Added SpanProcessor OnEnding callback (#6367)

Co-authored-by: jack-berg <34418638+jack-berg@users.noreply.github.com>

sdk/trace/src/main/java/io/opentelemetry/sdk/trace/MultiSpanProcessor.java

@@ -7,6 +7,7 @@ package io.opentelemetry.sdk.trace;
 
 import io.opentelemetry.context.Context;
 import io.opentelemetry.sdk.common.CompletableResultCode;
+import io.opentelemetry.sdk.trace.internal.ExtendedSpanProcessor;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
@@ -16,8 +17,9 @@ import java.util.concurrent.atomic.AtomicBoolean;
  * Implementation of the {@code SpanProcessor} that simply forwards all received events to a list of
  * {@code SpanProcessor}s.
  */
-final class MultiSpanProcessor implements SpanProcessor {
+final class MultiSpanProcessor implements ExtendedSpanProcessor {
   private final List<SpanProcessor> spanProcessorsStart;
+  private final List<ExtendedSpanProcessor> spanProcessorsEnding;
   private final List<SpanProcessor> spanProcessorsEnd;
   private final List<SpanProcessor> spanProcessorsAll;
   private final AtomicBoolean isShutdown = new AtomicBoolean(false);
@@ -58,6 +60,18 @@ final class MultiSpanProcessor implements SpanProcessor {
     return !spanProcessorsEnd.isEmpty();
   }
 
+  @Override
+  public void onEnding(ReadWriteSpan span) {
+    for (ExtendedSpanProcessor spanProcessor : spanProcessorsEnding) {
+      spanProcessor.onEnding(span);
+    }
+  }
+
+  @Override
+  public boolean isOnEndingRequired() {
+    return !spanProcessorsEnding.isEmpty();
+  }
+
   @Override
   public CompletableResultCode shutdown() {
     if (isShutdown.getAndSet(true)) {
@@ -83,10 +97,17 @@ final class MultiSpanProcessor implements SpanProcessor {
     this.spanProcessorsAll = spanProcessors;
     this.spanProcessorsStart = new ArrayList<>(spanProcessorsAll.size());
     this.spanProcessorsEnd = new ArrayList<>(spanProcessorsAll.size());
+    this.spanProcessorsEnding = new ArrayList<>(spanProcessorsAll.size());
     for (SpanProcessor spanProcessor : spanProcessorsAll) {
       if (spanProcessor.isStartRequired()) {
         spanProcessorsStart.add(spanProcessor);
       }
+      if (spanProcessor instanceof ExtendedSpanProcessor) {
+        ExtendedSpanProcessor extendedSpanProcessor = (ExtendedSpanProcessor) spanProcessor;
+        if (extendedSpanProcessor.isOnEndingRequired()) {
+          spanProcessorsEnding.add(extendedSpanProcessor);
+        }
+      }
       if (spanProcessor.isEndRequired()) {
         spanProcessorsEnd.add(spanProcessor);
       }
@@ -98,6 +119,8 @@ final class MultiSpanProcessor implements SpanProcessor {
     return "MultiSpanProcessor{"
         + "spanProcessorsStart="
         + spanProcessorsStart
+        + ", spanProcessorsEnding="
+        + spanProcessorsEnding
         + ", spanProcessorsEnd="
         + spanProcessorsEnd
         + ", spanProcessorsAll="

sdk/trace/src/main/java/io/opentelemetry/sdk/trace/SdkSpan.java

@@ -23,6 +23,7 @@ import io.opentelemetry.sdk.trace.data.EventData;
 import io.opentelemetry.sdk.trace.data.LinkData;
 import io.opentelemetry.sdk.trace.data.SpanData;
 import io.opentelemetry.sdk.trace.data.StatusData;
+import io.opentelemetry.sdk.trace.internal.ExtendedSpanProcessor;
 import io.opentelemetry.sdk.trace.internal.data.ExceptionEventData;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -95,9 +96,24 @@ final class SdkSpan implements ReadWriteSpan {
   @GuardedBy("lock")
   private long endEpochNanos;
 
-  // True if the span is ended.
+  private enum EndState {
+    NOT_ENDED,
+    ENDING,
+    ENDED
+  }
+
   @GuardedBy("lock")
-  private boolean hasEnded;
+  private EndState hasEnded;
+
+  /**
+   * The thread on which {@link #end()} is called and which will be invoking the {@link
+   * SpanProcessor}s. This field is used to ensure that only this thread may modify the span while
+   * it is in state {@link EndState#ENDING} to prevent concurrent updates outside of {@link
+   * ExtendedSpanProcessor#onEnding(ReadWriteSpan)}.
+   */
+  @GuardedBy("lock")
+  @Nullable
+  private Thread spanEndingThread;
 
   private SdkSpan(
       SpanContext context,
@@ -122,7 +138,7 @@ final class SdkSpan implements ReadWriteSpan {
     this.kind = kind;
     this.spanProcessor = spanProcessor;
     this.resource = resource;
-    this.hasEnded = false;
+    this.hasEnded = EndState.NOT_ENDED;
     this.clock = clock;
     this.startEpochNanos = startEpochNanos;
     this.attributes = attributes;
@@ -220,7 +236,7 @@ final class SdkSpan implements ReadWriteSpan {
           status,
           name,
           endEpochNanos,
-          hasEnded);
+          hasEnded == EndState.ENDED);
     }
   }
 
@@ -242,7 +258,7 @@ final class SdkSpan implements ReadWriteSpan {
   @Override
   public boolean hasEnded() {
     synchronized (lock) {
-      return hasEnded;
+      return hasEnded == EndState.ENDED;
     }
   }
 
@@ -288,7 +304,7 @@ final class SdkSpan implements ReadWriteSpan {
   @Override
   public long getLatencyNanos() {
     synchronized (lock) {
-      return (hasEnded ? endEpochNanos : clock.now()) - startEpochNanos;
+      return (hasEnded == EndState.NOT_ENDED ? clock.now() : endEpochNanos) - startEpochNanos;
     }
   }
 
@@ -303,7 +319,7 @@ final class SdkSpan implements ReadWriteSpan {
       return this;
     }
     synchronized (lock) {
-      if (hasEnded) {
+      if (!isModifiableByCurrentThread()) {
         logger.log(Level.FINE, "Calling setAttribute() on an ended Span.");
         return this;
       }
@@ -318,6 +334,12 @@ final class SdkSpan implements ReadWriteSpan {
     return this;
   }
 
+  @GuardedBy("lock")
+  private boolean isModifiableByCurrentThread() {
+    return hasEnded == EndState.NOT_ENDED
+        || (hasEnded == EndState.ENDING && Thread.currentThread() == spanEndingThread);
+  }
+
   @Override
   public ReadWriteSpan addEvent(String name) {
     if (name == null) {
@@ -380,7 +402,7 @@ final class SdkSpan implements ReadWriteSpan {
 
   private void addTimedEvent(EventData timedEvent) {
     synchronized (lock) {
-      if (hasEnded) {
+      if (!isModifiableByCurrentThread()) {
         logger.log(Level.FINE, "Calling addEvent() on an ended Span.");
         return;
       }
@@ -400,7 +422,7 @@ final class SdkSpan implements ReadWriteSpan {
       return this;
     }
     synchronized (lock) {
-      if (hasEnded) {
+      if (!isModifiableByCurrentThread()) {
         logger.log(Level.FINE, "Calling setStatus() on an ended Span.");
         return this;
       } else if (this.status.getStatusCode() == StatusCode.OK) {
@@ -438,7 +460,7 @@ final class SdkSpan implements ReadWriteSpan {
       return this;
     }
     synchronized (lock) {
-      if (hasEnded) {
+      if (!isModifiableByCurrentThread()) {
         logger.log(Level.FINE, "Calling updateName() on an ended Span.");
         return this;
       }
@@ -463,7 +485,7 @@ final class SdkSpan implements ReadWriteSpan {
                 spanLimits.getMaxNumberOfAttributesPerLink(),
                 spanLimits.getMaxAttributeValueLength()));
     synchronized (lock) {
-      if (hasEnded) {
+      if (!isModifiableByCurrentThread()) {
         logger.log(Level.FINE, "Calling addLink() on an ended Span.");
         return this;
       }
@@ -493,12 +515,22 @@ final class SdkSpan implements ReadWriteSpan {
 
   private void endInternal(long endEpochNanos) {
     synchronized (lock) {
-      if (hasEnded) {
-        logger.log(Level.FINE, "Calling end() on an ended Span.");
+      if (hasEnded != EndState.NOT_ENDED) {
+        logger.log(Level.FINE, "Calling end() on an ended or ending Span.");
         return;
       }
       this.endEpochNanos = endEpochNanos;
-      hasEnded = true;
+      spanEndingThread = Thread.currentThread();
+      hasEnded = EndState.ENDING;
+    }
+    if (spanProcessor instanceof ExtendedSpanProcessor) {
+      ExtendedSpanProcessor extendedSpanProcessor = (ExtendedSpanProcessor) spanProcessor;
+      if (extendedSpanProcessor.isOnEndingRequired()) {
+        extendedSpanProcessor.onEnding(this);
+      }
+    }
+    synchronized (lock) {
+      hasEnded = EndState.ENDED;
     }
     if (spanProcessor.isEndRequired()) {
       spanProcessor.onEnd(this);
@@ -508,7 +540,7 @@ final class SdkSpan implements ReadWriteSpan {
   @Override
   public boolean isRecording() {
     synchronized (lock) {
-      return !hasEnded;
+      return hasEnded != EndState.ENDED;
     }
   }
 
@@ -533,7 +565,7 @@ final class SdkSpan implements ReadWriteSpan {
 
     // if the span has ended, then the events are unmodifiable
     // so we can return them directly and save copying all the data.
-    if (hasEnded) {
+    if (hasEnded == EndState.ENDED) {
       return Collections.unmodifiableList(events);
     }
 
@@ -547,7 +579,7 @@ final class SdkSpan implements ReadWriteSpan {
     }
     // if the span has ended, then the attributes are unmodifiable,
     // so we can return them directly and save copying all the data.
-    if (hasEnded) {
+    if (hasEnded == EndState.ENDED) {
       return attributes;
     }
     // otherwise, make a copy of the data into an immutable container.

sdk/trace/src/main/java/io/opentelemetry/sdk/trace/internal/ExtendedSpanProcessor.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.sdk.trace.internal;
+
+import io.opentelemetry.api.trace.Span;
+import io.opentelemetry.sdk.trace.ReadWriteSpan;
+import io.opentelemetry.sdk.trace.ReadableSpan;
+import io.opentelemetry.sdk.trace.SpanProcessor;
+
+/**
+ * Extended {@link SpanProcessor} with experimental APIs.
+ *
+ * <p>This class is internal and is hence not for public use. Its APIs are unstable and can change
+ * at any time.
+ */
+public interface ExtendedSpanProcessor extends SpanProcessor {
+
+  /**
+   * Called when a {@link io.opentelemetry.api.trace.Span} is ended, but before {@link
+   * SpanProcessor#onEnd(ReadableSpan)} is invoked with an immutable variant of this span. This
+   * means that the span will still be mutable. Note that the span will only be modifiable
+   * synchronously from this callback, concurrent modifications from other threads will be
+   * prevented. Only called if {@link Span#isRecording()} returns true.
+   *
+   * <p>This method is called synchronously on the execution thread, should not throw or block the
+   * execution thread.
+   *
+   * @param span the {@code Span} that is just about to be ended.
+   */
+  void onEnding(ReadWriteSpan span);
+
+  /**
+   * Returns {@code true} if this {@link SpanProcessor} requires onEnding events.
+   *
+   * @return {@code true} if this {@link SpanProcessor} requires onEnding events.
+   */
+  boolean isOnEndingRequired();
+}

sdk/trace/src/test/java/io/opentelemetry/sdk/trace/MultiSpanProcessorTest.java

@@ -14,6 +14,7 @@ import static org.mockito.Mockito.when;
 
 import io.opentelemetry.context.Context;
 import io.opentelemetry.sdk.common.CompletableResultCode;
+import io.opentelemetry.sdk.trace.internal.ExtendedSpanProcessor;
 import java.util.Arrays;
 import java.util.Collections;
 import org.junit.jupiter.api.BeforeEach;
@@ -27,18 +28,20 @@ import org.mockito.quality.Strictness;
 @ExtendWith(MockitoExtension.class)
 @MockitoSettings(strictness = Strictness.LENIENT)
 class MultiSpanProcessorTest {
-  @Mock private SpanProcessor spanProcessor1;
-  @Mock private SpanProcessor spanProcessor2;
+  @Mock private ExtendedSpanProcessor spanProcessor1;
+  @Mock private ExtendedSpanProcessor spanProcessor2;
   @Mock private ReadableSpan readableSpan;
   @Mock private ReadWriteSpan readWriteSpan;
 
   @BeforeEach
   void setUp() {
     when(spanProcessor1.isStartRequired()).thenReturn(true);
+    when(spanProcessor1.isOnEndingRequired()).thenReturn(true);
     when(spanProcessor1.isEndRequired()).thenReturn(true);
     when(spanProcessor1.forceFlush()).thenReturn(CompletableResultCode.ofSuccess());
     when(spanProcessor1.shutdown()).thenReturn(CompletableResultCode.ofSuccess());
     when(spanProcessor2.isStartRequired()).thenReturn(true);
+    when(spanProcessor2.isOnEndingRequired()).thenReturn(true);
     when(spanProcessor2.isEndRequired()).thenReturn(true);
     when(spanProcessor2.forceFlush()).thenReturn(CompletableResultCode.ofSuccess());
     when(spanProcessor2.shutdown()).thenReturn(CompletableResultCode.ofSuccess());
@@ -61,12 +64,17 @@ class MultiSpanProcessorTest {
 
   @Test
   void twoSpanProcessor() {
-    SpanProcessor multiSpanProcessor =
+    ExtendedSpanProcessor multiSpanProcessor =
+        (ExtendedSpanProcessor)
             SpanProcessor.composite(Arrays.asList(spanProcessor1, spanProcessor2));
     multiSpanProcessor.onStart(Context.root(), readWriteSpan);
     verify(spanProcessor1).onStart(same(Context.root()), same(readWriteSpan));
     verify(spanProcessor2).onStart(same(Context.root()), same(readWriteSpan));
 
+    multiSpanProcessor.onEnding(readWriteSpan);
+    verify(spanProcessor1).onEnding(same(readWriteSpan));
+    verify(spanProcessor2).onEnding(same(readWriteSpan));
+
     multiSpanProcessor.onEnd(readableSpan);
     verify(spanProcessor1).onEnd(same(readableSpan));
     verify(spanProcessor2).onEnd(same(readableSpan));
@@ -83,8 +91,10 @@ class MultiSpanProcessorTest {
   @Test
   void twoSpanProcessor_DifferentRequirements() {
     when(spanProcessor1.isEndRequired()).thenReturn(false);
+    when(spanProcessor2.isOnEndingRequired()).thenReturn(false);
     when(spanProcessor2.isStartRequired()).thenReturn(false);
-    SpanProcessor multiSpanProcessor =
+    ExtendedSpanProcessor multiSpanProcessor =
+        (ExtendedSpanProcessor)
             SpanProcessor.composite(Arrays.asList(spanProcessor1, spanProcessor2));
 
     assertThat(multiSpanProcessor.isStartRequired()).isTrue();
@@ -94,6 +104,10 @@ class MultiSpanProcessorTest {
     verify(spanProcessor1).onStart(same(Context.root()), same(readWriteSpan));
     verify(spanProcessor2, times(0)).onStart(any(Context.class), any(ReadWriteSpan.class));
 
+    multiSpanProcessor.onEnding(readWriteSpan);
+    verify(spanProcessor1).onEnding(same(readWriteSpan));
+    verify(spanProcessor2, times(0)).onEnding(any(ReadWriteSpan.class));
+
     multiSpanProcessor.onEnd(readableSpan);
     verify(spanProcessor1, times(0)).onEnd(any(ReadableSpan.class));
     verify(spanProcessor2).onEnd(same(readableSpan));
@@ -117,6 +131,7 @@ class MultiSpanProcessorTest {
         .hasToString(
             "MultiSpanProcessor{"
                 + "spanProcessorsStart=[spanProcessor1, spanProcessor1], "
+                + "spanProcessorsEnding=[spanProcessor1, spanProcessor1], "
                 + "spanProcessorsEnd=[spanProcessor1, spanProcessor1], "
                 + "spanProcessorsAll=[spanProcessor1, spanProcessor1]}");
   }

sdk/trace/src/test/java/io/opentelemetry/sdk/trace/SdkSpanTest.java

@@ -18,6 +18,8 @@ import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatCode;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.same;
+import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
@@ -42,6 +44,7 @@ import io.opentelemetry.sdk.trace.data.EventData;
 import io.opentelemetry.sdk.trace.data.LinkData;
 import io.opentelemetry.sdk.trace.data.SpanData;
 import io.opentelemetry.sdk.trace.data.StatusData;
+import io.opentelemetry.sdk.trace.internal.ExtendedSpanProcessor;
 import io.opentelemetry.sdk.trace.internal.data.ExceptionEventData;
 import java.io.PrintWriter;
 import java.io.StringWriter;
@@ -58,6 +61,8 @@ import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicLong;
 import java.util.stream.IntStream;
 import javax.annotation.Nullable;
 import org.junit.jupiter.api.BeforeEach;
@@ -90,7 +95,7 @@ class SdkSpanTest {
   private final Map<AttributeKey, Object> attributes = new HashMap<>();
   private Attributes expectedAttributes;
   private final LinkData link = LinkData.create(spanContext);
-  @Mock private SpanProcessor spanProcessor;
+  @Mock private ExtendedSpanProcessor spanProcessor;
 
   private TestClock testClock;
 
@@ -107,6 +112,7 @@ class SdkSpanTest {
     expectedAttributes = builder.build();
     testClock = TestClock.create(Instant.ofEpochSecond(0, START_EPOCH_NANOS));
     when(spanProcessor.isStartRequired()).thenReturn(true);
+    when(spanProcessor.isOnEndingRequired()).thenReturn(true);
     when(spanProcessor.isEndRequired()).thenReturn(true);
   }
 
@@ -140,6 +146,92 @@ class SdkSpanTest {
     assertThat(span.hasEnded()).isTrue();
   }
 
+  @Test
+  void onEnding_spanStillMutable() {
+    SdkSpan span = createTestSpan(SpanKind.INTERNAL);
+
+    AttributeKey<String> dummyAttrib = AttributeKey.stringKey("processor_foo");
+
+    AtomicBoolean endedStateInProcessor = new AtomicBoolean();
+    doAnswer(
+            invocation -> {
+              ReadWriteSpan sp = invocation.getArgument(0, ReadWriteSpan.class);
+              assertThat(sp.hasEnded()).isFalse();
+              sp.end(); // should have no effect, nested end should be detected
+              endedStateInProcessor.set(sp.hasEnded());
+              sp.setAttribute(dummyAttrib, "bar");
+              return null;
+            })
+        .when(spanProcessor)
+        .onEnding(any());
+
+    span.end();
+    verify(spanProcessor).onEnding(same(span));
+    assertThat(span.hasEnded()).isTrue();
+    assertThat(endedStateInProcessor.get()).isFalse();
+    assertThat(span.getAttribute(dummyAttrib)).isEqualTo("bar");
+  }
+
+  @Test
+  void onEnding_concurrentModificationsPrevented() {
+    SdkSpan span = createTestSpan(SpanKind.INTERNAL);
+
+    AttributeKey<String> syncAttrib = AttributeKey.stringKey("sync_foo");
+    AttributeKey<String> concurrentAttrib = AttributeKey.stringKey("concurrent_foo");
+
+    doAnswer(
+            invocation -> {
+              ReadWriteSpan sp = invocation.getArgument(0, ReadWriteSpan.class);
+
+              Thread concurrent =
+                  new Thread(
+                      () -> {
+                        sp.setAttribute(concurrentAttrib, "concurrent_bar");
+                      });
+              concurrent.start();
+              concurrent.join();
+
+              sp.setAttribute(syncAttrib, "sync_bar");
+
+              return null;
+            })
+        .when(spanProcessor)
+        .onEnding(any());
+
+    span.end();
+    verify(spanProcessor).onEnding(same(span));
+    assertThat(span.getAttribute(concurrentAttrib)).isNull();
+    assertThat(span.getAttribute(syncAttrib)).isEqualTo("sync_bar");
+  }
+
+  @Test
+  void onEnding_latencyPinned() {
+    SdkSpan span = createTestSpan(SpanKind.INTERNAL);
+
+    AtomicLong spanLatencyInProcessor = new AtomicLong();
+    doAnswer(
+            invocation -> {
+              ReadWriteSpan sp = invocation.getArgument(0, ReadWriteSpan.class);
+
+              testClock.advance(Duration.ofSeconds(100));
+              spanLatencyInProcessor.set(sp.getLatencyNanos());
+              return null;
+            })
+        .when(spanProcessor)
+        .onEnding(any());
+
+    testClock.advance(Duration.ofSeconds(1));
+    long expectedDuration = testClock.now() - START_EPOCH_NANOS;
+
+    assertThat(span.getLatencyNanos()).isEqualTo(expectedDuration);
+
+    span.end();
+    verify(spanProcessor).onEnding(same(span));
+    assertThat(span.hasEnded()).isTrue();
+    assertThat(span.getLatencyNanos()).isEqualTo(expectedDuration);
+    assertThat(spanLatencyInProcessor.get()).isEqualTo(expectedDuration);
+  }
+
   @Test
   void toSpanData_ActiveSpan() {
     SdkSpan span = createTestSpan(SpanKind.INTERNAL);

sdk/trace/src/test/java/io/opentelemetry/sdk/trace/internal/ExtendedSpanProcessorUsageTest.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.sdk.trace.internal;
+
+import static io.opentelemetry.sdk.testing.assertj.OpenTelemetryAssertions.assertThat;
+
+import io.opentelemetry.api.common.AttributeKey;
+import io.opentelemetry.api.trace.Span;
+import io.opentelemetry.api.trace.Tracer;
+import io.opentelemetry.context.Context;
+import io.opentelemetry.sdk.testing.exporter.InMemorySpanExporter;
+import io.opentelemetry.sdk.trace.ReadWriteSpan;
+import io.opentelemetry.sdk.trace.ReadableSpan;
+import io.opentelemetry.sdk.trace.SdkTracerProvider;
+import io.opentelemetry.sdk.trace.export.SimpleSpanProcessor;
+import org.junit.jupiter.api.Test;
+
+/** Demonstrating usage of {@link ExtendedSpanProcessor}. */
+class ExtendedSpanProcessorUsageTest {
+
+  private static final AttributeKey<String> FOO_KEY = AttributeKey.stringKey("foo");
+  private static final AttributeKey<String> BAR_KEY = AttributeKey.stringKey("bar");
+
+  private static class CopyFooToBarProcessor implements ExtendedSpanProcessor {
+
+    @Override
+    public void onStart(Context parentContext, ReadWriteSpan span) {}
+
+    @Override
+    public boolean isStartRequired() {
+      return false;
+    }
+
+    @Override
+    public void onEnd(ReadableSpan span) {}
+
+    @Override
+    public boolean isEndRequired() {
+      return false;
+    }
+
+    @Override
+    public void onEnding(ReadWriteSpan span) {
+      String val = span.getAttribute(FOO_KEY);
+      span.setAttribute(BAR_KEY, val);
+    }
+
+    @Override
+    public boolean isOnEndingRequired() {
+      return true;
+    }
+  }
+
+  @Test
+  void extendedSpanProcessorUsage() {
+    InMemorySpanExporter exporter = InMemorySpanExporter.create();
+
+    try (SdkTracerProvider tracerProvider =
+        SdkTracerProvider.builder()
+            .addSpanProcessor(SimpleSpanProcessor.create(exporter))
+            .addSpanProcessor(new CopyFooToBarProcessor())
+            .build()) {
+
+      Tracer tracer = tracerProvider.get("dummy-tracer");
+      Span span = tracer.spanBuilder("my-span").startSpan();
+
+      span.setAttribute(FOO_KEY, "Hello!");
+
+      span.end();
+
+      assertThat(exporter.getFinishedSpanItems())
+          .hasSize(1)
+          .first()
+          .satisfies(
+              spanData -> {
+                assertThat(spanData.getAttributes())
+                    .containsEntry(FOO_KEY, "Hello!")
+                    .containsEntry(BAR_KEY, "Hello!");
+              });
+    }
+  }
+}