Fix NPE in ModelUtils.isFreeFormObject() (#1625)

Fixes #1696

An object schema containing no properties that also has additionalProperties
set to an object schema with no properties will cause
ModelUtils.isFreeFormObject to throw an NPE.
This PR adds additional checking to avoid the NPE.

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

@@ -547,20 +547,17 @@ public class ModelUtils {
         if ("object".equals(schema.getType())) {
             // no properties
             if ((schema.getProperties() == null || schema.getProperties().isEmpty())) {
-                if (schema.getAdditionalProperties() == null) {
+                Schema addlProps = getAdditionalProperties(schema);
+                // additionalProperties not defined
+                if (addlProps == null) {
                     return true;
                 } else {
-                    // additionalProperties set to true
+                    if (addlProps instanceof ObjectSchema) {
-                    if (schema.getAdditionalProperties() instanceof Boolean
+                        ObjectSchema objSchema = (ObjectSchema) addlProps;
-                            && (Boolean) schema.getAdditionalProperties()) {
+                        // additionalProperties defined as {}
-                        return true;
+                        if (objSchema.getProperties() == null || objSchema.getProperties().isEmpty()) {
-                    }
+                            return true;
-
+                        }
-                    // additionalProperties is set to {}
-                    if (schema.getAdditionalProperties() instanceof Schema && schema.getAdditionalProperties() != null
-                            && schema.getAdditionalProperties() instanceof ObjectSchema
-                            && ((Schema) schema.getAdditionalProperties()).getProperties().isEmpty()) {
-                        return true;
                     }
                 }
             }

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

@@ -199,4 +199,32 @@ public class ModelUtilsTest {
 
         Assert.assertEquals(refToComposedSchema, ModelUtils.unaliasSchema(allSchemas, refToComposedSchema));
     }
+
+    /**
+     * Issue https://github.com/OpenAPITools/openapi-generator/issues/1624.
+     * ModelUtils.isFreeFormObject() should not throw an NPE when passed an empty
+     * object schema that has additionalProperties defined as an empty object schema.
+     */
+    @Test
+    public void testIsFreeFormObject() {
+        // Create initial "empty" object schema.
+        ObjectSchema objSchema = new ObjectSchema();
+        Assert.assertTrue(ModelUtils.isFreeFormObject(objSchema));
+
+        // Set additionalProperties to an empty ObjectSchema.
+        objSchema.setAdditionalProperties(new ObjectSchema());
+        Assert.assertTrue(ModelUtils.isFreeFormObject(objSchema));
+
+        // Add a single property to the schema (no longer a free-form object).
+        Map<String, Schema> props = new HashMap<>();
+        props.put("prop1", new StringSchema());
+        objSchema.setProperties(props);
+        Assert.assertFalse(ModelUtils.isFreeFormObject(objSchema));
+
+        // Test a non-object schema
+        Assert.assertFalse(ModelUtils.isFreeFormObject(new StringSchema()));
+
+        // Test a null schema
+        Assert.assertFalse(ModelUtils.isFreeFormObject(null));
+    }
 }