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摘要: 原创出处 https://blog.csdn.net/apei830/article/details/78722209 「v墨竹v」欢迎转载,保留摘要,谢谢!


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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类型 当使用inheritable方法创建时,local对象为InheritableThreadLocal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

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

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文章目录
  1. 1. Tracing.Builder
  2. 2. Sampler
  3. 3. TraceContext
  4. 4. TraceContextOrSamplingFlags
  5. 5. CurrentTraceContext
  6. 6. CurrentTraceContext.Scope
  7. 7. ThreadContextCurrentTraceContext
  8. 8. Propagation
  9. 9. ExtraFieldPropagation
  • 666. 彩蛋