spring-cloud-sleuth+zipkin源碼探究

1.1. 前言

  粗略看了下spring cloud sleuth core源碼，發現內容真的有點多，它支持了不少類型的鏈路追蹤，我就找其中一個比較有表明性的深刻剖析下源碼結構和內容

1.2. spring-cloud-sleuth-core源碼解析

1.2.1. 結構

1. 能夠看到源碼中支持的追蹤類型有不少，支持async，hystrix，websocket，rxjava，Spring mvc，servlet，spring restTemplate，feign，zuul等等，這裏我着重探討spring web mvc的鏈路追蹤
2. 打開web包，找到TraceWebAutoConfiguration，這裏配置了主要的初始化類
   

1.2.2. 過濾器註冊

1. 當啓動初始化程序時，跟蹤代碼以下

@Bean
    public FilterRegistrationBean traceWebFilter(TraceFilter traceFilter) {
        FilterRegistrationBean filterRegistrationBean = new FilterRegistrationBean(
                traceFilter);
        filterRegistrationBean.setDispatcherTypes(ASYNC, ERROR, FORWARD, INCLUDE,
                REQUEST);
        filterRegistrationBean.setOrder(TraceFilter.ORDER);
        return filterRegistrationBean;
    }

    @Bean
    @ConditionalOnMissingBean
    public TraceFilter traceFilter(BeanFactory beanFactory,
            SkipPatternProvider skipPatternProvider) {
        return new TraceFilter(beanFactory, skipPatternProvider.skipPattern());
    }

1. 初始化traceFilter，進行過濾器註冊

1.2.3. 攔截器註冊

1. 而後看TraceWebMvcConfigurer類，它會進行攔截器的註冊

@Configuration
class TraceWebMvcConfigurer extends WebMvcConfigurerAdapter {
    @Autowired BeanFactory beanFactory;

    @Bean
    public TraceHandlerInterceptor traceHandlerInterceptor(BeanFactory beanFactory) {
        return new TraceHandlerInterceptor(beanFactory);
    }

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(this.beanFactory.getBean(TraceHandlerInterceptor.class));
    }
}

1. 在TraceHandlerInterceptor類中，preHandle,afterCompletion方法能夠看出，這是對請求進行攔截進行span的包裝

@Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response,
            Object handler) throws Exception {
        String spanName = spanName(handler);
        boolean continueSpan = getRootSpanFromAttribute(request) != null;
        Span span = continueSpan ? getRootSpanFromAttribute(request) : getTracer().createSpan(spanName);
        if (log.isDebugEnabled()) {
            log.debug("Handling span " + span);
        }
        addClassMethodTag(handler, span);
        addClassNameTag(handler, span);
        setSpanInAttribute(request, span);
        if (!continueSpan) {
            setNewSpanCreatedAttribute(request, span);
        }
        return true;
    }

@Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response,
            Object handler, Exception ex) throws Exception {
        if (isErrorControllerRelated(request)) {
            if (log.isDebugEnabled()) {
                log.debug("Skipping closing of a span for error controller processing");
            }
            return;
        }
        Span span = getRootSpanFromAttribute(request);
        if (ex != null) {
            getErrorParser().parseErrorTags(span, ex);
        }
        if (getNewSpanFromAttribute(request) != null) {
            if (log.isDebugEnabled()) {
                log.debug("Closing span " + span);
            }
            Span newSpan = getNewSpanFromAttribute(request);
            getTracer().continueSpan(newSpan);
            getTracer().close(newSpan);
            clearNewSpanCreatedAttribute(request);
        }
    }

1.2.4. zipkin端點提交

1. 這裏首先會初始化HttpZipkinSpanReporter類，，用來進行span端點提交，而後初始化ZipkinSpanListenerspan的監聽器，用來監聽並調用端點提交，以上配置再下圖位置
   

1.2.5. 調用http接口時，進入過濾器

1. 首先進入TraceFilter中的過濾方法doFilter，這裏會作span的建立

private Span createSpan(HttpServletRequest request,
            boolean skip, Span spanFromRequest, String name) {
        if (spanFromRequest != null) {
            if (log.isDebugEnabled()) {
                log.debug("Span has already been created - continuing with the previous one");
            }
            return spanFromRequest;
        }
        //加入調用鏈路ZipkinHttpSpanExtractor，此鏈路在TraceHttpAutoConfiguration中配置實例化，調用鏈尚未時，返回爲空，做爲頭節點
        Span parent = spanExtractor().joinTrace(new HttpServletRequestTextMap(request));
        if (parent != null) {
            if (log.isDebugEnabled()) {
                log.debug("Found a parent span " + parent + " in the request");
            }
            addRequestTagsForParentSpan(request, parent);
            spanFromRequest = parent;
            tracer().continueSpan(spanFromRequest);
            if (parent.isRemote()) {
                parent.logEvent(Span.SERVER_RECV);
            }
            request.setAttribute(TRACE_REQUEST_ATTR, spanFromRequest);
            if (log.isDebugEnabled()) {
                log.debug("Parent span is " + parent + "");
            }
        } else {
            if (skip) {
                spanFromRequest = tracer().createSpan(name, NeverSampler.INSTANCE);
            }
            else {
                String header = request.getHeader(Span.SPAN_FLAGS);
                if (Span.SPAN_SAMPLED.equals(header)) {
                    spanFromRequest = tracer().createSpan(name, new AlwaysSampler());
                } else {
                //建立span節點
                    spanFromRequest = tracer().createSpan(name);
                }
            }
            spanFromRequest.logEvent(Span.SERVER_RECV);
            request.setAttribute(TRACE_REQUEST_ATTR, spanFromRequest);
            if (log.isDebugEnabled()) {
                log.debug("No parent span present - creating a new span");
            }
        }
        return spanFromRequest;
    }

1.2.6. 進入攔截器

1. 在preHandle方法中，對span進行包裝，而後把span放入請求頭header中
2. 最後再DefaultTracer中進行span的關閉和spanReporter的提交