DataDog · mcculls · Jun 14, 2026
@@ -1,12 +1,17 @@
 package datadog.trace.bootstrap.instrumentation.classloading;
 
+import java.util.concurrent.atomic.AtomicBoolean;
+
 /** Provides a way for a single optional observer to be notified before a class is defined. */
 public final class ClassDefining {
+  private static final AtomicBoolean HAS_OBSERVER = new AtomicBoolean();
   private static volatile Observer OBSERVER = (loader, bytecode, offset, length) -> {};
 
   /** Registers the given observer to get notifications about class definitions. */
   public static void observe(Observer observer) {
-    OBSERVER = observer;
+    if (HAS_OBSERVER.compareAndSet(false, true)) {
+      OBSERVER = observer; // set once in premain
+    }
   }
 
   /** Called from advice added to j.l.ClassLoader by DefineClassInstrumentation. */

@@ -0,0 +1,95 @@
+package datadog.trace.bootstrap.instrumentation.classloading;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import java.lang.reflect.Field;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class ClassDefiningTest {
+
+  @BeforeEach
+  void resetStaticState() throws Exception {
+    Field hasObserver = ClassDefining.class.getDeclaredField("HAS_OBSERVER");
+    hasObserver.setAccessible(true);
+    ((AtomicBoolean) hasObserver.get(null)).set(false);
+
+    Field observer = ClassDefining.class.getDeclaredField("OBSERVER");
+    observer.setAccessible(true);
+    observer.set(null, (ClassDefining.Observer) (loader, bytecode, offset, length) -> {});
+  }
+
+  @Test
+  void beginWithNoObserverIsNoOp() {
+    assertDoesNotThrow(() -> ClassDefining.begin(null, new byte[10], 0, 10));
+  }
+
+  @Test
+  void beginCallsRegisteredObserverOnEachInvocation() {
+    AtomicInteger calls = new AtomicInteger();
+    ClassDefining.observe((loader, bytecode, offset, length) -> calls.incrementAndGet());
+
+    ClassDefining.begin(null, new byte[4], 0, 4);
+    ClassDefining.begin(null, new byte[4], 0, 4);
+
+    assertEquals(2, calls.get());
+  }
+
+  @Test
+  void observerReceivesCorrectArguments() {
+    ClassLoader loader = ClassLoader.getSystemClassLoader();
+    byte[] bytecode = {1, 2, 3, 4, 5};
+
+    ClassLoader[] capturedLoader = new ClassLoader[1];
+    byte[][] capturedBytecode = new byte[1][];
+    int[] capturedOffset = new int[1];
+    int[] capturedLength = new int[1];
+
+    ClassDefining.observe(
+        (l, b, o, len) -> {
+          capturedLoader[0] = l;
+          capturedBytecode[0] = b;
+          capturedOffset[0] = o;
+          capturedLength[0] = len;
+        });
+
+    ClassDefining.begin(loader, bytecode, 1, 3);
+
+    assertSame(loader, capturedLoader[0]);
+    assertSame(bytecode, capturedBytecode[0]);
+    assertEquals(1, capturedOffset[0]);
+    assertEquals(3, capturedLength[0]);
+  }
+
+  @Test
+  void secondObserveCallIsIgnoredFirstObserverRemains() {
+    AtomicInteger firstCalls = new AtomicInteger();
+    AtomicInteger secondCalls = new AtomicInteger();
+
+    ClassDefining.observe((loader, bytecode, offset, length) -> firstCalls.incrementAndGet());
+    ClassDefining.observe((loader, bytecode, offset, length) -> secondCalls.incrementAndGet());
+
+    ClassDefining.begin(null, new byte[4], 0, 4);
+
+    assertEquals(1, firstCalls.get());
+    assertEquals(0, secondCalls.get());
+  }
+
+  @Test
+  void observeIsIdempotentWhenCalledWithSameObserverRepeatedly() {
+    AtomicInteger calls = new AtomicInteger();
+    ClassDefining.Observer observer = (l, b, o, len) -> calls.incrementAndGet();
+
+    for (int i = 0; i < 5; i++) {
+      ClassDefining.observe(observer);
+    }
+
+    ClassDefining.begin(null, new byte[1], 0, 1);
+
+    assertEquals(1, calls.get());
+  }
+}