Avoid NPE when TraceContext has deferred sampling

Closes gh-33093

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/tracing/prometheus/PrometheusExemplarsAutoConfiguration.java

@@ -80,7 +80,11 @@ public class PrometheusExemplarsAutoConfiguration {
 		@Override
 		public boolean isSampled() {
 			Span currentSpan = currentSpan();
-			return currentSpan != null && currentSpan.context().sampled();
+			if (currentSpan == null) {
+				return false;
+			}
+			Boolean sampled = currentSpan.context().sampled();
+			return sampled != null && sampled;
 		}
 
 		private Span currentSpan() {

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/tracing/prometheus/LazyTracingSpanContextSupplierTests.java

@@ -0,0 +1,151 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.autoconfigure.tracing.prometheus;
+
+import io.micrometer.tracing.Span;
+import io.micrometer.tracing.TraceContext;
+import io.micrometer.tracing.Tracer;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.BeansException;
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.boot.actuate.autoconfigure.tracing.prometheus.PrometheusExemplarsAutoConfiguration.LazyTracingSpanContextSupplier;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link LazyTracingSpanContextSupplier}.
+ *
+ * @author Andy Wilkinson
+ */
+class LazyTracingSpanContextSupplierTests {
+
+	private final Tracer tracer = mock(Tracer.class);
+
+	private final ObjectProvider<Tracer> objectProvider = new ObjectProvider<>() {
+
+		@Override
+		public Tracer getObject() throws BeansException {
+			return LazyTracingSpanContextSupplierTests.this.tracer;
+		}
+
+		@Override
+		public Tracer getObject(Object... args) throws BeansException {
+			return LazyTracingSpanContextSupplierTests.this.tracer;
+		}
+
+		@Override
+		public Tracer getIfAvailable() throws BeansException {
+			return LazyTracingSpanContextSupplierTests.this.tracer;
+		}
+
+		@Override
+		public Tracer getIfUnique() throws BeansException {
+			return LazyTracingSpanContextSupplierTests.this.tracer;
+		}
+
+	};
+
+	private final LazyTracingSpanContextSupplier spanContextSupplier = new LazyTracingSpanContextSupplier(
+			this.objectProvider);
+
+	@Test
+	void whenCurrentSpanIsNullThenSpanIdIsNull() {
+		assertThat(this.spanContextSupplier.getSpanId()).isNull();
+	}
+
+	@Test
+	void whenCurrentSpanIsNullThenTraceIdIsNull() {
+		assertThat(this.spanContextSupplier.getTraceId()).isNull();
+	}
+
+	@Test
+	void whenCurrentSpanIsNullThenSampledIsFalse() {
+		assertThat(this.spanContextSupplier.isSampled()).isFalse();
+	}
+
+	@Test
+	void whenCurrentSpanHasSpanIdThenSpanIdIsFromSpan() {
+		Span span = mock(Span.class);
+		given(this.tracer.currentSpan()).willReturn(span);
+		TraceContext traceContext = mock(TraceContext.class);
+		given(traceContext.spanId()).willReturn("span-id");
+		given(span.context()).willReturn(traceContext);
+		assertThat(this.spanContextSupplier.getSpanId()).isEqualTo("span-id");
+	}
+
+	@Test
+	void whenCurrentSpanHasTraceIdThenTraceIdIsFromSpan() {
+		Span span = mock(Span.class);
+		given(this.tracer.currentSpan()).willReturn(span);
+		TraceContext traceContext = mock(TraceContext.class);
+		given(traceContext.traceId()).willReturn("trace-id");
+		given(span.context()).willReturn(traceContext);
+		assertThat(this.spanContextSupplier.getTraceId()).isEqualTo("trace-id");
+	}
+
+	@Test
+	void whenCurrentSpanHasNoSpanIdThenSpanIdIsNull() {
+		Span span = mock(Span.class);
+		given(this.tracer.currentSpan()).willReturn(span);
+		TraceContext traceContext = mock(TraceContext.class);
+		given(span.context()).willReturn(traceContext);
+		assertThat(this.spanContextSupplier.getSpanId()).isNull();
+	}
+
+	@Test
+	void whenCurrentSpanHasNoTraceIdThenTraceIdIsFNull() {
+		Span span = mock(Span.class);
+		given(this.tracer.currentSpan()).willReturn(span);
+		TraceContext traceContext = mock(TraceContext.class);
+		given(span.context()).willReturn(traceContext);
+		assertThat(this.spanContextSupplier.getTraceId()).isNull();
+	}
+
+	@Test
+	void whenCurrentSpanIsSampledThenSampledIsTrue() {
+		Span span = mock(Span.class);
+		given(this.tracer.currentSpan()).willReturn(span);
+		TraceContext traceContext = mock(TraceContext.class);
+		given(traceContext.sampled()).willReturn(true);
+		given(span.context()).willReturn(traceContext);
+		assertThat(this.spanContextSupplier.isSampled()).isTrue();
+	}
+
+	@Test
+	void whenCurrentSpanIsNotSampledThenSampledIsFalse() {
+		Span span = mock(Span.class);
+		given(this.tracer.currentSpan()).willReturn(span);
+		TraceContext traceContext = mock(TraceContext.class);
+		given(traceContext.sampled()).willReturn(false);
+		given(span.context()).willReturn(traceContext);
+		assertThat(this.spanContextSupplier.isSampled()).isFalse();
+	}
+
+	@Test
+	void whenCurrentSpanHasDeferredSamplingThenSampledIsFalse() {
+		Span span = mock(Span.class);
+		given(this.tracer.currentSpan()).willReturn(span);
+		TraceContext traceContext = mock(TraceContext.class);
+		given(traceContext.sampled()).willReturn(null);
+		given(span.context()).willReturn(traceContext);
+		assertThat(this.spanContextSupplier.isSampled()).isFalse();
+	}
+
+}