[Java] fix: map the OAS 3.1 null type to Object, not a never-generated ModelNull (#24935)

AbstractJavaCodegen has no "null" entry in typeMapping, so getSchemaType falls
through to toModelName("null"). "null" is a Java reserved word, so the model
name becomes ModelNull, a class the generator never emits, and the generated
client does not compile. Python, Go, Elixir and Xojo already map "null" in
their typeMapping; this adds the Java one.

The 3_1 java fixture gains third_property with the quoted form of the null
type. The existing second_property uses unquoted "type: null", which YAML
reads as a null scalar, so that property carries no type at all and already
generated Object. Only bin/configs/java-okhttp-gson-3.1.yaml consumes this
fixture, and samples-java-petsore-client-jdk11 builds that sample, so the
regression is now covered in CI.

Author: renechoi

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractJavaCodegen.java

@@ -304,6 +304,7 @@ public AbstractJavaCodegen() {
         typeMapping.put("date", "Date");
         typeMapping.put("file", "File");
         typeMapping.put("AnyType", "Object");
+        typeMapping.put("null", "Object");
         typeMapping.put("enum", "Enum");
 
         importMapping.put("BigDecimal", "java.math.BigDecimal");

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/AbstractJavaCodegenTest.java

@@ -1114,4 +1114,13 @@ public void removeAnnotationsTest() {
     public void testSanitizedDataType() {
         assertThat(codegen.sanitizeDataType("org.somepkg.DataType")).isEqualTo("orgsomepkgDataType");
     }
+
+    @Test(description = "the OAS 3.1 null type maps to Object instead of a never-generated ModelNull (issue 24520)")
+    public void testNullTypeMapsToObject() {
+        Schema<?> nullSchema = new Schema<>().type("null");
+
+        assertThat(codegen.getSchemaType(nullSchema)).isEqualTo("Object");
+        assertThat(codegen.getTypeDeclaration(nullSchema)).isEqualTo("Object");
+        assertThat(codegen.getTypeDeclaration(new ArraySchema().items(nullSchema))).isEqualTo("List<Object>");
+    }
 }

modules/openapi-generator/src/test/resources/3_1/java/petstore.yaml

@@ -1135,3 +1135,6 @@ components:
         second_property:
           description: just null type
           type: null
+        third_property:
+          description: null type declared as the OAS 3.1 quoted string
+          type: 'null'

samples/client/petstore/java/okhttp-gson-3.1/api/openapi.yaml

@@ -1215,6 +1215,8 @@ components:
           $ref: "#/components/schemas/Pet"
         second_property:
           description: just null type
+        third_property:
+          description: null type declared as the OAS 3.1 quoted string
     updatePetWithForm_request:
       properties:
         name:

samples/client/petstore/java/okhttp-gson-3.1/docs/PetWithTypesObjectNullAndRef.md

@@ -9,6 +9,7 @@
 |------------ | ------------- | ------------- | -------------|
 |**firstProperty** | [**Pet**](Pet.md) |  |  [optional] |
 |**secondProperty** | **Object** | just null type |  [optional] |
+|**thirdProperty** | **Object** | null type declared as the OAS 3.1 quoted string |  [optional] |
 
 
 

samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/model/PetWithTypesObjectNullAndRef.java

@@ -63,6 +63,11 @@ public class PetWithTypesObjectNullAndRef {
   @javax.annotation.Nullable
   private Object secondProperty = null;
 
+  public static final String SERIALIZED_NAME_THIRD_PROPERTY = "third_property";
+  @SerializedName(SERIALIZED_NAME_THIRD_PROPERTY)
+  @javax.annotation.Nullable
+  private Object thirdProperty = null;
+
   public PetWithTypesObjectNullAndRef() {
   }
 
@@ -103,6 +108,25 @@ public void setSecondProperty(@javax.annotation.Nullable Object secondProperty)
     this.secondProperty = secondProperty;
   }
 
+
+  public PetWithTypesObjectNullAndRef thirdProperty(@javax.annotation.Nullable Object thirdProperty) {
+    this.thirdProperty = thirdProperty;
+    return this;
+  }
+
+  /**
+   * null type declared as the OAS 3.1 quoted string
+   * @return thirdProperty
+   */
+  @javax.annotation.Nullable
+  public Object getThirdProperty() {
+    return thirdProperty;
+  }
+
+  public void setThirdProperty(@javax.annotation.Nullable Object thirdProperty) {
+    this.thirdProperty = thirdProperty;
+  }
+
   /**
    * A container for additional, undeclared properties.
    * This is a holder for any undeclared properties as specified with
@@ -159,7 +183,8 @@ public boolean equals(Object o) {
     }
     PetWithTypesObjectNullAndRef petWithTypesObjectNullAndRef = (PetWithTypesObjectNullAndRef) o;
     return Objects.equals(this.firstProperty, petWithTypesObjectNullAndRef.firstProperty) &&
-        Objects.equals(this.secondProperty, petWithTypesObjectNullAndRef.secondProperty)&&
+        Objects.equals(this.secondProperty, petWithTypesObjectNullAndRef.secondProperty) &&
+        Objects.equals(this.thirdProperty, petWithTypesObjectNullAndRef.thirdProperty)&&
         Objects.equals(this.additionalProperties, petWithTypesObjectNullAndRef.additionalProperties);
   }
 
@@ -169,7 +194,7 @@ private static <T> boolean equalsNullable(JsonNullable<T> a, JsonNullable<T> b)
 
   @Override
   public int hashCode() {
-    return Objects.hash(firstProperty, secondProperty, additionalProperties);
+    return Objects.hash(firstProperty, secondProperty, thirdProperty, additionalProperties);
   }
 
   private static <T> int hashCodeNullable(JsonNullable<T> a) {
@@ -185,6 +210,7 @@ public String toString() {
     sb.append("class PetWithTypesObjectNullAndRef {\n");
     sb.append("    firstProperty: ").append(toIndentedString(firstProperty)).append("\n");
     sb.append("    secondProperty: ").append(toIndentedString(secondProperty)).append("\n");
+    sb.append("    thirdProperty: ").append(toIndentedString(thirdProperty)).append("\n");
     sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
@@ -204,7 +230,7 @@ private String toIndentedString(Object o) {
 
   static {
     // a set of all properties/fields (JSON key names)
-    openapiFields = new HashSet<String>(Arrays.asList("first_property", "second_property"));
+    openapiFields = new HashSet<String>(Arrays.asList("first_property", "second_property", "third_property"));
 
     // a set of required properties/fields (JSON key names)
     openapiRequiredFields = new HashSet<String>(0);