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:

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();  //Slf4jCurrentTraceContext  final CurrentTraceContext currentTraceContext;  final Tracer tracer;  final HttpServerHandler
    
      handler;  //TraceContext的数据提取器  final TraceContext.Extractor
     
       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 component      this.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 sleuth    Tracing tracing(@Value("${spring.zipkin.service.name:${spring.application.name:default}}") String serviceName,            Propagation.Factory factory,            CurrentTraceContext currentTraceContext,            Reporter
    
      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：

@Override  public 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 request    TraceContext context = (TraceContext) request.getAttribute(TraceContext.class.getName());    if (context != null) {      // A forwarded request might end up on another thread, so make sure it is scoped      Scope 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 context    request.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 later        servlet.handleAsync(handler, httpRequest, httpResponse, span);      } else { // we have a synchronous response, so we can finish the span        handler.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)        @ConditionalOnMissingBean        public CurrentTraceContext slf4jSpanLogger() {            return Slf4jCurrentTraceContext.create();        }               ...     }

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

public static final class Default extends CurrentTraceContext {    static final ThreadLocal
    
      DEFAULT = new ThreadLocal<>();    // Inheritable as Brave 3's ThreadLocalServerClientAndLocalSpanState was inheritable    static final InheritableThreadLocal
     
       INHERITABLE = new InheritableThreadLocal<>();    final ThreadLocal
      
        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.     *     * 
       
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
        
          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scope  public Scope maybeScope(@Nullable TraceContext currentSpan) {    //获取当前TraceContext    TraceContext currentScope = get();    //如果传入的TraceContext为空，且当前TraceContext为空返回空scope    if (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