cloud源码-Ribbon

boot2.0.0.7release版本

什么是Ribbon
Ribbon是Netflix公司开源的一个负载均衡的项目，它属于上述的第二种，是一个客户端负载均衡器，运行在客户端上。它是一个经过了云端测试的IPC库，可以很好地控制HTTP和TCP客户端的一些行为。 Feign已经默认使用了Ribbon。

负载均衡
容错
多协议（HTTP，TCP，UDP）支持异步和反应模型
缓存和批处理

RestTemplate和Ribbon相结合

Ribbon在Netflix组件是非常重要的一个组件，在Zuul中使用Ribbon做负载均衡，以及Feign组件的结合等。在Spring Cloud 中，作为开发中，做的最多的可能是将RestTemplate和Ribbon相结合，你可能会这样写：

@Configuration
public class RibbonConfig {
    @Bean
    @LoadBalanced
    RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

消费另外一个的服务的接口，差不多是这样的：

@Service
public class RibbonService {
    @Autowired
    RestTemplate restTemplate;
    public String hi(String name) {
        return restTemplate.getForObject("http://eureka-client/hi?name="+name,String.class);
    }
}

LoadBalancerClient

在Riibon中一个非常重要的组件为LoadBalancerClient，它作为负载均衡的一个客户端。它在spring-cloud-commons包下：
的LoadBalancerClient是一个接口，它继承ServiceInstanceChooser，它的实现类是RibbonLoadBalancerClient，这三者之间的关系如下图：

其中LoadBalancerClient接口，有如下三个方法，其中excute()为执行请求，reconstructURI()用来重构url：

public interface LoadBalancerClient extends ServiceInstanceChooser {
  <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;
  <T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException;
  URI reconstructURI(ServiceInstance instance, URI original);
}

ServiceInstanceChooser接口，主要有一个方法，用来根据serviceId来获取ServiceInstance，代码如下:

public interface ServiceInstanceChooser {
    ServiceInstance choose(String serviceId);
}

LoadBalancerClient的实现类为RibbonLoadBalancerClient，这个类是非常重要的一个类，最终的负载均衡的请求处理，由它来执行。它的部分源码如下：

public class RibbonLoadBalancerClient implements LoadBalancerClient {
...//省略代码
@Override
    public ServiceInstance choose(String serviceId) {
        Server server = getServer(serviceId);
        if (server == null) {
            return null;
        }
        return new RibbonServer(serviceId, server, isSecure(server, serviceId),
                serverIntrospector(serviceId).getMetadata(server));
    }
protected Server getServer(String serviceId) {
        return getServer(getLoadBalancer(serviceId));
   }
protected Server getServer(ILoadBalancer loadBalancer) {
        if (loadBalancer == null) {
            return null;
        }
        return loadBalancer.chooseServer("default"); // TODO: better handling of key
    }
protected ILoadBalancer getLoadBalancer(String serviceId) {
        return this.clientFactory.getLoadBalancer(serviceId);
    }
    ...//省略代码

在RibbonLoadBalancerClient的源码中，其中choose()方法是选择具体服务实例的一个方法。该方法通过getServer()方法去获取实例，经过源码跟踪，最终交给了ILoadBalancer类去选择服务实例。

ILoadBalancer在ribbon-loadbalancer的jar包下,它是定义了实现软件负载均衡的一个接口，它需要一组可供选择的服务注册列表信息，以及根据特定方法去选择服务，它的源码如下 ：

public interface ILoadBalancer {
    public void addServers(List<Server> newServers);
    public Server chooseServer(Object key);
    public void markServerDown(Server server);
    public List<Server> getReachableServers();
    public List<Server> getAllServers();
}

其中，addServers()方法是添加一个Server集合；chooseServer()方法是根据key去获取Server；markServerDown()方法用来标记某个服务下线；getReachableServers()获取可用的Server集合；getAllServers()获取所有的Server集合。

DynamicServerListLoadBalancer

DynamicServerListLoadBalancer实现了ILoadBalancer接口，是负载均衡的真正实现类，
它的继承类为BaseLoadBalancer，它的实现类为DynamicServerListLoadBalancer，这三者之间的关系如下：

构造器

public DynamicServerListLoadBalancer(IClientConfig clientConfig, IRule rule, IPing ping,
                                         ServerList<T> serverList, ServerListFilter<T> filter,
                                         ServerListUpdater serverListUpdater) {
        super(clientConfig, rule, ping);
        this.serverListImpl = serverList;
        this.filter = filter;
        this.serverListUpdater = serverListUpdater;
        if (filter instanceof AbstractServerListFilter) {
            ((AbstractServerListFilter) filter).setLoadBalancerStats(getLoadBalancerStats());
        }
        restOfInit(clientConfig);
    }
    public DynamicServerListLoadBalancer() {
        super();
    }

查看上述三个类的源码，可用发现，配置以下信息，IClientConfig、IRule、IPing、ServerList、ServerListFilter和ILoadBalancer，查看BaseLoadBalancer类，它默认的情况下，实现了以下配置：

public class BaseLoadBalancer extends AbstractLoadBalancer implements
        PrimeConnections.PrimeConnectionListener, IClientConfigAware {
    private static Logger logger = LoggerFactory
            .getLogger(BaseLoadBalancer.class);
    private final static IRule DEFAULT_RULE = new RoundRobinRule();
    private final static SerialPingStrategy DEFAULT_PING_STRATEGY = new SerialPingStrategy();
    private static final String DEFAULT_NAME = "default";
    private static final String PREFIX = "LoadBalancer_";
    protected IRule rule = DEFAULT_RULE;
    protected IPingStrategy pingStrategy = DEFAULT_PING_STRATEGY;
    protected IPing ping = null;
    @Monitor(name = PREFIX + "AllServerList", type = DataSourceType.INFORMATIONAL)
    protected volatile List<Server> allServerList = Collections
            .synchronizedList(new ArrayList<Server>());
    @Monitor(name = PREFIX + "UpServerList", type = DataSourceType.INFORMATIONAL)
    protected volatile List<Server> upServerList = Collections
            .synchronizedList(new ArrayList<Server>());
    protected ReadWriteLock allServerLock = new ReentrantReadWriteLock();
    protected ReadWriteLock upServerLock = new ReentrantReadWriteLock();
    protected String name = DEFAULT_NAME;
    protected Timer lbTimer = null;
    protected int pingIntervalSeconds = 10;
    protected int maxTotalPingTimeSeconds = 5;
    protected Comparator<Server> serverComparator = new ServerComparator();
    protected AtomicBoolean pingInProgress = new AtomicBoolean(false);
    protected LoadBalancerStats lbStats;
    private volatile Counter counter = Monitors.newCounter("LoadBalancer_ChooseServer");
    private PrimeConnections primeConnections;
    private volatile boolean enablePrimingConnections = false;
    private IClientConfig config;
    private List<ServerListChangeListener> changeListeners = new CopyOnWriteArrayList<ServerListChangeListener>();
    private List<ServerStatusChangeListener> serverStatusListeners = new CopyOnWriteArrayList<ServerStatusChangeListener>();
    /**
     * Default constructor which sets name as "default", sets null ping, and
     * {@link RoundRobinRule} as the rule.
     * <p>
     * This constructor is mainly used by {@link ClientFactory}. Calling this
     * constructor must be followed by calling {@link #init()} or
     * {@link #initWithNiwsConfig(IClientConfig)} to complete initialization.
     * This constructor is provided for reflection. When constructing
     * programatically, it is recommended to use other constructors.
     */
    public BaseLoadBalancer() {
        this.name = DEFAULT_NAME;
        this.ping = null;
        setRule(DEFAULT_RULE);
        setupPingTask();
        lbStats = new LoadBalancerStats(DEFAULT_NAME);
    }

IClientConfig ribbonClientConfig: DefaultClientConfigImpl配置
IRule ribbonRule: RoundRobinRule 路由策略
IPing ribbonPing: DummyPing
ServerList ribbonServerList: ConfigurationBasedServerList
ServerListFilter ribbonServerListFilter: ZonePreferenceServerListFilter
ILoadBalancer ribbonLoadBalancer: ZoneAwareLoadBalancer
IClientConfig 用于对客户端或者负载均衡的配置，它的默认实现类为DefaultClientConfigImpl。

IRule

用于复杂均衡的策略，它有三个方法，其中choose()是根据key 来获取server,setLoadBalancer()和getLoadBalancer()是用来设置和获取ILoadBalancer的，它的源码如下：

public interface IRule{
    public Server choose(Object key);
    public void setLoadBalancer(ILoadBalancer lb);
    public ILoadBalancer getLoadBalancer();    
}

IRule有很多默认的实现类，这些实现类根据不同的算法和逻辑来处理负载均衡。Ribbon实现的IRule有一下。在大多数情况下，这些默认的实现类是可以满足需求的，如果有特性的需求，可以自己实现。

BestAvailableRule 选择最小请求数

ClientConfigEnabledRoundRobinRule 轮询

RandomRule 随机选择一个server

RoundRobinRule 轮询选择server
原子类递增计数器，对服务器个数取模，取到返回，否则最多取10次

public class RoundRobinRule extends AbstractLoadBalancerRule {
    private AtomicInteger nextServerCyclicCounter;
    private static final boolean AVAILABLE_ONLY_SERVERS = true;
    private static final boolean ALL_SERVERS = false;
    private static Logger log = LoggerFactory.getLogger(RoundRobinRule.class);
    public RoundRobinRule() {
        nextServerCyclicCounter = new AtomicInteger(0);
    }
    public RoundRobinRule(ILoadBalancer lb) {
        this();
        setLoadBalancer(lb);
    }
    public Server choose(ILoadBalancer lb, Object key) {
        if (lb == null) {
            log.warn("no load balancer");
            return null;
        }
        Server server = null;
        int count = 0;
        while (server == null && count++ < 10) {
            List<Server> reachableServers = lb.getReachableServers();
            List<Server> allServers = lb.getAllServers();
            int upCount = reachableServers.size();
            int serverCount = allServers.size();
            if ((upCount == 0) || (serverCount == 0)) {
                log.warn("No up servers available from load balancer: " + lb);
                return null;
            }
            int nextServerIndex = incrementAndGetModulo(serverCount);
            server = allServers.get(nextServerIndex);
            if (server == null) {
                /* Transient. */
                Thread.yield();
                continue;
            }
            if (server.isAlive() && (server.isReadyToServe())) {
                return (server);
            }
            // Next.
            server = null;
        }
        if (count >= 10) {
            log.warn("No available alive servers after 10 tries from load balancer: "
                    + lb);
        }
        return server;
    }
    /**
     * Inspired by the implementation of {@link AtomicInteger#incrementAndGet()}.
     *
     * @param modulo The modulo to bound the value of the counter.
     * @return The next value.
     */
    private int incrementAndGetModulo(int modulo) {
        for (;;) {
            int current = nextServerCyclicCounter.get();
            int next = (current + 1) % modulo;
            if (nextServerCyclicCounter.compareAndSet(current, next))
                return next;
        }
    }
    @Override
    public Server choose(Object key) {
        return choose(getLoadBalancer(), key);
    }
    @Override
    public void initWithNiwsConfig(IClientConfig clientConfig) {
    }
}

RetryRule 根据轮询的方式重试

WeightedResponseTimeRule 根据响应时间去分配一个weight ，weight越低，被选择的可能性就越低

ZoneAvoidanceRule 根据server的zone区域和可用性来轮询选择

IPing

是用来想server发生”ping”，来判断该server是否有响应，从而判断该server是否可用。它有一个isAlive()方法，它的源码如下：

public interface IPing {
    public boolean isAlive(Server server);
}

IPing的实现类有PingUrl、PingConstant、NoOpPing、DummyPing和NIWSDiscoveryPing。它门之间的关系如下：

PingUrl 真实的去ping 某个url，判断其是否alive
PingConstant 固定返回某服务是否可用，默认返回true，即可用

public class PingConstant implements IPing {
        boolean constant = true;
        public void setConstant(String constantStr) {
                constant = (constantStr != null) && (constantStr.toLowerCase().equals("true"));
        }
        public void setConstant(boolean constant) {
                this.constant = constant;
        }
        public boolean getConstant() {
                return constant;
        }
        public boolean isAlive(Server server) {
                return constant;
        }
}

NoOpPing 不去ping,直接返回true,即可用。

public class NoOpPing implements IPing {
    @Override
    public boolean isAlive(Server server) {
        return true;
    }
}

DummyPing 直接返回true，并实现了initWithNiwsConfig方法。

public class DummyPing extends AbstractLoadBalancerPing {
    public DummyPing() {
    }
    public boolean isAlive(Server server) {
        return true;
    }
    @Override
    public void initWithNiwsConfig(IClientConfig clientConfig) {
    }
}

NIWSDiscoveryPing，根据DiscoveryEnabledServer的InstanceInfo的InstanceStatus去判断，如果为InstanceStatus.UP，则为可用，否则不可用。

ServerList

是定义获取所有的server的注册列表信息的接口，它的代码如下：

public interface ServerList<T extends Server> {
    public List<T> getInitialListOfServers();
    public List<T> getUpdatedListOfServers();   
}

ServerListFilter接口，定于了可根据配置去过滤或者根据特性动态获取符合条件的server列表的方法，代码如下：

public interface ServerListFilter<T extends Server> {
    public List<T> getFilteredListOfServers(List<T> servers);
}

初始化配置

阅读DynamicServerListLoadBalancer的源码，DynamicServerListLoadBalancer的构造函数中有个initWithNiwsConfig()方法。在改方法中，经过一系列的初始化配置，最终执行了restOfInit()方法。其代码如下：

public DynamicServerListLoadBalancer(IClientConfig clientConfig) {
        initWithNiwsConfig(clientConfig);
    }
    @Override
    public void initWithNiwsConfig(IClientConfig clientConfig) {
        try {
            super.initWithNiwsConfig(clientConfig);
            String niwsServerListClassName = clientConfig.getPropertyAsString(
                    CommonClientConfigKey.NIWSServerListClassName,
                    DefaultClientConfigImpl.DEFAULT_SEVER_LIST_CLASS);
            ServerList<T> niwsServerListImpl = (ServerList<T>) ClientFactory
                    .instantiateInstanceWithClientConfig(niwsServerListClassName, clientConfig);
            this.serverListImpl = niwsServerListImpl;
            if (niwsServerListImpl instanceof AbstractServerList) {
                AbstractServerListFilter<T> niwsFilter = ((AbstractServerList) niwsServerListImpl)
                        .getFilterImpl(clientConfig);
                niwsFilter.setLoadBalancerStats(getLoadBalancerStats());
                this.filter = niwsFilter;
            }
            String serverListUpdaterClassName = clientConfig.getPropertyAsString(
                    CommonClientConfigKey.ServerListUpdaterClassName,
                    DefaultClientConfigImpl.DEFAULT_SERVER_LIST_UPDATER_CLASS
            );
            this.serverListUpdater = (ServerListUpdater) ClientFactory
                    .instantiateInstanceWithClientConfig(serverListUpdaterClassName, clientConfig);
            restOfInit(clientConfig);
        } catch (Exception e) {
            throw new RuntimeException(
                    "Exception while initializing NIWSDiscoveryLoadBalancer:"
                            + clientConfig.getClientName()
                            + ", niwsClientConfig:" + clientConfig, e);
        }
    }

在restOfInit()方法上，有一个 updateListOfServers()的方法，该方法是用来获取所有的ServerList的。

void restOfInit(IClientConfig clientConfig) {
        boolean primeConnection = this.isEnablePrimingConnections();
        // turn this off to avoid duplicated asynchronous priming done in BaseLoadBalancer.setServerList()
        this.setEnablePrimingConnections(false);
        enableAndInitLearnNewServersFeature();
        updateListOfServers();
        if (primeConnection && this.getPrimeConnections() != null) {
            this.getPrimeConnections()
                    .primeConnections(getReachableServers());
        }
        this.setEnablePrimingConnections(primeConnection);
        LOGGER.info("DynamicServerListLoadBalancer for client {} initialized: {}", clientConfig.getClientName(), this.toString());
    }

进一步跟踪updateListOfServers()方法的源码，最终由serverListImpl.getUpdatedListOfServers()获取所有的服务列表的，代码如下：

@VisibleForTesting
    public void updateListOfServers() {
        List<T> servers = new ArrayList<T>();
        if (serverListImpl != null) {
            servers = serverListImpl.getUpdatedListOfServers();
            LOGGER.debug("List of Servers for {} obtained from Discovery client: {}",
                    getIdentifier(), servers);
            if (filter != null) {
                servers = filter.getFilteredListOfServers(servers);
                LOGGER.debug("Filtered List of Servers for {} obtained from Discovery client: {}",
                        getIdentifier(), servers);
            }
        }
        updateAllServerList(servers);
    }

而serverListImpl是ServerList接口的具体实现类。跟踪代码，ServerList的实现类为DiscoveryEnabledNIWSServerList，在ribbon-eureka.jar的com.netflix.niws.loadbalancer下。其中DiscoveryEnabledNIWSServerList有 getInitialListOfServers()和getUpdatedListOfServers()方法，具体代码如下：

@Override
    public List<DiscoveryEnabledServer> getInitialListOfServers(){
        return obtainServersViaDiscovery();
    }
    @Override
    public List<DiscoveryEnabledServer> getUpdatedListOfServers(){
        return obtainServersViaDiscovery();
    }

继续跟踪源码，obtainServersViaDiscovery（），是根据eurekaClientProvider.get()来回去EurekaClient，再根据EurekaClient来获取注册列表信息，代码如下：

private List<DiscoveryEnabledServer> obtainServersViaDiscovery() {
        List<DiscoveryEnabledServer> serverList = new ArrayList<DiscoveryEnabledServer>();
        if (eurekaClientProvider == null || eurekaClientProvider.get() == null) {
            logger.warn("EurekaClient has not been initialized yet, returning an empty list");
            return new ArrayList<DiscoveryEnabledServer>();
        }
        EurekaClient eurekaClient = eurekaClientProvider.get();
        if (vipAddresses!=null){
            for (String vipAddress : vipAddresses.split(",")) {
                // if targetRegion is null, it will be interpreted as the same region of client
                List<InstanceInfo> listOfInstanceInfo = eurekaClient.getInstancesByVipAddress(vipAddress, isSecure, targetRegion);
                for (InstanceInfo ii : listOfInstanceInfo) {
                    if (ii.getStatus().equals(InstanceStatus.UP)) {
                        if(shouldUseOverridePort){
                            if(logger.isDebugEnabled()){
                                logger.debug("Overriding port on client name: " + clientName + " to " + overridePort);
                            }
                            // copy is necessary since the InstanceInfo builder just uses the original reference,
                            // and we don't want to corrupt the global eureka copy of the object which may be
                            // used by other clients in our system
                            InstanceInfo copy = new InstanceInfo(ii);
                            if(isSecure){
                                ii = new InstanceInfo.Builder(copy).setSecurePort(overridePort).build();
                            }else{
                                ii = new InstanceInfo.Builder(copy).setPort(overridePort).build();
                            }
                        }
                        DiscoveryEnabledServer des = new DiscoveryEnabledServer(ii, isSecure, shouldUseIpAddr);
                        des.setZone(DiscoveryClient.getZone(ii));
                        serverList.add(des);
                    }
                }
                if (serverList.size()>0 && prioritizeVipAddressBasedServers){
                    break; // if the current vipAddress has servers, we dont use subsequent vipAddress based servers
                }
            }
        }
        return serverList;
    }

其中eurekaClientProvider的实现类是LegacyEurekaClientProvider，它是一个获取eurekaClient类，通过静态的方法去获取eurekaClient，其代码如下：

class LegacyEurekaClientProvider implements Provider<EurekaClient> {
    private volatile EurekaClient eurekaClient;
    @Override
    public synchronized EurekaClient get() {
        if (eurekaClient == null) {
            eurekaClient = DiscoveryManager.getInstance().getDiscoveryClient();
        }
        return eurekaClient;
    }
}

EurekaClient的实现类为DiscoveryClient，在之前已经分析了它具有服务注册、获取服务注册列表等的全部功能。

public class DiscoveryClient implements EurekaClient {

由此可见，负载均衡器是从EurekaClient获取服务信息，并根据IRule去路由，并且根据IPing去判断服务的可用性。

那么现在还有个问题，负载均衡器多久一次去获取一次从Eureka Client获取注册信息呢。

获取服务

在BaseLoadBalancer类下，BaseLoadBalancer的构造函数，该构造函数开启了一个PingTask任务，代码如下：

public BaseLoadBalancer(String name, IRule rule, LoadBalancerStats stats,
            IPing ping, IPingStrategy pingStrategy) {
        ...//代码省略
        setupPingTask();
         ...//代码省略
    }

setupPingTask()的具体代码逻辑，它开启了ShutdownEnabledTimer执行PingTask任务，在默认情况下pingIntervalSeconds为10，即每10秒钟，想EurekaClient发送一次”ping”。

void setupPingTask() {
        if (canSkipPing()) {
            return;
        }
        if (lbTimer != null) {
            lbTimer.cancel();
        }
        lbTimer = new ShutdownEnabledTimer("NFLoadBalancer-PingTimer-" + name,
                true);
        lbTimer.schedule(new PingTask(), 0, pingIntervalSeconds * 1000);
        forceQuickPing();
    }

PingTask源码，即new一个Pinger对象，并执行runPinger()方法。

class PingTask extends TimerTask {
        public void run() {
            try {
                new Pinger(pingStrategy).runPinger();
            } catch (Exception e) {
                logger.error("LoadBalancer [{}]: Error pinging", name, e);
            }
        }
    }

查看Pinger的runPinger()方法，最终根据 pingerStrategy.pingServers(ping, allServers)来获取服务的可用性，如果该返回结果，如之前相同，则不去向EurekaClient获取注册列表，如果不同则通知ServerStatusChangeListener或者changeListeners发生了改变，进行更新或者重新拉取。

public void runPinger() throws Exception {
            if (!pingInProgress.compareAndSet(false, true)) { 
                return; // Ping in progress - nothing to do
            }
            // we are "in" - we get to Ping
            Server[] allServers = null;
            boolean[] results = null;
            Lock allLock = null;
            Lock upLock = null;
            try {
                /*
                 * The readLock should be free unless an addServer operation is
                 * going on...
                 */
                allLock = allServerLock.readLock();
                allLock.lock();
                allServers = allServerList.toArray(new Server[allServerList.size()]);
                allLock.unlock();
                int numCandidates = allServers.length;
                results = pingerStrategy.pingServers(ping, allServers);
                final List<Server> newUpList = new ArrayList<Server>();
                final List<Server> changedServers = new ArrayList<Server>();
                for (int i = 0; i < numCandidates; i++) {
                    boolean isAlive = results[i];
                    Server svr = allServers[i];
                    boolean oldIsAlive = svr.isAlive();
                    svr.setAlive(isAlive);
                    if (oldIsAlive != isAlive) {
                        changedServers.add(svr);
                        logger.debug("LoadBalancer [{}]:  Server [{}] status changed to {}", 
                            name, svr.getId(), (isAlive ? "ALIVE" : "DEAD"));
                    }
                    if (isAlive) {
                        newUpList.add(svr);
                    }
                }
                upLock = upServerLock.writeLock();
                upLock.lock();
                upServerList = newUpList;
                upLock.unlock();
                notifyServerStatusChangeListener(changedServers);
            } finally {
                pingInProgress.set(false);
            }
        }

由此可见，LoadBalancerClient是在初始化的时候，会向Eureka回去服务注册列表，并且向通过10s一次向EurekaClient发送“ping”，来判断服务的可用性，如果服务的可用性发生了改变或者服务数量和之前的不一致，则更新或者重新拉取。LoadBalancerClient有了这些服务注册列表，就可以根据具体的IRule来进行负载均衡。

RestTemplate是如何和Ribbon结合的

最后，回答问题的本质，为什么在RestTemplate加一个@LoadBalance注解就可可以开启负载均衡呢？

@LoadBalanced
    RestTemplate restTemplate() {
        return new RestTemplate();
    }

搜索@LoadBalanced有哪些类用到了LoadBalanced有哪些类用到了， 发现LoadBalancerAutoConfiguration类，即LoadBalancer自动配置类。

@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {
@LoadBalanced
    @Autowired(required = false)
    private List<RestTemplate> restTemplates = Collections.emptyList();
}
    @Bean
    public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
            final List<RestTemplateCustomizer> customizers) {
        return new SmartInitializingSingleton() {
            @Override
            public void afterSingletonsInstantiated() {
                for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
                    for (RestTemplateCustomizer customizer : customizers) {
                        customizer.customize(restTemplate);
                    }
                }
            }
        };
    }
    @Configuration
    @ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
    static class LoadBalancerInterceptorConfig {
        @Bean
        public LoadBalancerInterceptor ribbonInterceptor(
                LoadBalancerClient loadBalancerClient,
                LoadBalancerRequestFactory requestFactory) {
            return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
        }
        @Bean
        @ConditionalOnMissingBean
        public RestTemplateCustomizer restTemplateCustomizer(
                final LoadBalancerInterceptor loadBalancerInterceptor) {
            return new RestTemplateCustomizer() {
                @Override
                public void customize(RestTemplate restTemplate) {
                    List<ClientHttpRequestInterceptor> list = new ArrayList<>(
                            restTemplate.getInterceptors());
                    list.add(loadBalancerInterceptor);
                    restTemplate.setInterceptors(list);
                }
            };
        }
    }
}

在该类中，首先维护了一个被@LoadBalanced修饰的RestTemplate对象的List，在初始化的过程中，通过调用customizer.customize(restTemplate)方法来给RestTemplate增加拦截器LoadBalancerInterceptor。

而LoadBalancerInterceptor，用于实时拦截，在LoadBalancerInterceptor这里实现来负载均衡。LoadBalancerInterceptor的拦截方法如下：

@Override
    public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
            final ClientHttpRequestExecution execution) throws IOException {
        final URI originalUri = request.getURI();
        String serviceName = originalUri.getHost();
        Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
        return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution));
    }

总结

综上所述，Ribbon的负载均衡，主要通过LoadBalancerClient来实现的，而LoadBalancerClient具体交给了ILoadBalancer来处理，ILoadBalancer通过配置IRule、IPing等信息，并向EurekaClient获取注册列表的信息，并默认10秒一次向EurekaClient发送“ping”,进而检查是否更新服务列表，最后，得到注册列表后，ILoadBalancer根据IRule的策略进行负载均衡。

而RestTemplate 被@LoadBalance注解后，能过用负载均衡，主要是维护了一个被@LoadBalance注解的RestTemplate列表，并给列表中的RestTemplate添加拦截器，进而交给负载均衡器去处理。