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 fields and accessors when a model redeclares an inherited fixed discriminator.

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/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);
@@ -389,7 +391,33 @@ private static void passPolymorphicDiscriminatorToChildren(ClientModelProperty p
         // 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);
+        // Merge a matching child declaration to avoid emitting the inherited discriminator twice.
+        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.");
+            }
+
+            discriminatorForChild = childProperty.newBuilder()
+                .name(parentDiscriminator.getName())
+                .readOnly(true)
+                .required(false)
+                .polymorphicDiscriminator(true)
+                .build();
+            child.getProperties().remove(i);
+            break;
+        }
+
+        child.getParentPolymorphicDiscriminators().add(0, discriminatorForChild);
 
         for (ClientModel derived : child.getDerivedModels()) {
             passPolymorphicDiscriminatorToChildren(parentDiscriminator, derived);

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/enumnesteddiscriminator/models/GoblinShark.java

@@ -20,7 +20,7 @@ public final class GoblinShark extends Shark {
      * discriminator property
      */
     @Generated
-    private FishKind kind = FishKind.SHARK;
+    private final FishKind kind = FishKind.SHARK;
 
     /*
      * The sharktype property.

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

@@ -17,10 +17,10 @@
 @Immutable
 public final class SawShark extends Shark {
     /*
-     * discriminator property
+     * The kind property.
      */
     @Generated
-    private FishKind kind = FishKind.SHARK;
+    private final FishKind kind = FishKind.SHARK;
 
     /*
      * The sharktype property.
@@ -39,7 +39,7 @@ public SawShark(int age) {
     }
 
     /**
-     * Get the kind property: discriminator property.
+     * Get the kind property: The kind property.
      * 
      * @return the kind value.
      */

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

@@ -20,7 +20,7 @@ public class Shark extends Fish {
      * discriminator property
      */
     @Generated
-    private FishKind kind = FishKind.SHARK;
+    private final FishKind kind = FishKind.SHARK;
 
     /*
      * The sharktype property.

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

@@ -23,7 +23,7 @@ public final class SawShark extends Shark {
      * Discriminator property for Fish.
      */
     @Generated
-    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/patch/models/Shark.java

@@ -23,7 +23,7 @@ public class Shark extends Fish {
      * Discriminator property for Fish.
      */
     @Generated
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

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

@@ -20,7 +20,7 @@ public final class GoblinShark extends Shark {
      * Discriminator property for Fish.
      */
     @Generated
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

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

@@ -20,7 +20,7 @@ public final class SawShark extends Shark {
      * Discriminator property for Fish.
      */
     @Generated
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

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

@@ -20,7 +20,7 @@ public class Shark extends Fish {
      * Discriminator property for Fish.
      */
     @Generated
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-test/src/main/resources/META-INF/tsptest-enumnesteddiscriminator_metadata.json

@@ -1 +1 @@
-{"flavor":"Azure","apiVersions":{},"crossLanguagePackageId":"TspTest.EnumNestedDiscriminator","crossLanguageVersion":"a7268a26bdc4","crossLanguageDefinitions":{"tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient":"TspTest.EnumNestedDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getMissingDiscriminator":"TspTest.EnumNestedDiscriminator.getMissingDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getMissingDiscriminatorWithResponse":"TspTest.EnumNestedDiscriminator.getMissingDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getModel":"TspTest.EnumNestedDiscriminator.getModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getModelWithResponse":"TspTest.EnumNestedDiscriminator.getModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getRecursiveModel":"TspTest.EnumNestedDiscriminator.getRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getRecursiveModelWithResponse":"TspTest.EnumNestedDiscriminator.getRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getWrongDiscriminator":"TspTest.EnumNestedDiscriminator.getWrongDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getWrongDiscriminatorWithResponse":"TspTest.EnumNestedDiscriminator.getWrongDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.putModel":"TspTest.EnumNestedDiscriminator.putModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.putModelWithResponse":"TspTest.EnumNestedDiscriminator.putModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.putRecursiveModel":"TspTest.EnumNestedDiscriminator.putRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.putRecursiveModelWithResponse":"TspTest.EnumNestedDiscriminator.putRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient":"TspTest.EnumNestedDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getMissingDiscriminator":"TspTest.EnumNestedDiscriminator.getMissingDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getMissingDiscriminatorWithResponse":"TspTest.EnumNestedDiscriminator.getMissingDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getModel":"TspTest.EnumNestedDiscriminator.getModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getModelWithResponse":"TspTest.EnumNestedDiscriminator.getModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getRecursiveModel":"TspTest.EnumNestedDiscriminator.getRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getRecursiveModelWithResponse":"TspTest.EnumNestedDiscriminator.getRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getWrongDiscriminator":"TspTest.EnumNestedDiscriminator.getWrongDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getWrongDiscriminatorWithResponse":"TspTest.EnumNestedDiscriminator.getWrongDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.putModel":"TspTest.EnumNestedDiscriminator.putModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.putModelWithResponse":"TspTest.EnumNestedDiscriminator.putModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.putRecursiveModel":"TspTest.EnumNestedDiscriminator.putRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.putRecursiveModelWithResponse":"TspTest.EnumNestedDiscriminator.putRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClientBuilder":"TspTest.EnumNestedDiscriminator","tsptest.enumnesteddiscriminator.models.Fish":"TspTest.EnumNestedDiscriminator.Fish","tsptest.enumnesteddiscriminator.models.FishKind":"TspTest.EnumNestedDiscriminator.FishKind","tsptest.enumnesteddiscriminator.models.GoblinShark":"TspTest.EnumNestedDiscriminator.GoblinShark","tsptest.enumnesteddiscriminator.models.Salmon":"TspTest.EnumNestedDiscriminator.Salmon","tsptest.enumnesteddiscriminator.models.SawShark":"TspTest.EnumNestedDiscriminator.SawShark","tsptest.enumnesteddiscriminator.models.Shark":"TspTest.EnumNestedDiscriminator.Shark","tsptest.enumnesteddiscriminator.models.SharkKind":"TspTest.EnumNestedDiscriminator.SharkKind"},"generatedFiles":["src/main/java/module-info.java","src/main/java/tsptest/enumnesteddiscriminator/EnumNestedDiscriminatorAsyncClient.java","src/main/java/tsptest/enumnesteddiscriminator/EnumNestedDiscriminatorClient.java","src/main/java/tsptest/enumnesteddiscriminator/EnumNestedDiscriminatorClientBuilder.java","src/main/java/tsptest/enumnesteddiscriminator/implementation/EnumNestedDiscriminatorClientImpl.java","src/main/java/tsptest/enumnesteddiscriminator/implementation/package-info.java","src/main/java/tsptest/enumnesteddiscriminator/models/Fish.java","src/main/java/tsptest/enumnesteddiscriminator/models/FishKind.java","src/main/java/tsptest/enumnesteddiscriminator/models/GoblinShark.java","src/main/java/tsptest/enumnesteddiscriminator/models/Salmon.java","src/main/java/tsptest/enumnesteddiscriminator/models/SawShark.java","src/main/java/tsptest/enumnesteddiscriminator/models/Shark.java","src/main/java/tsptest/enumnesteddiscriminator/models/SharkKind.java","src/main/java/tsptest/enumnesteddiscriminator/models/package-info.java","src/main/java/tsptest/enumnesteddiscriminator/package-info.java"]}
+{"flavor":"Azure","apiVersions":{},"crossLanguagePackageId":"TspTest.EnumNestedDiscriminator","crossLanguageVersion":"e2c89125d0a8","crossLanguageDefinitions":{"tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient":"TspTest.EnumNestedDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getMissingDiscriminator":"TspTest.EnumNestedDiscriminator.getMissingDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getMissingDiscriminatorWithResponse":"TspTest.EnumNestedDiscriminator.getMissingDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getModel":"TspTest.EnumNestedDiscriminator.getModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getModelWithResponse":"TspTest.EnumNestedDiscriminator.getModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getRecursiveModel":"TspTest.EnumNestedDiscriminator.getRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getRecursiveModelWithResponse":"TspTest.EnumNestedDiscriminator.getRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getWrongDiscriminator":"TspTest.EnumNestedDiscriminator.getWrongDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.getWrongDiscriminatorWithResponse":"TspTest.EnumNestedDiscriminator.getWrongDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.putModel":"TspTest.EnumNestedDiscriminator.putModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.putModelWithResponse":"TspTest.EnumNestedDiscriminator.putModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.putRecursiveModel":"TspTest.EnumNestedDiscriminator.putRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorAsyncClient.putRecursiveModelWithResponse":"TspTest.EnumNestedDiscriminator.putRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient":"TspTest.EnumNestedDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getMissingDiscriminator":"TspTest.EnumNestedDiscriminator.getMissingDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getMissingDiscriminatorWithResponse":"TspTest.EnumNestedDiscriminator.getMissingDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getModel":"TspTest.EnumNestedDiscriminator.getModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getModelWithResponse":"TspTest.EnumNestedDiscriminator.getModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getRecursiveModel":"TspTest.EnumNestedDiscriminator.getRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getRecursiveModelWithResponse":"TspTest.EnumNestedDiscriminator.getRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getWrongDiscriminator":"TspTest.EnumNestedDiscriminator.getWrongDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.getWrongDiscriminatorWithResponse":"TspTest.EnumNestedDiscriminator.getWrongDiscriminator","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.putModel":"TspTest.EnumNestedDiscriminator.putModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.putModelWithResponse":"TspTest.EnumNestedDiscriminator.putModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.putRecursiveModel":"TspTest.EnumNestedDiscriminator.putRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClient.putRecursiveModelWithResponse":"TspTest.EnumNestedDiscriminator.putRecursiveModel","tsptest.enumnesteddiscriminator.EnumNestedDiscriminatorClientBuilder":"TspTest.EnumNestedDiscriminator","tsptest.enumnesteddiscriminator.models.Fish":"TspTest.EnumNestedDiscriminator.Fish","tsptest.enumnesteddiscriminator.models.FishKind":"TspTest.EnumNestedDiscriminator.FishKind","tsptest.enumnesteddiscriminator.models.GoblinShark":"TspTest.EnumNestedDiscriminator.GoblinShark","tsptest.enumnesteddiscriminator.models.Salmon":"TspTest.EnumNestedDiscriminator.Salmon","tsptest.enumnesteddiscriminator.models.SawShark":"TspTest.EnumNestedDiscriminator.SawShark","tsptest.enumnesteddiscriminator.models.Shark":"TspTest.EnumNestedDiscriminator.Shark","tsptest.enumnesteddiscriminator.models.SharkKind":"TspTest.EnumNestedDiscriminator.SharkKind"},"generatedFiles":["src/main/java/module-info.java","src/main/java/tsptest/enumnesteddiscriminator/EnumNestedDiscriminatorAsyncClient.java","src/main/java/tsptest/enumnesteddiscriminator/EnumNestedDiscriminatorClient.java","src/main/java/tsptest/enumnesteddiscriminator/EnumNestedDiscriminatorClientBuilder.java","src/main/java/tsptest/enumnesteddiscriminator/implementation/EnumNestedDiscriminatorClientImpl.java","src/main/java/tsptest/enumnesteddiscriminator/implementation/package-info.java","src/main/java/tsptest/enumnesteddiscriminator/models/Fish.java","src/main/java/tsptest/enumnesteddiscriminator/models/FishKind.java","src/main/java/tsptest/enumnesteddiscriminator/models/GoblinShark.java","src/main/java/tsptest/enumnesteddiscriminator/models/Salmon.java","src/main/java/tsptest/enumnesteddiscriminator/models/SawShark.java","src/main/java/tsptest/enumnesteddiscriminator/models/Shark.java","src/main/java/tsptest/enumnesteddiscriminator/models/SharkKind.java","src/main/java/tsptest/enumnesteddiscriminator/models/package-info.java","src/main/java/tsptest/enumnesteddiscriminator/package-info.java"]}

packages/http-client-java/generator/http-client-generator-test/tsp/nested-enum-discriminator.tsp

@@ -55,6 +55,7 @@ model Salmon extends Fish {
 
 @doc("The third level model SawShark in polymorphic multiple levels inheritance.")
 model SawShark extends Shark {
+  kind: FishKind.Shark;
   sharktype: SharkKind.Saw;
 }