(Key.EntityType.Application, ALL_APPS, type, v, EurekaAccept.full),
                        new Key(Key.EntityType.Application, ALL_APPS, type, v, EurekaAccept.compact),
                        new Key(Key.EntityType.Application, ALL_APPS_DELTA, type, v, EurekaAccept.full),
                        new Key(Key.EntityType.Application, ALL_APPS_DELTA, type, v, EurekaAccept.compact)
                );
                if (null != vipAddress) { invalidate(new Key(Key.EntityType.VIP, vipAddress, type, v, EurekaAccept.full));
                }
                if (null != secureVipAddress) { invalidate(new Key(Key.EntityType.SVIP, secureVipAddress, type, v, EurekaAccept.full));
                }
            }
        }
    }
	//主动失效读写缓存里的数据
    public void invalidate(Key... keys) { for (Key key : keys) { logger.debug("Invalidating the response cache key : {} {} {} {}, {}",
                    key.getEntityType(), key.getName(), key.getVersion(), key.getType(), key.getEurekaAccept());
            readWriteCacheMap.invalidate(key);
            Collection keysWithRegions = regionSpecificKeys.get(key);
            if (null != keysWithRegions && !keysWithRegions.isEmpty()) { for (Key keysWithRegion : keysWithRegions) { logger.debug("Invalidating the response cache key : {} {} {} {} {}",
                            key.getEntityType(), key.getName(), key.getVersion(), key.getType(), key.getEurekaAccept());
                    readWriteCacheMap.invalidate(keysWithRegion);
                }
            }
        }
    }

1.2、服务续约

InstanceRegistry # renew() ->

PeerAwareInstanceRegistry # renew()->

AbstractInstanceRegistry # renew()

先从注册表获取该服务的实例列表（gMap），再从gMap中通过实例的id 获取具体的 要续约的实例。之后根据服务实例的InstanceStatus判断是否处于宕机状态，以及是否和之前状态相同。如果一切状态正常，最终调用Lease中的renew方法：

可以看出，其实服务续约的操作非常简单，它的本质就是修改服务的最后的更新时间。将最后更新时间改为系统当前时间加上服务的过期时间。

值得提一下的是，lastUpdateTimestamp这个变量是被volatile关键字修饰的。

 public boolean renew(String appName, String id, boolean isReplication) { RENEW.increment(isReplication);
        Map> gMap = registry.get(appName);
        Lease leaseToRenew = null;
        if (gMap != null) { leaseToRenew = gMap.get(id);
        }
        if (leaseToRenew == null) { RENEW_NOT_FOUND.increment(isReplication);
            logger.warn("DS: Registry: lease doesn't exist, registering resource: {} - {}", appName, id);
            return false;
        } else { InstanceInfo instanceInfo = leaseToRenew.getHolder();
            if (instanceInfo != null) { // touchASGCache(instanceInfo.getASGName());
                InstanceStatus overriddenInstanceStatus = this.getOverriddenInstanceStatus(
                        instanceInfo, leaseToRenew, isReplication);
                if (overriddenInstanceStatus == InstanceStatus.UNKNOWN) { logger.info("Instance status UNKNOWN possibly due to deleted override for instance {}"
                            + "; re-register required", instanceInfo.getId());
                    RENEW_NOT_FOUND.increment(isReplication);
                    return false;
                }
                if (!instanceInfo.getStatus().equals(overriddenInstanceStatus)) { logger.info(
                            "The instance status {} is different from overridden instance status {} for instance {}. "
                                    + "Hence setting the status to overridden status", instanceInfo.getStatus().name(),
                                    instanceInfo.getOverriddenStatus().name(),
                                    instanceInfo.getId());
                    instanceInfo.setStatusWithoutDirty(overriddenInstanceStatus);
                }
            }
            renewsLastMin.increment();
            leaseToRenew.renew();
            return true;
        }
    }

1.3、服务剔除

当Eureka-server发现有的实例没有续约超过一定时间，则将该服务从注册列表剔除，该项工作由一个定时任务完成的。该任务的定义过程比较复杂，仅列出其调用过程：

EurekaServerInitializerConfiguration # start() ->

EurekaServerBootstrap # contextInitialized() ->

# initEurekaServerContext() ->

PeerAwareInstanceRegistryImpl # openForTraffic() ->

AbstractInstanceRegistry # postInit()

在AbstractInstanceRegistry的postInit方法中，定义EvictionTask定时任务，构建定时器启动该任务，执行任务中剔除方法 evict()。

private long evictionIntervalTimerInMs = 60 * 1000;

任务的时间被定义为60秒，即默认每分钟执行一次。

具体查看evit()剔除方法：

 protected void postInit() { renewsLastMin.start();
        if (evictionTaskRef.get() != null) { evictionTaskRef.get().cancel();
        }
        evictionTaskRef.set(new EvictionTask());
        evictionTimer.schedule(evictionTaskRef.get(),
                serverConfig.getEvictionIntervalTimerInMs(),
                serverConfig.getEvictionIntervalTimerInMs());
    }

 public void evict(long additionalLeaseMs) { logger.debug("Running the evict task");
        if (!isLeaseExpirationEnabled()) { logger.debug("DS: lease expiration is currently disabled.");
            return;
        }
        // We collect first all expired items, to evict them in random order. For large eviction sets,
        // if we do not that, we might wipe out whole apps before self preservation kicks in. By randomizing it,
        // the impact should be evenly distributed across all applications.
        //新建实例列表expiredLeases，用来存放过期的实例。
        List> expiredLeases = new ArrayList<>();
        //遍历registry注册表，对实例进行检测工作，使用isExpired方法判断实例是否过期
        //evictionTimestamp：剔除时间，当剔除节点的时候，将系统当前时间赋值给这个		evictionTimestamp
	//additionalLeaseMs：集群同步产生的预留时间，这个时间是程序中传过来的
        for (Entry>> groupEntry : registry.entrySet()) { Map> leaseMap = groupEntry.getValue();
            if (leaseMap != null) { for (Entry> leaseEntry : leaseMap.entrySet()) { Lease lease = leaseEntry.getValue();
                    if (lease.isExpired(additionalLeaseMs) && lease.getHolder() != null) { expiredLeases.add(lease);
                    }
                }
            }
        }
        // To compensate for GC pauses or drifting local time, we need to use current registry size as a base for
        // triggering self-preservation. Without that we would wipe out full registry.
        int registrySize = (int) getLocalRegistrySize();
        //根据阈值计算可以被剔除的服务数量最大值
        int registrySizeThreshold = (int) (registrySize * serverConfig.getRenewalPercentThreshold());
        //剔除后剩余最小数量
        int evictionLimit = registrySize - registrySizeThreshold;
		//expiredLeases.size() 剔除列表的数量
        int toEvict = Math.min(expiredLeases.size(), evictionLimit);
        if (toEvict > 0) { logger.info("Evicting {} items (expired={}, evictionLimit={})", toEvict, expiredLeases.size(), evictionLimit);
			//上面的代码中根据自我保护机制进行了判断，使用Min函数计算两者的最小值，剔除较小数量的服务实例。举个例子，假如当前共有100个服务，那么剔除阈值为85，如果list中有60个服务，那么就会剔除该60个服务。但是如果list中有95个服务，那么只会剔除其中的85个服务，在这种情况下，又会产生一个问题，eureka-server该如何判断去剔除哪些服务，保留哪些服务呢？
            Random random = new Random(System.currentTimeMillis());
            for (int i = 0; i < toEvict; i++) { // Pick a random item (Knuth shuffle algorithm)
                int next = i + random.nextInt(expiredLeases.size() - i);
                Collections.swap(expiredLeases, i, next);
                Lease lease = expiredLeases.get(i);
                String appName = lease.getHolder().getAppName();
                String id = lease.getHolder().getId();
                EXPIRED.increment();
                logger.warn("DS: Registry: expired lease for {}/{}", appName, id);
                //这里使用了随机算法进行剔除，保证不会连续剔除某个微服务的全部实例。最终调用internalCancel方法，实际执行剔除。
               internalCancel(appName, id, false);
            }
        }
    }

系统当前时间 >最后更新时间 + 过期时间 + 预留时间

这里进行判断：

系统当前时间 >最后更新时间 + 过期时间 + 预留时间

当该条件成立时，认为服务过期。在Eureka中过期时间默认定义为3个心跳的时间，一个心跳是30秒，因此过期时间是90秒。

当以上两个条件之一成立时，判断该实例过期，将该过期实例放入上面创建的列表中。注意这里仅仅是将实例放入List中，并没有实际剔除。

在实际剔除任务前，需要提一下eureka的自我保护机制：

当15分钟内，心跳失败的服务大于一定比例时，会触发自我保护机制。

这个值在Eureka中被定义为85%，一旦触发自我保护机制，Eureka会尝试保护其服务注册表中的信息，不再删除服务注册表中的数据。

1.4、服务下线

客户端发送http请求告诉eureka-server自己下线,调用 AbstractInstanceRegistry 中 cancel方法：

 @Override
    public boolean cancel(String appName, String id, boolean isReplication) { return internalCancel(appName, id, isReplication);
    }

最终还是调用了和服务剔除中一样的方法，remove掉了gMap中的实例。

1.5、集群信息同步

集群信息同步发生在Eureka-server之间，之前提到在PeerAwareInstanceRegistryImpl类中，在执行register方法注册微服务实例完成后，执行了集群信息同步方法replicateToPeers，具体分析一下该方法：

 private void replicateToPeers(Action action, String appName, String id,
                                  InstanceInfo info /* optional */,
                                  InstanceStatus newStatus /* optional */, boolean isReplication) { Stopwatch tracer = action.getTimer().start();
        try { if (isReplication) { numberOfReplicationsLastMin.increment();
            }
            // If it is a replication already, do not replicate again as this will create a poison replication
            if (peerEurekaNodes == Collections.EMPTY_LIST || isReplication) { return;
            }
            for (final PeerEurekaNode node : peerEurekaNodes.getPeerEurekaNodes()) { // If the url represents this host, do not replicate to yourself.
                if (peerEurekaNodes.isThisMyUrl(node.getServiceUrl())) { continue;
                }
                //这
                replicateInstanceActionsToPeers(action, appName, id, info, newStatus, node);
            }
        } finally { tracer.stop();
        }
    }
   private void replicateInstanceActionsToPeers(Action action, String appName,
  String id, InstanceInfo info, InstanceStatus newStatus,
  PeerEurekaNode node) { try { InstanceInfo infoFromRegistry;
            CurrentRequestVersion.set(Version.V2);
            switch (action) { case Cancel:
                    node.cancel(appName, id);
                    break;
                case Heartbeat:
                    InstanceStatus overriddenStatus = overriddenInstanceStatusMap.get(id);
                    infoFromRegistry = getInstanceByAppAndId(appName, id, false);
                    node.heartbeat(appName, id, infoFromRegistry, overriddenStatus, false);
                    break;
                case Register: //这
                    node.register(info);
                    break;
                case StatusUpdate:
                    infoFromRegistry = getInstanceByAppAndId(appName, id, false);
                    node.statusUpdate(appName, id, newStatus, infoFromRegistry);
                    break;
                case DeleteStatusOverride:
                    infoFromRegistry = getInstanceByAppAndId(appName, id, false);
                    node.deleteStatusOverride(appName, id, infoFromRegistry);
                    break;
            }
        } catch (Throwable t) { logger.error("Cannot replicate information to {} for action {}", node.getServiceUrl(), action.name(), t);
        } finally { CurrentRequestVersion.remove();
        }
    }

首先，遍历集群节点，用以给各个集群信息节点进行信息同步。

然后，调用replicateInstanceActionsToPeers方法，在该方法中根据具体的操作类型Action，选择分支，最终调用PeerEurekaNode的register方法：

 public void register(final InstanceInfo info) throws Exception { long expiryTime = System.currentTimeMillis() + getLeaseRenewalOf(info);
        batchingDispatcher.process(
                taskId("register", info),
                new InstanceReplicationTask(targetHost, Action.Register, info, null, true) { public EurekaHttpResponse execute() { //z这
                        return replicationClient.register(info);
                    }
                },
                expiryTime
        );
    }

最终发送http请求，但是与普通注册操作不同的时，这时将集群同步的标识置为true，说明注册信息是来自集群同步。

在注册过程中运行到addInstance方法时，单独注册时isReplication的值为false，集群同步时为true。通过该值，能够避免集群间出现死循环，进行循环同步的问题。

1.6、获取注册信息接口

com.netflix.eureka.resources.ApplicationsResource#getContainers

1、从缓存中获取responseCache.get(cacheKey)

2、首先从只读缓存里获取

 @GET
    public Response getContainers(@PathParam("version") String version,
                                  @HeaderParam(HEADER_ACCEPT) String acceptHeader,
                                  @HeaderParam(HEADER_ACCEPT_ENCODING) String acceptEncoding,
                                  @HeaderParam(EurekaAccept.HTTP_X_EUREKA_ACCEPT) String eurekaAccept,
                                  @Context UriInfo uriInfo,
                                  @Nullable @QueryParam("regions") String regionsStr) { boolean isRemoteRegionRequested = null != regionsStr && !regionsStr.isEmpty();
        String[] regions = null;
        if (!isRemoteRegionRequested) { EurekaMonitors.GET_ALL.increment();
        } else { regions = regionsStr.toLowerCase().split(",");
            Arrays.sort(regions); // So we don't have different caches for same regions queried in different order.
            EurekaMonitors.GET_ALL_WITH_REMOTE_REGIONS.increment();
        }
        // Check if the server allows the access to the registry. The server can
        // restrict access if it is not
        // ready to serve traffic depending on various reasons.
        if (!registry.shouldAllowAccess(isRemoteRegionRequested)) { return Response.status(Status.FORBIDDEN).build();
        }
        CurrentRequestVersion.set(Version.toEnum(version));
        KeyType keyType = Key.KeyType.JSON;
        String returnMediaType = MediaType.APPLICATION_JSON;
        if (acceptHeader == null || !acceptHeader.contains(HEADER_JSON_VALUE)) { keyType = Key.KeyType.XML;
            returnMediaType = MediaType.APPLICATION_XML;
        }
        Key cacheKey = new Key(Key.EntityType.Application,
                ResponseCacheImpl.ALL_APPS,
                keyType, CurrentRequestVersion.get(), EurekaAccept.fromString(eurekaAccept), regions
        );
        Response response;
        if (acceptEncoding != null && acceptEncoding.contains(HEADER_GZIP_VALUE)) { response = Response.ok(responseCache.getGZIP(cacheKey))
                    .header(HEADER_CONTENT_ENCODING, HEADER_GZIP_VALUE)
                    .header(HEADER_CONTENT_TYPE, returnMediaType)
                    .build();
        } else { //1、从缓存中获取responseCache.get(cacheKey)
            response = Response.ok(responseCache.get(cacheKey))
                    .build();
        }
        CurrentRequestVersion.remove();
        return response;
    }

com.netflix.eureka.registry.ResponseCacheImpl#get(com.netflix.eureka.registry.Key)

 public String get(final Key key) { //shouldUseReadOnlyResponseCache = true
        return get(key, shouldUseReadOnlyResponseCache);
    }
    @VisibleForTesting
    String get(final Key key, boolean useReadOnlyCache) { Value payload = getValue(key, useReadOnlyCache);
        if (payload == null || payload.getPayload().equals(EMPTY_PAYLOAD)) { return null;
        } else { return payload.getPayload();
        }
    }
    @VisibleForTesting
    Value getValue(final Key key, boolean useReadOnlyCache) { Value payload = null;
        try { if (useReadOnlyCache) { //2、首先从只读缓存里获取
                final Value currentPayload = readOnlyCacheMap.get(key);
                if (currentPayload != null) { payload = currentPayload;
                } else { //3、只读缓存中没有就从读写缓存中获取 
                	//读写缓存默认180秒会自动过期
                    payload = readWriteCacheMap.get(key);
                    //回写只读缓存 
                    //timeSchedule每隔30秒执行getCacheUpdate定时任务将读写缓存中的数据更新到只读缓存中
                    readOnlyCacheMap.put(key, payload);
                }
            } else { payload = readWriteCacheMap.get(key);
            }
        } catch (Throwable t) { logger.error("Cannot get value for key : {}", key, t);
        }
        return payload;
    }

ResponseCacheImpl

 ResponseCacheImpl(EurekaServerConfig serverConfig, ServerCodecs serverCodecs, AbstractInstanceRegistry registry) { this.serverConfig = serverConfig;
        this.serverCodecs = serverCodecs;
        this.shouldUseReadOnlyResponseCache = serverConfig.shouldUseReadOnlyResponseCache();
        this.registry = registry;
        long responseCacheUpdateIntervalMs = serverConfig.getResponseCacheUpdateIntervalMs();
        //读写缓存
        this.readWriteCacheMap =
                CacheBuilder.newBuilder().initialCapacity(serverConfig.getInitialCapacityOfResponseCache())
                        //180秒自动过期
                        .expireAfterWrite(serverConfig.getResponseCacheAutoExpirationInSeconds(), TimeUnit.SECONDS)
                        .removalListener(new RemovalListener() { @Override
                            public void onRemoval(RemovalNotification notification) { Key removedKey = notification.getKey();
                                if (removedKey.hasRegions()) { Key cloneWithNoRegions = removedKey.cloneWithoutRegions();
                                    regionSpecificKeys.remove(cloneWithNoRegions, removedKey);
                                }
                            }
                        })
                        .build(new CacheLoader() { @Override
                            public Value load(Key key) throws Exception { if (key.hasRegions()) { Key cloneWithNoRegions = key.cloneWithoutRegions();
                                    regionSpecificKeys.put(cloneWithNoRegions, key);
                                }
                                Value value = generatePayload(key);
                                return value;
                            }
                        });
		//每隔30秒执行getCacheUpdateTask定时任务将读写缓存中数据更新到只读缓存中
        if (shouldUseReadOnlyResponseCache) { timer.schedule(getCacheUpdateTask(),
                    new Date(((System.currentTimeMillis() / responseCacheUpdateIntervalMs) * responseCacheUpdateIntervalMs)
                            + responseCacheUpdateIntervalMs),
                    responseCacheUpdateIntervalMs);
        }
        try { Monitors.registerObject(this);
        } catch (Throwable e) { logger.warn("Cannot register the JMX monitor for the InstanceRegistry", e);
        }
    }