解决SpringBoot 3.3.x集成Micrometer和Prometheus，导致项目阻塞，无法启动

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使用SpringBoot 3.3.x集成Micrometer和Prometheus，导致项目无法启动，因为集成的组件特别多，具体什么组件有问题，无法得知。但是，作为一名程序员，基本排查问题的思路，还是需要具备的！

排查问题方向

1.版本不兼容【代码冲突】

2.错误使用

因此，按照这两个方向进行排查，首先，排除 错误使用 剩下就是版本问题，因为是引入micrometer-registry-prometheus才导致项目阻塞，而对于组件，一般有两种解决方案，一种是降低版本【治标不治本】，另一种阅读源码，找出具体代码，然后改掉【治本】

所以，就有以下两种解决方案

1.降低micrometer的版本至1.11.12，但是需要注意的是spring boot 3.5会删除 PrometheusSimpleclientMetricsExportAutoConfiguration 因此，这种方式不推荐

1  <dependencies>
    <dependency>
      <groupId>io.micrometer</groupId>
      <artifactId>micrometer-registry-prometheus</artifactId>
      <version>1.11.12</version>
    </dependency>
  </dependencies>

2.直接阅读源码，找出问题的根源，经过我debug，找出造成阻塞的代码。注意，使用ObjectProvider，会延迟加载，需要被实际调用，才会完成初始化

源码里面有这么一段注释，大致意思是：MeterRegistry依赖于Tracer,Tracer又依赖于MeterRegistry，所以通过延迟加载的方式来打破循环

Since the MeterRegistry can depend on the {@link Tracer} (Exemplars) and the {@link Tracer} can depend on the MeterRegistry (recording metrics),

this {@link SpanContext} breaks the cycle by lazily loading the {@link Tracer}.

 注意：ObjectProvider.getObject() 本身不会直接造成项目阻塞，但如果Bean的获取耗时过长，可能会导致调用该方法的线程被阻塞，此外，如果存在循环依赖，还会造成死锁的情况

@AutoConfiguration(before = PrometheusMetricsExportAutoConfiguration.class,
		after = MicrometerTracingAutoConfiguration.class)
@ConditionalOnBean(Tracer.class)
@ConditionalOnClass({ Tracer.class, SpanContext.class })
public class PrometheusExemplarsAutoConfiguration {

	@Bean
	@ConditionalOnMissingBean
    // 延迟加载
	SpanContext spanContext(ObjectProvider<Tracer> tracerProvider) {
		return new LazyTracingSpanContext(tracerProvider);
	}

	/**
	 * Since the MeterRegistry can depend on the {@link Tracer} (Exemplars) and the
	 * {@link Tracer} can depend on the MeterRegistry (recording metrics), this
	 * {@link SpanContext} breaks the cycle by lazily loading the {@link Tracer}.
	 */
	static class LazyTracingSpanContext implements SpanContext {

		private final SingletonSupplier<Tracer> tracer;

		LazyTracingSpanContext(ObjectProvider<Tracer> tracerProvider) {
			this.tracer = SingletonSupplier.of(tracerProvider::getObject);
		}

		@Override
		public String getCurrentTraceId() {
			Span currentSpan = currentSpan();
			return (currentSpan != null) ? currentSpan.context().traceId() : null;
		}

		@Override
		public String getCurrentSpanId() {
			Span currentSpan = currentSpan();
			return (currentSpan != null) ? currentSpan.context().spanId() : null;
		}

		@Override
		public boolean isCurrentSpanSampled() {
			Span currentSpan = currentSpan();
			if (currentSpan == null) {
				return false;
			}
			Boolean sampled = currentSpan.context().sampled();
			return sampled != null && sampled;
		}

		@Override
		public void markCurrentSpanAsExemplar() {
		}

		private Span currentSpan() {
			return this.tracer.obtain().currentSpan();
		}

	}

}

因此，改为直接从IOC容器，获取Bean，源码改动如下

@AutoConfiguration(before = PrometheusMetricsExportAutoConfiguration.class,
		after = MicrometerTracingAutoConfiguration.class)
@ConditionalOnBean(Tracer.class)
@ConditionalOnClass({ Tracer.class, SpanContext.class })
public class PrometheusExemplarsAutoConfiguration {

	@Bean
	@ConditionalOnMissingBean
	SpanContext spanContext() {
		return new TracingSpanContext();
	}

	/**
	 * Since the MeterRegistry can depend on the {@link Tracer} (Exemplars) and the
	 * {@link Tracer} can depend on the MeterRegistry (recording metrics), this
	 * {@link SpanContext} breaks the cycle by lazily loading the {@link Tracer}.
	 */
	static class TracingSpanContext implements SpanContext {

		@Override
		public String getCurrentTraceId() {
			Span currentSpan = currentSpan();
			return ObjectUtil.isNotNull(currentSpan) ? currentSpan.context().traceId() : null;
		}

		@Override
		public String getCurrentSpanId() {
			Span currentSpan = currentSpan();
			return ObjectUtil.isNotNull(currentSpan) ? currentSpan.context().spanId() : null;
		}

		@Override
		public boolean isCurrentSpanSampled() {
			Span currentSpan = currentSpan();
			if (ObjectUtil.isNull(currentSpan)) {
				return false;
			}
			return currentSpan.context().sampled();
		}

		@Override
		public void markCurrentSpanAsExemplar() {
		}

		private Span currentSpan() {
			try {
				return SpringContextUtil.getBean(Tracer.class).currentSpan();
			}
			catch (Exception e) {
				return null;
			}
		}

	}

}

大功告成

这个问题，可是困扰了我好几周呢，中间可是折腾好久，换好多种实现，其中，不外乎换组件，换架构！终于，硬着头皮解决这个问题啦，哈哈哈~

我是老寇，我们后会有期