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。
- 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();
//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