fix: ExampleGenerator correctly produces YYYY-MM-dd format for date with examples (#17495)

When an example was specified for a property with the `date` format, it's example would be generated to something like `2024-01-01T00:00:00.000+00:00` or `Sat Jan 30 01:00:00 CET 2021`.

It's because the resolved type for a Date example is a DateSchema, and a DateSchema uses `java.util.Date` to represents the Date. Which gets printed using .toString() not in the YYYY-MM-dd format.

Fixes #17494 #15342

modules/openapi-generator/src/main/java/org/openapitools/codegen/examples/ExampleGenerator.java

@@ -27,6 +27,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.math.BigDecimal;
+import java.text.SimpleDateFormat;
 import java.util.*;
 
 public class ExampleGenerator {
@@ -36,6 +37,8 @@ public class ExampleGenerator {
     private static final String MIME_TYPE_JSON = "application/json";
     private static final String MIME_TYPE_XML = "application/xml";
 
+    protected final SimpleDateFormat DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd", Locale.ROOT);
+
     private static final String EXAMPLE = "example";
     private static final String CONTENT_TYPE = "contentType";
     private static final String GENERATED_CONTENT_TYPE = "generatedContentType";
@@ -54,6 +57,7 @@ public class ExampleGenerator {
         this.openAPI = openAPI;
         // use a fixed seed to make the "random" numbers reproducible.
         this.random = new Random("ExampleGenerator".hashCode());
+        DATE_FORMAT.setTimeZone(TimeZone.getTimeZone("UTC"));
     }
 
     public List<Map<String, String>> generateFromResponseSchema(String statusCode, Schema responseSchema, Set<String> producesInfo) {
@@ -231,6 +235,11 @@ public class ExampleGenerator {
         LOGGER.debug("Resolving example for property {}...", property);
         if (property.getExample() != null) {
             LOGGER.debug("Example set in openapi spec, returning example: '{}'", property.getExample().toString());
+            // When a property is of type Date, we want to ensure that we're returning a formatted Date.
+            // And not returning the Date object directly.
+            if (property.getExample() instanceof Date) {
+                return DATE_FORMAT.format(property.getExample());
+            }
             return property.getExample();
         } else if (ModelUtils.isBooleanSchema(property)) {
             Object defaultValue = property.getDefault();

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

@@ -39,6 +39,35 @@ public class ExampleGeneratorTest {
         assertEquals("200", examples.get(0).get("statusCode"));
     }
 
+    @Test
+    public void generateFromResponseSchemaWithDateFormat() {
+        OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/example_generator_test.yaml");
+
+        new InlineModelResolver().flatten(openAPI);
+
+        ExampleGenerator exampleGenerator = new ExampleGenerator(openAPI.getComponents().getSchemas(), openAPI);
+        Set<String> mediaTypeKeys = new TreeSet<>();
+        mediaTypeKeys.add("application/json");
+        List<Map<String, String>> examples = exampleGenerator.generateFromResponseSchema(
+                "200",
+                openAPI
+                        .getPaths()
+                        .get("/generate_from_response_schema_with_date_format")
+                        .getGet()
+                        .getResponses()
+                        .get("200")
+                        .getContent()
+                        .get("application/json")
+                        .getSchema(),
+                mediaTypeKeys
+        );
+
+        assertEquals(1, examples.size());
+        assertEquals("application/json", examples.get(0).get("contentType"));
+        assertEquals(String.format(Locale.ROOT, "{%n  \"date_with_example\" : \"2024-01-01\",%n  \"date_without_example\" : \"2000-01-23\"%n}"), examples.get(0).get("example"));
+        assertEquals("200", examples.get(0).get("statusCode"));
+    }
+
     @Test
     public void generateFromResponseSchemaWithNoExample() {
         OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/example_generator_test.yaml");

modules/openapi-generator/src/test/resources/3_0/example_generator_test.yaml

@@ -26,6 +26,16 @@ paths:
               schema:
                 type: string
                 example: primitive type example value
+  /generate_from_response_schema_with_date_format:
+    get:
+      operationId: generateFromResponseSchemaWithDateFormat
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/DateSchema'
   /generate_from_response_schema_with_array_of_model:
     get:
       operationId: generateFromResponseSchemaWithArrayOfModel
@@ -93,6 +103,16 @@ paths:
                 $ref: '#/components/schemas/ExampleOneOfSchema'
 components:
   schemas:
+    DateSchema:
+      type: object
+      properties:
+        date_with_example:
+          type: string
+          format: date
+          example: 2024-01-01
+        date_without_example:
+          type: string
+          format: date
     StringSchema:
       type: string
       example: string schema example value