Capture context class loader during async callback registration

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/internal/state/CallbackRegistration.java

@@ -14,6 +14,7 @@
 import java.util.List;
 import java.util.logging.Level;
 import java.util.logging.Logger;
+import javax.annotation.Nullable;
 
 /**
  * A registered callback.
@@ -29,11 +30,13 @@ public final class CallbackRegistration {
   private final Runnable callback;
   private final List<InstrumentDescriptor> instrumentDescriptors;
   private final boolean hasStorages;
+  @Nullable private final ClassLoader contextClassLoader;
 
   private CallbackRegistration(
       List<SdkObservableMeasurement> observableMeasurements, Runnable callback) {
     this.observableMeasurements = observableMeasurements;
     this.callback = callback;
+    this.contextClassLoader = Thread.currentThread().getContextClassLoader();
     this.instrumentDescriptors =
         observableMeasurements.stream()
             .map(SdkObservableMeasurement::getInstrumentDescriptor)
@@ -80,13 +83,18 @@ public void invokeCallback(RegisteredReader reader, long startEpochNanos, long e
     observableMeasurements.forEach(
         observableMeasurement ->
             observableMeasurement.setActiveReader(reader, startEpochNanos, epochNanos));
+    // Restore the context class loader that was active when the callback was registered.
+    Thread currentThread = Thread.currentThread();
+    ClassLoader previousContextClassLoader = currentThread.getContextClassLoader();
+    currentThread.setContextClassLoader(contextClassLoader);
     try {
       callback.run();
     } catch (Throwable e) {
       propagateIfFatal(e);
       throttlingLogger.log(
           Level.WARNING, "An exception occurred invoking callback for " + this + ".", e);
     } finally {
+      currentThread.setContextClassLoader(previousContextClassLoader);
       observableMeasurements.forEach(SdkObservableMeasurement::unsetActiveReader);
     }
   }

sdk/metrics/src/test/java/io/opentelemetry/sdk/metrics/internal/state/CallbackRegistrationTest.java

@@ -30,6 +30,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.atomic.AtomicReference;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -219,6 +220,59 @@ void invokeCallback_NoStorage() {
     assertThat(counter.get()).isEqualTo(0);
   }
 
+  @Test
+  void invokeCallback_RestoresContextClassLoader() {
+    // Simulate the context class loader at registration time
+    ClassLoader registrationClassLoader = new ClassLoader() {};
+    ClassLoader originalClassLoader = Thread.currentThread().getContextClassLoader();
+
+    Thread.currentThread().setContextClassLoader(registrationClassLoader);
+    AtomicReference<ClassLoader> observedClassLoader = new AtomicReference<>();
+    Runnable callback =
+        () -> observedClassLoader.set(Thread.currentThread().getContextClassLoader());
+    CallbackRegistration callbackRegistration =
+        CallbackRegistration.create(Collections.singletonList(measurement2), callback);
+    Thread.currentThread().setContextClassLoader(originalClassLoader);
+
+    // Simulate invocation on a thread with null context class loader (like DaemonThreadFactory)
+    Thread.currentThread().setContextClassLoader(null);
+    callbackRegistration.invokeCallback(registeredReader, 0, 1);
+
+    // Callback should have seen the registration-time classloader
+    assertThat(observedClassLoader.get()).isSameAs(registrationClassLoader);
+
+    // After invocation, the thread's context classloader should be restored to null
+    assertThat(Thread.currentThread().getContextClassLoader()).isNull();
+
+    // Clean up
+    Thread.currentThread().setContextClassLoader(originalClassLoader);
+  }
+
+  @Test
+  void invokeCallback_RestoresContextClassLoaderOnException() {
+    ClassLoader registrationClassLoader = new ClassLoader() {};
+    ClassLoader originalClassLoader = Thread.currentThread().getContextClassLoader();
+
+    Thread.currentThread().setContextClassLoader(registrationClassLoader);
+    Runnable callback =
+        () -> {
+          throw new RuntimeException("Error!");
+        };
+    CallbackRegistration callbackRegistration =
+        CallbackRegistration.create(Collections.singletonList(measurement2), callback);
+    Thread.currentThread().setContextClassLoader(originalClassLoader);
+
+    // Simulate invocation on a thread with null context class loader
+    Thread.currentThread().setContextClassLoader(null);
+    callbackRegistration.invokeCallback(registeredReader, 0, 1);
+
+    // Context classloader should still be restored even after exception
+    assertThat(Thread.currentThread().getContextClassLoader()).isNull();
+
+    // Clean up
+    Thread.currentThread().setContextClassLoader(originalClassLoader);
+  }
+
   @Test
   void invokeCallback_MultipleMeasurements_ThrowsException() {
     Runnable callback =