从Spring Cloud Sleuth切换到Micrometer后，Spring Cloud Gateway中Tracer.getCurrentSpan()为何失效？

核心原因 #

1. 过滤器执行顺序过早
   Spring Cloud Gateway中负责初始化Trace Span的TraceWebFilter默认优先级为Ordered.HIGHEST_PRECEDENCE + 10，如果你的自定义全局过滤器执行顺序早于这个值，此时Trace Span还未被创建，调用Tracer.getCurrentSpan()自然返回null。

2. WebFlux环境的上下文特性
   Micrometer Tracing在WebFlux（Reactor）环境中，Span是绑定在Reactor Context而非ThreadLocal中的。你当前直接在filter方法的主线程调用tracer.currentSpan()，此时Reactor Context还未与当前请求的Span绑定，无法获取到有效Span。

解决方案 #

步骤1：调整过滤器执行顺序

给自定义过滤器添加@Order注解，确保它在Trace过滤器之后执行：

@Component
@RequiredArgsConstructor
@Order(Ordered.HIGHEST_PRECEDENCE + 20) // 优先级低于TraceWebFilter
public class GlobalFilterComponent implements GlobalFilter {
    // ... 原有代码
}

步骤2：正确从Reactor Context中获取Span

修改代码，在Reactor上下文绑定阶段获取Span，示例如下：

@Component
@RequiredArgsConstructor
@Order(Ordered.HIGHEST_PRECEDENCE + 20)
public class GlobalFilterComponent implements GlobalFilter {

    private final Tracer tracer;

    @Override
    public Mono<Void> filter(final ServerWebExchange exchange, final GatewayFilterChain chain) {
        return chain.filter(exchange)
                .transformDeferredContextual((mono, contextView) -> {
                    // 在Reactor上下文绑定后获取当前Span
                    Optional<String> traceId = Optional.ofNullable(tracer.currentSpan())
                            .map(Span::context)
                            .map(TraceContext::traceIdString);
                    
                    return mono.doFinally(signal -> {
                        traceId.ifPresent(id -> exchange.getResponse().getHeaders().add("trace-id", id));
                    });
                });
    }
}

或者用deferContextual简化实现：

@Override
public Mono<Void> filter(final ServerWebExchange exchange, final GatewayFilterChain chain) {
    return chain.filter(exchange)
            .then(Mono.deferContextual(contextView -> {
                Span currentSpan = tracer.currentSpan();
                Optional<String> traceId = Optional.ofNullable(currentSpan)
                        .map(Span::context)
                        .map(TraceContext::traceIdString);
                
                traceId.ifPresent(id -> exchange.getResponse().getHeaders().add("trace-id", id));
                return Mono.empty();
            }));
}

额外检查 #

确认依赖完整性，确保引入以下核心依赖（以Maven为例）：

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-gateway</artifactId>
</dependency>
<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-tracing-bridge-brave</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

内容的提问来源于stack exchange，提问作者sge