一直没弄明白sleuth的tracerContext是如何创建和传递的，闲来无事研究了一下。由于对sleuth的源码不熟悉，准备通过debug brave.Tracer的nextId()方法，查看方法调用栈来找来龙去脉。

首先创建两个service A和B，记作srvA、srvB，在srvA中添加testA controller，sevB中添加testB controller，testA中通过Feign调用testB。

1. 先看当用户通过浏览器调用srvA的时候，srvA是作为server的。

configuration:
TraceWebServletAutoConfiguration==>TracingFilter
TraceHttpAutoConfiguration==>HttpTracing
TraceAutoConfiguration==>Tracing
SleuthLogAutoConfiguration.Slf4jConfiguration==>CurrentTraceContext

配置中，TracingFilter在实例化时需要一个HttpTracing：

  public static Filter create(HttpTracing httpTracing) {return new TracingFilter(httpTracing);}//Servlet运行时类final ServletRuntime servlet = ServletRuntime.get();//Slf4jCurrentTraceContextfinal CurrentTraceContext currentTraceContext;final Tracer tracer;final HttpServerHandler<HttpServletRequest, HttpServletResponse> handler;//TraceContext的数据提取器final TraceContext.Extractor<HttpServletRequest> extractor;TracingFilter(HttpTracing httpTracing) {tracer = httpTracing.tracing().tracer();currentTraceContext = httpTracing.tracing().currentTraceContext();handler = HttpServerHandler.create(httpTracing, ADAPTER);extractor = httpTracing.tracing().propagation().extractor(GETTER);}

HttpTracing Builder模式构造时接收一个Tracing：

    Tracing tracing;//客户端span解析器HttpClientParser clientParser;String serverName;//服务端span解析器HttpServerParser serverParser;HttpSampler clientSampler, serverSampler;Builder(Tracing tracing) {if (tracing == null) throw new NullPointerException("tracing == null");final ErrorParser errorParser = tracing.errorParser();this.tracing = tracing;this.serverName = "";// override to re-use any custom error parser from the tracing componentthis.clientParser = new HttpClientParser() {@Override protected ErrorParser errorParser() {return errorParser;}};this.serverParser = new HttpServerParser() {@Override protected ErrorParser errorParser() {return errorParser;}};this.clientSampler = HttpSampler.TRACE_ID;this.serverSampler(HttpSampler.TRACE_ID);}

Tracing实例化：

    @Bean@ConditionalOnMissingBean// NOTE: stable bean name as might be used outside sleuthTracing tracing(@Value("${spring.zipkin.service.name:${spring.application.name:default}}") String serviceName,Propagation.Factory factory,CurrentTraceContext currentTraceContext,Reporter<zipkin2.Span> reporter,Sampler sampler,ErrorParser errorParser,SleuthProperties sleuthProperties) {return Tracing.newBuilder().sampler(sampler).errorParser(errorParser).localServiceName(serviceName)//ExtraFieldPropagation.Factory.propagationFactory(factory).currentTraceContext(currentTraceContext).spanReporter(adjustedReporter(reporter)).traceId128Bit(sleuthProperties.isTraceId128()).supportsJoin(sleuthProperties.isSupportsJoin()).build();}

下面看TracingFilter的doFilter：

  @Overridepublic void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)throws IOException, ServletException {HttpServletRequest httpRequest = (HttpServletRequest) request;HttpServletResponse httpResponse = servlet.httpResponse(response);// Prevent duplicate spans for the same requestTraceContext context = (TraceContext) request.getAttribute(TraceContext.class.getName());if (context != null) {// A forwarded request might end up on another thread, so make sure it is scopedScope scope = currentTraceContext.maybeScope(context);try {chain.doFilter(request, response);} finally {scope.close();}return;}Span span = handler.handleReceive(extractor, httpRequest);// Add attributes for explicit access to customization or span contextrequest.setAttribute(SpanCustomizer.class.getName(), span.customizer());request.setAttribute(TraceContext.class.getName(), span.context());Throwable error = null;Scope scope = currentTraceContext.newScope(span.context());try {// any downstream code can see Tracer.currentSpan() or use Tracer.currentSpanCustomizer()chain.doFilter(httpRequest, httpResponse);} catch (IOException | ServletException | RuntimeException | Error e) {error = e;throw e;} finally {scope.close();if (servlet.isAsync(httpRequest)) { // we don't have the actual response, handle laterservlet.handleAsync(handler, httpRequest, httpResponse, span);} else { // we have a synchronous response, so we can finish the spanhandler.handleSend(ADAPTER.adaptResponse(httpRequest, httpResponse), error, span);}}}

在SleuthLogAutoConfiguration中如果有slfj的包，则注入CurrentTraceContext：

    @Configuration@ConditionalOnClass(MDC.class)@EnableConfigurationProperties(SleuthSlf4jProperties.class)protected static class Slf4jConfiguration {@Bean@ConditionalOnProperty(value = "spring.sleuth.log.slf4j.enabled", matchIfMissing = true)@ConditionalOnMissingBeanpublic CurrentTraceContext slf4jSpanLogger() {return Slf4jCurrentTraceContext.create();}...}

Slf4jCurrentTraceContext中，delegate就是CurrentTraceContext.Default.inheritable()：

  public static final class Default extends CurrentTraceContext {static final ThreadLocal<TraceContext> DEFAULT = new ThreadLocal<>();// Inheritable as Brave 3's ThreadLocalServerClientAndLocalSpanState was inheritablestatic final InheritableThreadLocal<TraceContext> INHERITABLE = new InheritableThreadLocal<>();final ThreadLocal<TraceContext> local;//静态方法create，local对象为ThreadLocal类型 /** Uses a non-inheritable static thread local */public static CurrentTraceContext create() {return new Default(DEFAULT);}//local对象为InheritableThreadLocal类型//官方文档指出，inheritable方法在线程池的环境中需谨慎使用，可能会取出错误的TraceContext，这样会导致Span等信息会记录并关联到错误的traceId上/*** Uses an inheritable static thread local which allows arbitrary calls to {@link* Thread#start()} to automatically inherit this context. This feature is available as it is was* the default in Brave 3, because some users couldn't control threads in their applications.** <p>This can be a problem in scenarios such as thread pool expansion, leading to data being* recorded in the wrong span, or spans with the wrong parent. If you are impacted by this,* switch to {@link #create()}.*/public static CurrentTraceContext inheritable() {return new Default(INHERITABLE);}Default(ThreadLocal<TraceContext> local) {if (local == null) throw new NullPointerException("local == null");this.local = local;}@Override public TraceContext get() {return local.get();}//替换当前TraceContext，close方法将之前的TraceContext设置回去//Scope接口继承了Closeable接口，在try中使用会自动调用close方法，为了避免用户忘记close方法，还提供了Runnable，Callable，Executor，ExecutorService包装方法@Override public Scope newScope(@Nullable TraceContext currentSpan) {final TraceContext previous = local.get();local.set(currentSpan);class DefaultCurrentTraceContextScope implements Scope {@Override public void close() {local.set(previous);}}return new DefaultCurrentTraceContextScope();}}

Slf4jCurrentTraceContext的delegate使用的就是一个InheritableThreadLocal，InheritableThreadLocal在创建子线程的时候，会将父线程的inheritableThreadLocals继承下来。这样就实现了TraceContext在父子线程中的传递。

看一下CurrentTraceContext的maybeScope：

  //返回一个新的scope，如果当前scope就是传入的scope，返回一个空scopepublic Scope maybeScope(@Nullable TraceContext currentSpan) {//获取当前TraceContextTraceContext currentScope = get();//如果传入的TraceContext为空，且当前TraceContext为空返回空scopeif (currentSpan == null) {if (currentScope == null) return Scope.NOOP;return newScope(null);}return currentSpan.equals(currentScope) ? Scope.NOOP : newScope(currentSpan);}

TracingFilter中HttpServerHandler解析Request：

请输入代码

2.srvA请求到servB时作为Client。

TraceLoadBalancerFeignClient-->LoadBalancerFeignClient-->FeignLoadBalancer-->LazyTracingFeignClient-->Client