前面的幾節(jié)我們重點(diǎn)分析了Hystrix的整個(gè)流程以及工作原理汇在，以及基于@HystrixCommand注解使用方式之后的流程分析，這一節(jié)我們主要結(jié)合實(shí)際應(yīng)用Zuul網(wǎng)關(guān)層來進(jìn)一步分析Zuul網(wǎng)關(guān)里面是如何應(yīng)用Hystrix熔斷的脏答！

ZuulFilter過濾器生命周期

這里我們引用Zuul官方的一張過濾器周期圖糕殉，Zuul的核心功能就是filter，這里我們也能直觀的看出殖告，請求下游服務(wù)(Origin Server)是在routing filters這類路由過濾器進(jìn)行阿蝶，而Zuul的Filters過濾器核心執(zhí)行邏輯在ZuulServlet，ZuulServlet繼承HttpServlet作為總的執(zhí)行入口黄绩，內(nèi)部就會(huì)執(zhí)行上述的ZuulFilter過濾器生命周期赡磅，那么下面我們就直接進(jìn)入重點(diǎn)，看看我們的Hystrix是怎么被利用起來的宝与！

EnableZuulProxy入口配置

@EnableCircuitBreaker
@EnableDiscoveryClient
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(ZuulProxyConfiguration.class)
public @interface EnableZuulProxy {
}

當(dāng)我們使用某一個(gè)框架的時(shí)候焚廊，我們關(guān)心最多的一個(gè)問題莫過于它的入口在哪里？從入口進(jìn)行分析無疑會(huì)減少很多工作量习劫，@EnableZuulProxy直接作用在應(yīng)用入口類咆瘟，然后我們看看引入了ZuulProxyConfiguration這個(gè)配置類，那我們接著看看ZuulProxyConfiguration配置

ZuulProxyConfiguration

@Configuration
@Import({ RibbonCommandFactoryConfiguration.RestClientRibbonConfiguration.class,
        RibbonCommandFactoryConfiguration.OkHttpRibbonConfiguration.class,
        RibbonCommandFactoryConfiguration.HttpClientRibbonConfiguration.class })
public class ZuulProxyConfiguration extends ZuulConfiguration {
    ......
    // route filters
    @Bean
    public RibbonRoutingFilter ribbonRoutingFilter(ProxyRequestHelper helper,
            RibbonCommandFactory<?> ribbonCommandFactory) {
        RibbonRoutingFilter filter = new RibbonRoutingFilter(helper, ribbonCommandFactory, this.requestCustomizers);
        return filter;
    }
    ......
}

因?yàn)檫@一節(jié)我們是分析Hystrix如何被使用起來的诽里，所以我們只需要關(guān)心RibbonRoutingFilter這個(gè)route類型的filters袒餐，因?yàn)檫@種filter才會(huì)請求下游服務(wù)

RibbonRoutingFilter

public class RibbonRoutingFilter extends ZuulFilter {
    ......
    //當(dāng)前過濾器是否生效
    @Override
    public boolean shouldFilter() {
        RequestContext ctx = RequestContext.getCurrentContext();
        //配置的routes路由為使用serviceId方式，不是直連的url配置方式
        return (ctx.getRouteHost() == null && ctx.get(SERVICE_ID_KEY) != null
                && ctx.sendZuulResponse());
    }
    //過濾器執(zhí)行方法
    @Override
    public Object run() {
        RequestContext context = RequestContext.getCurrentContext();
        //添加忽略的頭信息,zuul的ignored-headers配置,會(huì)過濾這些頭信息參數(shù)
        this.helper.addIgnoredHeaders();
        try {
            //構(gòu)建Ribbon命令執(zhí)行的上下文，保存相關(guān)參數(shù)的對象
            RibbonCommandContext commandContext = buildCommandContext(context);
            //轉(zhuǎn)發(fā)請求灸眼，調(diào)用下游服務(wù)
            ClientHttpResponse response = forward(commandContext);
            setResponse(response);
            return response;
        }
        catch (ZuulException ex) {
            throw new ZuulRuntimeException(ex);
        }
        catch (Exception ex) {
            throw new ZuulRuntimeException(ex);
        }
    }
    //主要就是封裝參數(shù)卧檐，封裝頭信息、請求參數(shù)信息...
    protected RibbonCommandContext buildCommandContext(RequestContext context) {
        HttpServletRequest request = context.getRequest();
        MultiValueMap<String, String> headers = this.helper
                .buildZuulRequestHeaders(request);
        MultiValueMap<String, String> params = this.helper
                .buildZuulRequestQueryParams(request);
        String verb = getVerb(request);
        InputStream requestEntity = getRequestBody(request);
        if (request.getContentLength() < 0) {
            context.setChunkedRequestBody();
        }
        String serviceId = (String) context.get(SERVICE_ID_KEY);
        Boolean retryable = (Boolean) context.get(RETRYABLE_KEY);
        String uri = this.helper.buildZuulRequestURI(request);
        uri = uri.replace("http://", "/");
        long contentLength = useServlet31 ? request.getContentLengthLong(): request.getContentLength();
        return new RibbonCommandContext(serviceId, verb, uri, retryable, headers, params,
                requestEntity, this.requestCustomizers, contentLength);
    }
    //轉(zhuǎn)發(fā)請求/調(diào)用下游服務(wù)
    protected ClientHttpResponse forward(RibbonCommandContext context) throws Exception {
        Map<String, Object> info = this.helper.debug(context.getMethod(),
                context.getUri(), context.getHeaders(), context.getParams(),
                context.getRequestEntity());
        //封裝RibbonCommand對象焰宣，內(nèi)部繼承HystrixExecutable霉囚，這樣就可以使用Hystrix的功能
        RibbonCommand command = this.ribbonCommandFactory.create(context);
        try {
            //執(zhí)行命令，調(diào)用下游服務(wù)匕积，最終調(diào)用到HystrixCommand#execute()盈罐，
            //這樣實(shí)現(xiàn)類通過實(shí)現(xiàn)Hystrix的run()、getFallback()方法就可以使用Hystrix的熔斷功能了
            ClientHttpResponse response = command.execute();
            this.helper.appendDebug(info, response.getStatusCode().value(),
                    response.getHeaders());
            return response;
        }
        catch (HystrixRuntimeException ex) {
            return handleException(info, ex);
        }
    }
    ......
}

在RibbonRoutingFilter這個(gè)過濾器通過片段中的代碼注釋信息闪唆，我們得知其實(shí)這個(gè)過濾器就做了一件事盅粪，那就是使用Hystrix的功能，把Hystrix引入進(jìn)來悄蕾，這樣就把調(diào)用下游服務(wù)的任務(wù)交給了Hystrix票顾，通過Hystrix來判斷下游是否需要熔斷等情況！

HttpClientRibbonCommandFactory

我們回過頭繼續(xù)看下ZuulProxyConfiguration配置類中通過@Import方式引入了RestClientRibbonConfiguration帆调、OkHttpRibbonConfiguration奠骄、HttpClientRibbonConfiguration，此處我們的應(yīng)用基于httpclient方式贷帮，那我們就只分析下HttpClientRibbonConfiguration的配置戚揭，其他方式大致都是一樣的邏輯

public class HttpClientRibbonCommandFactory extends AbstractRibbonCommandFactory {
    ......
    //創(chuàng)建RibbonCommand
    @Override
    public HttpClientRibbonCommand create(final RibbonCommandContext context) {
        //降級實(shí)現(xiàn)類诱告，默認(rèn)null
        ZuulFallbackProvider zuulFallbackProvider = getFallbackProvider(context.getServiceId());
        //獲取serviceId撵枢，如@FeignClient注解中的value字段
        final String serviceId = context.getServiceId();
        //獲取RibbonLoadBalancingHttpClient客戶端
        //默認(rèn)在Ribbon的RibbonClientConfiguration#HttpClientRibbonConfiguration中注冊
        final RibbonLoadBalancingHttpClient client = this.clientFactory.getClient(
                serviceId, RibbonLoadBalancingHttpClient.class);
        //設(shè)置負(fù)載均衡器ILoadBalancer
        client.setLoadBalancer(this.clientFactory.getLoadBalancer(serviceId));
        //返回封裝了參數(shù)的HttpClientRibbonCommand，內(nèi)部繼承AbstractRibbonCommand
        return new HttpClientRibbonCommand(serviceId, client, context, zuulProperties, zuulFallbackProvider,
                clientFactory.getClientConfig(serviceId));
    }
}

上面HttpClientRibbonCommandFactory中最重要的一件事情精居，獲取Ribbon的RibbonLoadBalancingHttpClient然后設(shè)置負(fù)載均衡器锄禽！最后返回了HttpClientRibbonCommand，其繼承了AbstractRibbonCommand靴姿，內(nèi)部就實(shí)現(xiàn)了Hystrix的run()沃但、getFallback()方法

AbstractRibbonCommand

public abstract class AbstractRibbonCommand<LBC extends AbstractLoadBalancerAwareClient<RQ, RS>, RQ extends ClientRequest, RS extends HttpResponse>
        extends HystrixCommand<ClientHttpResponse> implements RibbonCommand {
    ......
    //HystrixCommand的run方法實(shí)現(xiàn)
    @Override
    protected ClientHttpResponse run() throws Exception {
        final RequestContext context = RequestContext.getCurrentContext();
        //創(chuàng)建一個(gè)請求，此處對應(yīng)RibbonApacheHttpRequest請求
        RQ request = createRequest();
        //負(fù)載均衡客戶端執(zhí)行負(fù)載均衡請求
        RS response = this.client.executeWithLoadBalancer(request, config);
        context.set("ribbonResponse", response);
        //Hystrix執(zhí)行命令超時(shí)佛吓，則顯示關(guān)閉Http請求
        if (this.isResponseTimedOut()) {
            if (response != null) {
                response.close();
            }
        }
        return new RibbonHttpResponse(response);
    }
    //HystrixCommand的getFallback降級方法
    @Override
    protected ClientHttpResponse getFallback() {
        if(zuulFallbackProvider != null) {
            return zuulFallbackProvider.fallbackResponse();
        }
        return super.getFallback();
    }
    ......
}

通過分析我們能得知最終的一個(gè)請求流程為：客戶端發(fā)起請求->HttpServlet-> ZuulServlet->routing filters->Hystrix->Ribbon->返回結(jié)果給客戶端,至此我們分析了Hystrix在實(shí)際應(yīng)用中（Zuul網(wǎng)關(guān)）的一個(gè)大致實(shí)現(xiàn)宵晚，當(dāng)然Zuul的功能也遠(yuǎn)非如此，Zuul的功能十分強(qiáng)大,在后續(xù)的總結(jié)里會(huì)單獨(dú)分析Zuul網(wǎng)關(guān)维雇！