Fix Jackson3JsonPayloadConverter API mismatch in the multi-release jar

temporal-sdk/build.gradle

@@ -63,6 +63,9 @@ dependencies {
     java17Implementation files(sourceSets.main.output.classesDirs) { builtBy compileJava }
     java17Implementation files({ sourceSets.main.compileClasspath })
     java17CompileOnly "tools.jackson.core:jackson-databind:$jackson3Version"
+    // The Java 8 stub must declare the same public API as the Java 17 class (JEP 238), so it needs
+    // the Jackson 3 types at compile time only.
+    compileOnly "tools.jackson.core:jackson-databind:$jackson3Version"
 
     java21Implementation files(sourceSets.main.output.classesDirs) { builtBy compileJava }
 }
@@ -181,9 +184,8 @@ test {
 }
 
 // On Java 17+, prepend java17 classes to all test classpaths so that Class.forName finds
-// the real Jackson3JsonPayloadConverter instead of the Java 8 stub. This lets us test
-// the present-java17-but-absent-jackson3 behavior (NoClassDefFoundError) in the same
-// test that tests the Java 8 stub behavior (UnsupportedOperationException).
+// the real Jackson3JsonPayloadConverter instead of the Java 8 stub, matching what the
+// multi-release jar resolves to at runtime.
 tasks.withType(Test).configureEach {
     dependsOn compileJava17Java
     doFirst {
@@ -236,9 +238,8 @@ testing {
         jackson3Tests(JvmTestSuite) {
             dependencies {
                 // java17 output must come before project() (added by configureEach) so that
-                // the compiler and runtime see the real Jackson3JsonPayloadConverter — which
-                // has a wider API than the Java 8 stub (newDefaultJsonMapper, JsonMapper
-                // constructor) because the stub can't reference Jackson 3 types.
+                // the compiler and runtime see the real Jackson3JsonPayloadConverter rather
+                // than the Java 8 stub, which only throws.
                 implementation files(sourceSets.java17.output.classesDirs) { builtBy compileJava17Java }
                 implementation "tools.jackson.core:jackson-databind:$jackson3Version"
             }
@@ -250,6 +251,7 @@ testing {
                                 languageVersion = JavaLanguageVersion.of(project.property("testJavaVersion") as int)
                             }
                         }
+                        systemProperty 'temporal.sdk.mainClassesDirs', sourceSets.main.output.classesDirs.asPath
                         shouldRunAfter(test)
                     }
                 }

temporal-sdk/src/jackson3Tests/java/io/temporal/common/converter/Jackson3JsonPayloadConverterApiTest.java

@@ -0,0 +1,64 @@
+package io.temporal.common.converter;
+
+import static org.junit.Assert.assertEquals;
+
+import java.io.File;
+import java.io.IOException;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
+import java.nio.file.Files;
+import java.util.Set;
+import java.util.TreeSet;
+import org.junit.Test;
+
+/**
+ * Verifies that the Java 8 stub of {@link Jackson3JsonPayloadConverter} declares the same public
+ * API as the Java 17 class, as required for versioned entries of a multi-release jar. Tools that
+ * compile against the base entry otherwise cannot resolve members that exist only in the Java 17
+ * class.
+ */
+public class Jackson3JsonPayloadConverterApiTest {
+
+  private static final String CLASS_FILE =
+      Jackson3JsonPayloadConverter.class.getName().replace('.', '/') + ".class";
+
+  @Test
+  public void testStubDeclaresTheSamePublicApi() throws Exception {
+    byte[] stub = readStub();
+    Class<?> stubClass =
+        new ClassLoader(getClass().getClassLoader()) {
+          Class<?> define() {
+            return defineClass(null, stub, 0, stub.length);
+          }
+        }.define();
+
+    assertEquals(publicApi(Jackson3JsonPayloadConverter.class), publicApi(stubClass));
+  }
+
+  private static byte[] readStub() throws IOException {
+    String dirs = System.getProperty("temporal.sdk.mainClassesDirs");
+    for (String dir : dirs.split(File.pathSeparator, -1)) {
+      File classFile = new File(dir, CLASS_FILE);
+      if (classFile.isFile()) {
+        return Files.readAllBytes(classFile.toPath());
+      }
+    }
+    throw new AssertionError("stub not found in " + dirs);
+  }
+
+  private static Set<String> publicApi(Class<?> clazz) {
+    Set<String> api = new TreeSet<>();
+    for (Constructor<?> constructor : clazz.getDeclaredConstructors()) {
+      if (Modifier.isPublic(constructor.getModifiers())) {
+        api.add(constructor.toGenericString());
+      }
+    }
+    for (Method method : clazz.getDeclaredMethods()) {
+      if (Modifier.isPublic(method.getModifiers()) && !method.isSynthetic()) {
+        api.add(method.toGenericString());
+      }
+    }
+    return api;
+  }
+}

temporal-sdk/src/main/java/io/temporal/common/converter/Jackson3JsonPayloadConverter.java

@@ -4,6 +4,7 @@
 import io.temporal.common.Experimental;
 import java.lang.reflect.Type;
 import java.util.Optional;
+import tools.jackson.databind.json.JsonMapper;
 
 /**
  * A {@link PayloadConverter} that uses Jackson 3.x for JSON serialization/deserialization. This
@@ -32,6 +33,14 @@ public Jackson3JsonPayloadConverter(boolean jackson2Compat) {
     throw new UnsupportedOperationException(UNSUPPORTED_MSG);
   }
 
+  public Jackson3JsonPayloadConverter(JsonMapper mapper) {
+    throw new UnsupportedOperationException(UNSUPPORTED_MSG);
+  }
+
+  public static JsonMapper newDefaultJsonMapper(boolean jackson2Compat) {
+    throw new UnsupportedOperationException(UNSUPPORTED_MSG);
+  }
+
   @Override
   public String getEncodingType() {
     throw new UnsupportedOperationException(UNSUPPORTED_MSG);

temporal-sdk/src/test/java/io/temporal/common/converter/JacksonJsonPayloadConverterTest.java

@@ -5,7 +5,6 @@
 import static org.junit.Assert.fail;
 
 import io.temporal.api.common.v1.Payloads;
-import java.lang.reflect.InvocationTargetException;
 import java.time.Instant;
 import java.util.Objects;
 import java.util.Optional;
@@ -25,27 +24,11 @@ public void testSetDefaultAsJackson3ThrowsWithoutJackson3() {
       JacksonJsonPayloadConverter.setDefaultAsJackson3(true, true);
       fail("Expected IllegalStateException");
     } catch (IllegalStateException e) {
-      // On Java 8: Class.forName finds the stub, whose constructor throws
-      //   UnsupportedOperationException → wrapped in InvocationTargetException by reflection.
-      // On Java 17+: Class.forName finds the real impl (java17 classes are on the classpath)
-      //   but Jackson 3 types are absent, so class loading throws NoClassDefFoundError directly.
+      // Both the Java 8 stub and the Java 17 class reference Jackson 3 types, so reflective
+      // constructor lookup fails while linking when Jackson 3 is absent.
       Throwable cause = e.getCause();
-      String specVersion = System.getProperty("java.specification.version");
-      int majorVersion =
-          specVersion.startsWith("1.")
-              ? Integer.parseInt(specVersion.substring(2))
-              : Integer.parseInt(specVersion);
-      if (majorVersion >= 17) {
-        assertTrue(
-            "Expected NoClassDefFoundError, got: " + cause, cause instanceof NoClassDefFoundError);
-      } else {
-        assertTrue(
-            "Expected InvocationTargetException, got: " + cause,
-            cause instanceof InvocationTargetException);
-        assertTrue(
-            "Expected UnsupportedOperationException, got: " + cause.getCause(),
-            cause.getCause() instanceof UnsupportedOperationException);
-      }
+      assertTrue(
+          "Expected NoClassDefFoundError, got: " + cause, cause instanceof NoClassDefFoundError);
     }
   }