Fix duplicate inherited discriminators in Java models

.chronus/changes/fix-java-duplicate-discriminators-2026-09-02.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-java"
+---
+
+Prevent duplicate Java discriminator members while preserving inherited discriminators in stream-style XML serialization.

packages/http-client-java/generator/http-client-generator-clientcore-test/src/main/java/type/model/inheritance/nesteddiscriminator/GoblinShark.java

@@ -16,7 +16,7 @@ public final class GoblinShark extends Shark {
      * Discriminator property for Fish.
      */
     @Metadata(properties = { MetadataProperties.GENERATED })
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-clientcore-test/src/main/java/type/model/inheritance/nesteddiscriminator/SawShark.java

@@ -16,7 +16,7 @@ public final class SawShark extends Shark {
      * Discriminator property for Fish.
      */
     @Metadata(properties = { MetadataProperties.GENERATED })
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-clientcore-test/src/main/java/type/model/inheritance/nesteddiscriminator/Shark.java

@@ -16,7 +16,7 @@ public class Shark extends Fish {
      * Discriminator property for Fish.
      */
     @Metadata(properties = { MetadataProperties.GENERATED })
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/implementation/ClientModelPropertiesManager.java

@@ -22,6 +22,7 @@
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 /**
  * Manages metadata about properties in a {@link ClientModel} and how they correlate with model class generation.
@@ -124,10 +125,13 @@ public ClientModelPropertiesManager(ClientModel model, JavaSettings settings) {
             xmlRootElementNamespace = model.getXmlNamespace();
         }
 
-        Set<String> thisModelPropertySerializeNames = model.getProperties()
-            .stream()
+        Set<String> thisModelPropertySerializeNames = Stream.concat(
             // discriminator property is known to be redefined in subclass
-            .filter(property -> !property.isPolymorphicDiscriminator())
+            model.getProperties().stream().filter(property -> !property.isPolymorphicDiscriminator()),
+            // For example, after a child's fixed "type" property is removed from model.getProperties(), the fixed
+            // "type" entry in model.getParentPolymorphicDiscriminators() is included here so the inherited parent
+            // "type" property is masked. Otherwise, the generated child has two "type" members.
+            model.getParentPolymorphicDiscriminators().stream())
             .map(ClientModelProperty::getSerializedName)
             .filter(name -> Objects.nonNull(name) && !name.isEmpty())
             .collect(Collectors.toSet());

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/implementation/PolymorphicDiscriminatorHandler.java

@@ -12,6 +12,7 @@
 import com.microsoft.typespec.http.client.generator.core.model.javamodel.JavaFile;
 import com.microsoft.typespec.http.client.generator.core.model.javamodel.JavaVisibility;
 import com.microsoft.typespec.http.client.generator.core.util.ClientModelUtil;
+import java.util.Objects;
 import java.util.function.Consumer;
 import java.util.function.Function;
 
@@ -116,7 +117,13 @@ private static void declareFieldInternal(ClientModelProperty discriminator, Clie
                 && settings.isShareJsonSerializableCode()) {
                 classBlock.memberVariable(JavaVisibility.PackagePrivate, fieldSignature);
             } else if (!allPolymorphicModelsInSamePackage || !settings.isShareJsonSerializableCode()) {
-                classBlock.privateMemberVariable(fieldSignature);
+                // Active discriminators stay mutable to preserve unknown values during fallback deserialization.
+                if (discriminator.isConstant()
+                    && !Objects.equals(discriminator.getSerializedName(), model.getPolymorphicDiscriminatorName())) {
+                    classBlock.privateFinalMemberVariable(fieldSignature);
+                } else {
+                    classBlock.privateMemberVariable(fieldSignature);
+                }
             }
         }
     }

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/mapper/ModelMapper.java

@@ -363,11 +363,13 @@ public ClientModel map(ObjectSchema compositeType) {
                 // the correct serialization in multi-level polymorphic structures.
                 for (ClientModel derivedType : derivedTypes) {
                     if (!Objects.equals(polymorphicDiscriminator, derivedType.getPolymorphicDiscriminatorName())) {
+                        // The child hierarchy stays in one fixed parent discriminator branch.
                         ClientModelProperty parentDiscriminator = result.getPolymorphicDiscriminator()
                             .newBuilder()
                             .defaultValue(result.getPolymorphicDiscriminator()
                                 .getClientType()
                                 .defaultValueExpression(derivedType.getSerializedName()))
+                            .constant(true)
                             .build();
 
                         passPolymorphicDiscriminatorToChildren(parentDiscriminator, derivedType);
@@ -383,13 +385,47 @@ public ClientModel map(ObjectSchema compositeType) {
 
     private static void passPolymorphicDiscriminatorToChildren(ClientModelProperty parentDiscriminator,
         ClientModel child) {
-        // Due to the execution order of ModelMapper, where children models complete mapping before the parent model,
-        // the parent polymorphic discriminator needs to be added at index 0. Reason, given an example where there are
-        // three models, where model #1 is the root parent with discriminator type, model #2 is a child of model #2 with
-        // discriminator kind, and model #3 is a child of model #3 with discriminator form. The order if this running
-        // will have model #2 add its discriminator to model #3 before model #1 runs adding its discriminator to #2 and
-        // #3. We want #3 to have the ordering of [type, kind], to represent the ordering of the parent models.
-        child.getParentPolymorphicDiscriminators().add(0, parentDiscriminator);
+        // A child that introduces a different discriminator still needs the fixed discriminator value selected by the
+        // parent hierarchy. For example, a parent may discriminate on "type", while a child fixes type="message" and
+        // discriminates its children on "role". The child branch must retain type="message" while dispatching by
+        // "role".
+        //
+        // If child.getProperties() contains a fixed property with the same serialized name and value as
+        // parentDiscriminator, use the matching property to build the parent discriminator entry. Then remove the
+        // matching property from child.getProperties() so the generated model does not contain "type" as both a normal
+        // property and a parent discriminator.
+        ClientModelProperty discriminatorForChild = parentDiscriminator;
+        for (int i = 0; i < child.getProperties().size(); i++) {
+            ClientModelProperty childProperty = child.getProperties().get(i);
+            if (!Objects.equals(parentDiscriminator.getSerializedName(), childProperty.getSerializedName())) {
+                continue;
+            }
+
+            if (!childProperty.isConstant()
+                || !Objects.equals(parentDiscriminator.getWireType(), childProperty.getWireType())
+                || !Objects.equals(parentDiscriminator.getClientType(), childProperty.getClientType())
+                || !Objects.equals(parentDiscriminator.getDefaultValue(), childProperty.getDefaultValue())) {
+                throw new IllegalStateException("Property '" + childProperty.getSerializedName() + "' on model '"
+                    + child.getName() + "' does not match its inherited polymorphic discriminator. Expected (type="
+                    + parentDiscriminator.getClientType() + ", value="
+                    + String.valueOf(parentDiscriminator.getDefaultValue()) + "), but found (type="
+                    + childProperty.getClientType() + ", value=" + String.valueOf(childProperty.getDefaultValue())
+                    + ").");
+            }
+
+            discriminatorForChild = childProperty.newBuilder()
+                .name(parentDiscriminator.getName())
+                .readOnly(true)
+                .required(false)
+                .polymorphicDiscriminator(true)
+                .build();
+            child.getProperties().remove(i);
+            break;
+        }
+
+        // Children are mapped before their parents, so insert at index 0 to preserve outer-to-inner discriminator
+        // order.
+        child.getParentPolymorphicDiscriminators().add(0, discriminatorForChild);
 
         for (ClientModel derived : child.getDerivedModels()) {
             passPolymorphicDiscriminatorToChildren(parentDiscriminator, derived);

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/template/ModelTemplate.java

@@ -886,18 +886,18 @@ private void addModelConstructor(ClientModel model, ClientModelPropertiesManager
                          *
                          * we use the property in this model to initiate the superclass
                          */
-                        ClientModelProperty propertyInThisModel = model.getProperties()
-                            .stream()
+                        ClientModelProperty overridingProperty = Stream
+                            .concat(model.getProperties().stream(), model.getParentPolymorphicDiscriminators().stream())
                             .filter(p -> Objects.equals(p.getSerializedName(), property.getSerializedName()))
                             .findFirst()
                             .orElse(null);
-                        if (propertyInThisModel != null) {
-                            if (propertyInThisModel.isConstant() && !property.isConstant()) {
+                        if (overridingProperty != null) {
+                            if (overridingProperty.isConstant() && !property.isConstant()) {
                                 // property changed to constant in this model, use constant value to initiate super
                                 // class
-                                superProperties.append(propertyInThisModel.getDefaultValue());
+                                superProperties.append(overridingProperty.getDefaultValue());
                             } else {
-                                superProperties.append(propertyInThisModel.getName());
+                                superProperties.append(overridingProperty.getName());
                             }
                         } else {
                             // this should not happen

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/template/StreamSerializationModelTemplate.java

@@ -2059,8 +2059,16 @@ private void writeToXml(JavaClass classBlock) {
                             + propertiesManager.getXmlNamespaceConstant(namespace) + ");"));
 
                     // Assumption for XML is polymorphic discriminators are attributes.
-                    if (propertiesManager.getDiscriminatorProperty() != null) {
-                        serializeXml(methodBlock, propertiesManager.getDiscriminatorProperty().getProperty(), false);
+                    ClientModelPropertyWithMetadata discriminatorProperty
+                        = propertiesManager.getDiscriminatorProperty();
+                    model.getParentPolymorphicDiscriminators()
+                        .stream()
+                        .filter(discriminator -> discriminatorProperty == null
+                            || !Objects.equals(discriminator.getSerializedName(),
+                                discriminatorProperty.getProperty().getSerializedName()))
+                        .forEach(discriminator -> serializeXml(methodBlock, discriminator, false));
+                    if (discriminatorProperty != null) {
+                        serializeXml(methodBlock, discriminatorProperty.getProperty(), false);
                     }
 
                     propertiesManager.forEachSuperXmlAttribute(property -> serializeXml(methodBlock, property, true));
@@ -2214,7 +2222,7 @@ private void writeSuperTypeFromXml(JavaClass classBlock) {
                         + propertiesManager.getXmlNamespaceConstant(discriminatorProperty.getXmlNamespace()) + ", "
                         + "\"" + discriminatorProperty.getSerializedName() + "\");");
                 } else {
-                    methodBlock.line("String discriminatorValue = reader.getStringAttribute(" + "\""
+                    methodBlock.line("String discriminatorValue = reader.getStringAttribute(null, " + "\""
                         + discriminatorProperty.getSerializedName() + "\");");
                 }
 
@@ -2226,12 +2234,18 @@ private void writeSuperTypeFromXml(JavaClass classBlock) {
                 // Add deserialization for all child types.
                 List<ClientModel> childTypes = getAllChildTypes(model, new ArrayList<>());
                 for (ClientModel childType : childTypes) {
+                    boolean sameDiscriminator = Objects.equals(childType.getPolymorphicDiscriminatorName(),
+                        model.getPolymorphicDiscriminatorName());
+                    if (!sameDiscriminator && !Objects.equals(childType.getParentModelName(), model.getName())) {
+                        continue;
+                    }
+
+                    String deserializationMethod = (isSuperTypeWithDiscriminator(childType) && sameDiscriminator)
+                        ? ".fromXmlInternal(reader, finalRootElementName)"
+                        : ".fromXml(reader, finalRootElementName)";
                     ifBlock = ifOrElseIf(methodBlock, ifBlock,
                         "\"" + childType.getSerializedName() + "\".equals(discriminatorValue)",
-                        ifStatement -> ifStatement
-                            .methodReturn(childType.getName() + (isSuperTypeWithDiscriminator(childType)
-                                ? ".fromXmlInternal(reader, finalRootElementName)"
-                                : ".fromXml(reader, finalRootElementName)")));
+                        ifStatement -> ifStatement.methodReturn(childType.getName() + deserializationMethod));
                 }
 
                 if (ifBlock == null) {
@@ -2439,6 +2453,10 @@ private void writeFromXmlDeserialization(JavaBlock methodBlock) {
         }
 
         private void deserializeXmlAttribute(JavaBlock methodBlock, ClientModelProperty attribute, boolean fromSuper) {
+            if (attribute.isRequired() && attribute.isConstant() && !attribute.isPolymorphicDiscriminator()) {
+                return;
+            }
+
             String xmlAttributeDeserialization = getSimpleXmlDeserialization(attribute.getWireType(), null,
                 attribute.getXmlName(), propertiesManager.getXmlNamespaceConstant(attribute.getXmlNamespace()), true);
 

packages/http-client-java/generator/http-client-generator-test/src/main/java/tsptest/armstreamstyleserialization/models/GoblinShark.java

@@ -21,7 +21,7 @@ public final class GoblinShark extends Shark {
     /*
      * Discriminator property for Fish.
      */
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-test/src/main/java/tsptest/armstreamstyleserialization/models/SawShark.java

@@ -21,7 +21,7 @@ public final class SawShark extends Shark {
     /*
      * Discriminator property for Fish.
      */
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-test/src/main/java/tsptest/armstreamstyleserialization/models/Shark.java

@@ -22,7 +22,7 @@ public class Shark extends FishInner {
     /*
      * Discriminator property for Fish.
      */
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-test/src/main/java/tsptest/discriminatoredgecases/models/ChildWithRequiredPropertyAsDiscriminator.java

@@ -103,6 +103,8 @@ public static ChildWithRequiredPropertyAsDiscriminator fromJson(JsonReader jsonR
                 // Use the discriminator value to determine which subtype should be deserialized.
                 if ("aValue".equals(discriminatorValue)) {
                     return GrandChildWithRequiredProperty.fromJson(readerToUse.reset());
+                } else if ("nested".equals(discriminatorValue)) {
+                    return GrandChildWithNestedDiscriminator.fromJson(readerToUse.reset());
                 } else {
                     return fromJsonKnownDiscriminator(readerToUse.reset());
                 }