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上一篇博文中，我们了解了Brave框架的基本使用，并且分析了跟Tracer相关的部分源代码。这篇博文我们接着看看Tracing的初始化及相关类的源代码

public class TraceDemo {      public static void main(String[] args) { 	Sender sender = OkHttpSender.create("http://localhost:9411/api/v2/spans"); 	AsyncReporter asyncReporter = AsyncReporter.builder(sender) 		.closeTimeout(500, TimeUnit.MILLISECONDS) 		.build(SpanBytesEncoder.JSON_V2);  	Tracing tracing = Tracing.newBuilder() 		.localServiceName("tracer-demo") 		.spanReporter(asyncReporter) 		.propagationFactory(ExtraFieldPropagation.newFactory(B3Propagation.FACTORY, "user-name")) 		.currentTraceContext(ThreadContextCurrentTraceContext.create()) 		.build(); 	Tracer tracer = tracing.tracer(); 	// ...     } } 

Brave中各个组件创建大量使用的builder设计模式，Tacing也不例外，先来看下Tracing.Builder

Tracing.Builder

public static final class Tracing.Builder {     String localServiceName;     Endpoint localEndpoint;     Reporter<zipkin2.Span> reporter;     Clock clock;     Sampler sampler = Sampler.ALWAYS_SAMPLE;     CurrentTraceContext currentTraceContext = CurrentTraceContext.Default.inheritable();     boolean traceId128Bit = false;     boolean supportsJoin = true;     Propagation.Factory propagationFactory = Propagation.Factory.B3;          public Tracing build() {       if (clock == null) clock = Platform.get();       if (localEndpoint == null) {         localEndpoint = Platform.get().localEndpoint();         if (localServiceName != null) {           localEndpoint = localEndpoint.toBuilder().serviceName(localServiceName).build();         }       }       if (reporter == null) reporter = Platform.get();       return new Default(this);     }      Builder() {     } } 

Tracing中依赖的几个重要类

• Endpoint - IP，端口和应用服务名等信息
• Sampler - 采样器，根据traceId来判断是否一条trace需要被采样，即上报到zipkin
• TraceContext - 包含TraceId，SpanId，是否采样等数据
• CurrentTraceContext - 是一个辅助类，可以用于获得当前线程的TraceContext
• Propagation - 是一个可以向数据携带的对象carrier上注入（inject）和提取（extract）数据的接口
• Propagation.Factory - Propagation的工厂类

前面TraceDemo例子中，我们初始化Tracing时设置了localServiceName，spanReporter，propagationFactory，currentTraceContext 其中spanReporter为AsyncReporter我们上一篇已经分析过其源代码了，在build方法中可以看到，其默认实现是Platform，默认会将Span信息用logger进行输出，而不是上报到zipkin中

  @Override public void report(zipkin2.Span span) {     if (!logger.isLoggable(Level.INFO)) return;     if (span == null) throw new NullPointerException("span == null");     logger.info(span.toString());   } 

Sampler

采样器，根据traceId来判断是否一条trace需要被采样，即上报到zipkin

public abstract class Sampler {    public static final Sampler ALWAYS_SAMPLE = new Sampler() {     @Override public boolean isSampled(long traceId) {       return true;     }      @Override public String toString() {       return "AlwaysSample";     }   };    public static final Sampler NEVER_SAMPLE = new Sampler() {     @Override public boolean isSampled(long traceId) {       return false;     }      @Override public String toString() {       return "NeverSample";     }   };    /** Returns true if the trace ID should be measured. */   public abstract boolean isSampled(long traceId);    /**    * Returns a sampler, given a rate expressed as a percentage.    *    * <p>The sampler returned is good for low volumes of traffic (<100K requests), as it is precise.    * If you have high volumes of traffic, consider {@link BoundarySampler}.    *    * @param rate minimum sample rate is 0.01, or 1% of traces    */   public static Sampler create(float rate) {     return CountingSampler.create(rate);   } } 

Sampler.ALWAYS_SAMPLE 永远需要被采样 Sampler.NEVER_SAMPLE 永远不采样

Sampler还有一个实现类 CountingSampler可以指定采样率，如CountingSampler.create(0.5f)则对50%的请求数据进行采样，里面用到了一个算法，这里不展开分析了。

TraceContext

包含TraceId，SpanId，是否采样等数据

在Tracer的newRootContext方法中有这样一段代码，通过newBuilder来构建TraceContext对象

  TraceContext newRootContext(SamplingFlags samplingFlags, List<Object> extra) {     long nextId = Platform.get().randomLong();     Boolean sampled = samplingFlags.sampled();     if (sampled == null) sampled = sampler.isSampled(nextId);     return TraceContext.newBuilder()         .sampled(sampled)         .traceIdHigh(traceId128Bit ? Platform.get().nextTraceIdHigh() : 0L).traceId(nextId)         .spanId(nextId)         .debug(samplingFlags.debug())         .extra(extra).build();   } 

TraceContext中有以下一些属性

• traceIdHigh - 唯一标识trace的16字节id，即128-bit
• traceId - 唯一标识trace的8字节id
• parentId - 父级Span的spanId
• spanId - 在某个trace中唯一标识span的8字节id
• shared - 如果为true，则表明需要从其他tracer上共享span信息
• extra - 在某个trace中相关的额外数据集

还有继承自SamplingFlags的两个属性

• sampled - 是否采样
• debug - 是否为调试，如果为true时，就算sampled为false，也表明该trace需要采样（即可以覆盖sampled的值）

TraceContext中还定义了两个接口Injector，Extractor

  public interface Injector<C> {     void inject(TraceContext traceContext, C carrier);   }    public interface Extractor<C> {     TraceContextOrSamplingFlags extract(C carrier);   } 

• Injector - 用于将TraceContext中的各种数据注入到carrier中，这里的carrier一般在RPC中指的是类似于Http Headers的可以携带额外信息的对象
• Extractor - 用于在carrier中提取TraceContext相关信息或者采样标记信息TraceContextOrSamplingFlags

TraceContextOrSamplingFlags

TraceContextOrSamplingFlags是三种数据的联合类型，即TraceContext，TraceIdContext，SamplingFlags，官方文档上说

• 当有traceId和spanId时，需用create(TraceContext)来创建
• 当只有spanId时，需用create(TraceIdContext)来创建
• 其他情况下，需用create(SamplingFlags)来创建 TraceContextOrSamplingFlags里的代码比较简单，这里不展开分析了

CurrentTraceContext

CurrentTraceContext是一个辅助类，可以用于获得当前线程的TraceContext，它的默认实现类是CurrentTraceContext.Default

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;      /** @deprecated prefer {@link #create()} as it isn't inheritable, so can't leak contexts. */     @Deprecated     public Default() {       this(INHERITABLE);     }      /** Uses a non-inheritable static thread local */     public static CurrentTraceContext create() {       return new Default(DEFAULT);     }      /**      * 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();     } } 

CurrentTraceContext.Default提供了两个静态方法，即create()和inheritable() 当使用create方法创建时，local对象为ThreadLocal<TraceContext>类型 当使用inheritable方法创建时，local对象为InheritableThreadLocal<TraceContext>类型 ThreadLocal可以理解为JVM为同一个线程开辟的一个共享内存空间，在同一个线程中不同方法调用，可以从该空间中取出放入的对象 而当使用InheritableThreadLocal获取线程绑定对象时，当前线程没有，则向当前线程的父线程的共享内存中获取

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

CurrentTraceContext.Scope

  public abstract Scope newScope(@Nullable TraceContext currentSpan);    /** A span remains in the scope it was bound to until close is called. */   public interface Scope extends Closeable {     /** No exceptions are thrown when unbinding a span scope. */     @Override void close();   } 

CurrentTraceContext中还定义了一个Scope接口，该接口继承自Closeable接口 自JDK7开始，凡是实现了Closeable接口的对象，只要在try语句中定义的，当finally执行的时候，JVM都会主动调用其close方法来回收资源，所以CurrentTraceContext中就提供了一个newScope方法，我们在代码里可以这样来用

try (Scope scope = newScope(invocationContext)) {   // do somthing } 

再来看看CurrentTraceContext.Default中是如何实现newScope的

@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();     } 

首先会将当前线程的TraceContext赋值给previous变量，然后设置新的TraceContext到当前线程，当Scope的close方法调用时，会还原previous的值到当前线程中

用两个嵌套的try代码块来演示下上面做法的意义

TraceContext traceContext1; TraceContext traceContext2; try (Scope scope = newScope(traceContext1)) {   // 1.此处CurrentTraceContext.get()能获得traceContext1   try (Scope scope = newScope(traceContext2)) {   // 2.此处CurrentTraceContext.get()能获得traceContext2   }   // 3.此处CurrentTraceContext.get()能获得traceContext1 } 

1. 在进入内层try代码块前，通过CurrentTraceContext.get()获取到的traceContext1
2. 在进入内层try代码块后，通过CurrentTraceContext.get()获取到的traceContext2
3. 在运行完内层try代码块，通过CurrentTraceContext.get()获取到的traceContext1

这种处理方式确实比较灵活优雅，不过对使用的人来说，也有点过于隐晦，不知道JDK7新特性的同学刚开始看到这种用法可能会一脸茫然

当然这种用法必须得让使用的人将scope对象new在try语句中，每个人都能按照这种约定的规则来写，容易出错，所以CurrentTraceContext中提供了几个对Callable，Runnable的封装方法wrap方法

  /** Wraps the input so that it executes with the same context as now. */   public <C> Callable<C> wrap(Callable<C> task) {     final TraceContext invocationContext = get();     class CurrentTraceContextCallable implements Callable<C> {       @Override public C call() throws Exception {         try (Scope scope = newScope(invocationContext)) {           return task.call();         }       }     }     return new CurrentTraceContextCallable();   }    /** Wraps the input so that it executes with the same context as now. */   public Runnable wrap(Runnable task) {     final TraceContext invocationContext = get();     class CurrentTraceContextRunnable implements Runnable {       @Override public void run() {         try (Scope scope = newScope(invocationContext)) {           task.run();         }       }     }     return new CurrentTraceContextRunnable();   } 

CurrentTraceContext还对Executor，及ExecuteService提供了包装方法

  /**    * Decorates the input such that the {@link #get() current trace context} at the time a task is    * scheduled is made current when the task is executed.    */   public Executor executor(Executor delegate) {     class CurrentTraceContextExecutor implements Executor {       @Override public void execute(Runnable task) {         delegate.execute(CurrentTraceContext.this.wrap(task));       }     }     return new CurrentTraceContextExecutor();   }    /**    * Decorates the input such that the {@link #get() current trace context} at the time a task is    * scheduled is made current when the task is executed.    */   public ExecutorService executorService(ExecutorService delegate) {     class CurrentTraceContextExecutorService extends brave.internal.WrappingExecutorService {        @Override protected ExecutorService delegate() {         return delegate;       }        @Override protected <C> Callable<C> wrap(Callable<C> task) {         return CurrentTraceContext.this.wrap(task);       }        @Override protected Runnable wrap(Runnable task) {         return CurrentTraceContext.this.wrap(task);       }     }     return new CurrentTraceContextExecutorService();   } 

这几个方法都用的是装饰器设计模式，属于比较常用的设计模式，此处就不再展开分析了

ThreadContextCurrentTraceContext

可以看到TraceDemo中，我们设置的CurrentTraceContext是ThreadContextCurrentTraceContext.create()

ThreadContextCurrentTraceContext是为log4j2封装的，是brave-context-log4j2包中的一个类，在ThreadContext中放置traceId和spanId两个属性，我们可以在log4j2的配置文件中配置日志打印的pattern，使用占位符%X{traceId}和%X{spanId}，让每行日志都能打印当前的traceId和spanId

zipkin-learning\Chapter1\servlet25\src\main\resources\log4j2.properties

appender.console.layout.pattern = %d{ABSOLUTE} [%X{traceId}/%X{spanId}] %-5p [%t] %C{2} - %m%n 

pom.xml中需要添加日志相关的jar

    <brave.version>4.9.1</brave.version>     <log4j.version>2.8.2</log4j.version>      <dependency>       <groupId>io.zipkin.brave</groupId>       <artifactId>brave-context-log4j2</artifactId>       <version>${brave.version}</version>     </dependency>      <dependency>       <groupId>org.apache.logging.log4j</groupId>       <artifactId>log4j-core</artifactId>       <version>${log4j.version}</version>     </dependency>     <dependency>       <groupId>org.apache.logging.log4j</groupId>       <artifactId>log4j-jul</artifactId>       <version>${log4j.version}</version>     </dependency>     <dependency>       <groupId>org.apache.logging.log4j</groupId>       <artifactId>log4j-jcl</artifactId>       <version>${log4j.version}</version>     </dependency>     <dependency>       <groupId>org.apache.logging.log4j</groupId>       <artifactId>log4j-slf4j-impl</artifactId>       <version>${log4j.version}</version>     </dependency> 

在Chapter1的例子中，如果你观察frontend和backend的控制台，会有如下输出0cabad9917e767ab为traceId，0cabad9917e767ab和e96a226ce75d30b4为spanId

10:11:05,731 [0cabad9917e767ab/0cabad9917e767ab] INFO  [qtp1441410416-17] servlet.FrontendServlet - frontend receive request 

10:11:05,820 [0cabad9917e767ab/e96a226ce75d30b4] INFO  [qtp1441410416-15] servlet.BackendServlet - backend receive request 

public final class ThreadContextCurrentTraceContext extends CurrentTraceContext {   public static ThreadContextCurrentTraceContext create() {     return create(CurrentTraceContext.Default.inheritable());   }    public static ThreadContextCurrentTraceContext create(CurrentTraceContext delegate) {     return new ThreadContextCurrentTraceContext(delegate);   }    final CurrentTraceContext delegate;    ThreadContextCurrentTraceContext(CurrentTraceContext delegate) {     if (delegate == null) throw new NullPointerException("delegate == null");     this.delegate = delegate;   }    @Override public TraceContext get() {     return delegate.get();   }    @Override public Scope newScope(@Nullable TraceContext currentSpan) {     final String previousTraceId = ThreadContext.get("traceId");     final String previousSpanId = ThreadContext.get("spanId");      if (currentSpan != null) {       ThreadContext.put("traceId", currentSpan.traceIdString());       ThreadContext.put("spanId", HexCodec.toLowerHex(currentSpan.spanId()));     } else {       ThreadContext.remove("traceId");       ThreadContext.remove("spanId");     }      Scope scope = delegate.newScope(currentSpan);     class ThreadContextCurrentTraceContextScope implements Scope {       @Override public void close() {         scope.close();         ThreadContext.put("traceId", previousTraceId);         ThreadContext.put("spanId", previousSpanId);       }     }     return new ThreadContextCurrentTraceContextScope();   } } 

ThreadContextCurrentTraceContext继承了CurrentTraceContext，覆盖了其newScope方法，提取了currentSpan中的traceId和spanId放到log4j2的上下文对象ThreadContext中

在https://github.com/openzipkin/brave/tree/master/context中还能找到对slf4j和log4j的支持 brave-context-slf4j中的brave.context.slf4j.MDCCurrentTraceContext brave-context-log4j12中的brave.context.log4j12.MDCCurrentTraceContext 代码都比较类似，这里不细说了

Propagation

Propagation，英文翻译传播器，是一个可以向数据携带的对象carrier上注入（inject）和提取（extract）数据的接口。 对于Http协议来说，通常carrier就是指http request对象，它的http headers可以携带trace信息，一般来说http的客户端会在headers里注入（inject）trace信息，而服务端则会在headers提取（extract）trace信息 Propagation.Setter和Propagation.Getter可以在carrier中设置和获取值 另外还有injector和extractor方法分别返回TraceContext.Injector和TraceContext.Extractor

  interface Setter<C, K> {     void put(C carrier, K key, String value);   }   interface Getter<C, K> {     @Nullable String get(C carrier, K key);   }    <C> TraceContext.Injector<C> injector(Setter<C, K> setter);   <C> TraceContext.Extractor<C> extractor(Getter<C, K> getter); 

Propagation中还有一个工厂类Propagation.Factory，有一个工厂方法create，通过KeyFactory来创建Propagation对象

abstract class Factory {     public static final Factory B3 = B3Propagation.FACTORY;      public boolean supportsJoin() {       return false;     }      public boolean requires128BitTraceId() {       return false;     }      public abstract <K> Propagation<K> create(KeyFactory<K> keyFactory); }  interface KeyFactory<K> {     KeyFactory<String> STRING = name -> name;      K create(String name); }  

Propagation的默认实现是B3Propagation B3Propagation用下面这些http headers来传播trace信息

• X-B3-TraceId - 128位或者64位的traceId，被编码成32位和16位的小写16进制形式
• X-B3-SpanId - 64位的spanId，被编码成16位的小写16进制形式
• X-B3-ParentSpanId - 64位的父级spanId，被编码成16位的小写16进制形式
• X-B3-Sampled - 1代表采样，0代表不采样，如果没有这个key，则留给header接受端，即服务端自行判断
• X-B3-Flags - debug，如果为1代表采样

  @Override public <C> TraceContext.Injector<C> injector(Setter<C, K> setter) {     if (setter == null) throw new NullPointerException("setter == null");     return new B3Injector<>(this, setter);   }    static final class B3Injector<C, K> implements TraceContext.Injector<C> {     final B3Propagation<K> propagation;     final Setter<C, K> setter;      B3Injector(B3Propagation<K> propagation, Setter<C, K> setter) {       this.propagation = propagation;       this.setter = setter;     }      @Override public void inject(TraceContext traceContext, C carrier) {       setter.put(carrier, propagation.traceIdKey, traceContext.traceIdString());       setter.put(carrier, propagation.spanIdKey, toLowerHex(traceContext.spanId()));       if (traceContext.parentId() != null) {         setter.put(carrier, propagation.parentSpanIdKey, toLowerHex(traceContext.parentId()));       }       if (traceContext.debug()) {         setter.put(carrier, propagation.debugKey, "1");       } else if (traceContext.sampled() != null) {         setter.put(carrier, propagation.sampledKey, traceContext.sampled() ? "1" : "0");       }     }   } 

inject方法中很简单，就是利用Setter将trace信息设置在carrier中

  @Override public <C> TraceContext.Extractor<C> extractor(Getter<C, K> getter) {     if (getter == null) throw new NullPointerException("getter == null");     return new B3Extractor(this, getter);   }    static final class B3Extractor<C, K> implements TraceContext.Extractor<C> {     final B3Propagation<K> propagation;     final Getter<C, K> getter;      B3Extractor(B3Propagation<K> propagation, Getter<C, K> getter) {       this.propagation = propagation;       this.getter = getter;     }      @Override public TraceContextOrSamplingFlags extract(C carrier) {       if (carrier == null) throw new NullPointerException("carrier == null");        String traceId = getter.get(carrier, propagation.traceIdKey);       String sampled = getter.get(carrier, propagation.sampledKey);       String debug = getter.get(carrier, propagation.debugKey);       if (traceId == null && sampled == null && debug == null) {         return TraceContextOrSamplingFlags.EMPTY;       }        // Official sampled value is 1, though some old instrumentation send true       Boolean sampledV = sampled != null           ? sampled.equals("1") || sampled.equalsIgnoreCase("true")           : null;       boolean debugV = "1".equals(debug);        String spanId = getter.get(carrier, propagation.spanIdKey);       if (spanId == null) { // return early if there's no span ID         return TraceContextOrSamplingFlags.create(             debugV ? SamplingFlags.DEBUG : SamplingFlags.Builder.build(sampledV)         );       }        TraceContext.Builder result = TraceContext.newBuilder().sampled(sampledV).debug(debugV);       result.traceIdHigh(           traceId.length() == 32 ? lowerHexToUnsignedLong(traceId, 0) : 0       );       result.traceId(lowerHexToUnsignedLong(traceId));       result.spanId(lowerHexToUnsignedLong(spanId));       String parentSpanIdString = getter.get(carrier, propagation.parentSpanIdKey);       if (parentSpanIdString != null) result.parentId(lowerHexToUnsignedLong(parentSpanIdString));       return TraceContextOrSamplingFlags.create(result.build());     }   } 

extract方法则利用Getter从carrier中获取trace信息

在TraceDemo中我们设置的propagationFactory是ExtraFieldPropagation.newFactory(B3Propagation.FACTORY, "user-name")

ExtraFieldPropagation

ExtraFieldPropagation可以用来传输额外的信息

运行Chapter1中的Frontend和Backend服务，在控制台输入

curl http://localhost:8081 --header "user-name: zhangsan" 

可以看到控制台输出了user-name的值zhangsan

Wed Nov 15 11:42:02 GMT+08:00 2017 zhangsan 

  static final class ExtraFieldInjector<C, K> implements Injector<C> {     final Injector<C> delegate;     final Propagation.Setter<C, K> setter;     final Map<String, K> nameToKey;      ExtraFieldInjector(Injector<C> delegate, Setter<C, K> setter, Map<String, K> nameToKey) {       this.delegate = delegate;       this.setter = setter;       this.nameToKey = nameToKey;     }      @Override public void inject(TraceContext traceContext, C carrier) {       for (Object extra : traceContext.extra()) {         if (extra instanceof Extra) {           ((Extra) extra).setAll(carrier, setter, nameToKey);           break;         }       }       delegate.inject(traceContext, carrier);     }   } 

ExtraFieldInjector的inject方法中，将traceContext的extra数据，set到carrier中，这里的Extra对象，其实就是key-value，有One和Many两种，Many时就相当于Map结构 在Extra中setAll方法中，先用extra的name去nameToKey里找，如果没有就不设置，如果找到就调用setter的put方法将值设置到carrier中。

  static final class One extends Extra {     String name, value;      @Override void put(String name, String value) {       this.name = name;       this.value = value;     }      @Override String get(String name) {       return name.equals(this.name) ? value : null;     }      @Override <C, K> void setAll(C carrier, Setter<C, K> setter, Map<String, K> nameToKey) {       K key = nameToKey.get(name);       if (key == null) return;       setter.put(carrier, key, value);     }      @Override public String toString() {       return "ExtraFieldPropagation{" + name + "=" + value + "}";     }   }    static final class Many extends Extra {     final LinkedHashMap<String, String> fields = new LinkedHashMap<>();      @Override void put(String name, String value) {       fields.put(name, value);     }      @Override String get(String name) {       return fields.get(name);     }      @Override <C, K> void setAll(C carrier, Setter<C, K> setter, Map<String, K> nameToKey) {       for (Map.Entry<String, String> field : fields.entrySet()) {         K key = nameToKey.get(field.getKey());         if (key == null) continue;         setter.put(carrier, nameToKey.get(field.getKey()), field.getValue());       }     }      @Override public String toString() {       return "ExtraFieldPropagation" + fields;     }   } 

ExtraFieldExtractor的extract方法中，循环names去carrier里找，然后构造Extra数据放入delegate执行extract方法后的结果中

  static final class ExtraFieldExtractor<C, K> implements Extractor<C> {     final Extractor<C> delegate;     final Propagation.Getter<C, K> getter;     final Map<String, K> names;      ExtraFieldExtractor(Extractor<C> delegate, Getter<C, K> getter, Map<String, K> names) {       this.delegate = delegate;       this.getter = getter;       this.names = names;     }      @Override public TraceContextOrSamplingFlags extract(C carrier) {       TraceContextOrSamplingFlags result = delegate.extract(carrier);        Extra extra = null;       for (Map.Entry<String, K> field : names.entrySet()) {         String maybeValue = getter.get(carrier, field.getValue());         if (maybeValue == null) continue;         if (extra == null) {           extra = new One();         } else if (extra instanceof One) {           One one = (One) extra;           extra = new Many();           extra.put(one.name, one.value);         }         extra.put(field.getKey(), maybeValue);       }       if (extra == null) return result;       return result.toBuilder().addExtra(extra).build();     }   } 

至此，Tracing类相关的源代码已分析的差不多了，后续博文中，我们会继续分析Brave跟各大框架整合的源代码