public static Filter create(HttpTracing httpTracing) {
  return new TracingFilter(httpTracing);
 }

 //Servlet運(yùn)行時類
 final ServletRuntime servlet = ServletRuntime.get();
 //Slf4jCurrentTraceContext
 final CurrentTraceContext currentTraceContext;
 final Tracer tracer;
 final HttpServerHandler<HttpServletRequest, HttpServletResponse> handler;
 //TraceContext的數(shù)據(jù)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);
 }

  Tracing tracing;
  //客戶端span解析器
  HttpClientParser clientParser;
  String serverName;
  //服務(wù)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);
  }

  @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<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();
  }

  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);
   }
  }
 }

 @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();
    }
    
    ...
   }

 public static final class Default extends CurrentTraceContext {
  static final ThreadLocal<TraceContext> DEFAULT = new ThreadLocal<>();
  // Inheritable as Brave 3's ThreadLocalServerClientAndLocalSpanState was inheritable
  static final InheritableThreadLocal<TraceContext> INHERITABLE = new InheritableThreadLocal<>();

  final ThreadLocal<TraceContext> local;

  //靜態(tài)方法create，local對象為ThreadLocal類型 
  /** Uses a non-inheritable static thread local */
  public static CurrentTraceContext create() {
   return new Default(DEFAULT);
  }

  //local對象為InheritableThreadLocal類型
  //官方文檔指出，inheritable方法在線程池的環(huán)境中需謹(jǐn)慎使用，可能會取出錯誤的TraceContext，這樣會導(dǎo)致Span等信息會記錄并關(guān)聯(lián)到錯誤的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();
  }

  //替換當(dāng)前TraceContext，close方法將之前的TraceContext設(shè)置回去
  //Scope接口繼承了Closeable接口，在try中使用會自動調(diào)用close方法，為了避免用戶忘記close方法，還提供了Runnable，Callable，Executor，ExecutorService包裝方法
  @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();
  }
 }

 //返回一個新的scope，如果當(dāng)前scope就是傳入的scope，返回一個空scope
 public Scope maybeScope(@Nullable TraceContext currentSpan) {
  //獲取當(dāng)前TraceContext
  TraceContext currentScope = get();
  //如果傳入的TraceContext為空，且當(dāng)前TraceContext為空返回空scope
  if (currentSpan == null) {
   if (currentScope == null) return Scope.NOOP;
   return newScope(null);
  }
  return currentSpan.equals(currentScope) ? Scope.NOOP : newScope(currentSpan);
 }