[Java] fix Jackson 3 createDefaultMapper(null) losing the default date format (restclient, webclient)

modules/openapi-generator/src/main/resources/Java/libraries/restclient/ApiClient.mustache

@@ -206,6 +206,9 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
 
     {{#useJackson3}}
     public static JsonMapper createDefaultMapper(@Nullable DateFormat dateFormat) {
+        if (null == dateFormat) {
+            dateFormat = createDefaultDateFormat();
+        }
         return JsonMapper.builder()
             .defaultDateFormat(dateFormat)
             {{#failOnUnknownProperties}}

modules/openapi-generator/src/main/resources/Java/libraries/webclient/ApiClient.mustache

@@ -183,6 +183,9 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
 
     {{#useJackson3}}
     public static JsonMapper createDefaultMapper(@Nullable DateFormat dateFormat) {
+        if (null == dateFormat) {
+            dateFormat = createDefaultDateFormat();
+        }
         return JsonMapper.builder()
             .defaultDateFormat(dateFormat)
             {{#failOnUnknownProperties}}

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

@@ -3671,6 +3671,37 @@ public void testRestClientWithXMLAndJackson3AndOpenApiNullable_issue_23860() {
         );
     }
 
+    @Test(description = "Regression test for issue #24588: with useJackson3=true the generated"
+            + " createDefaultMapper must fall back to createDefaultDateFormat() when called with"
+            + " null, like the Jackson 2 branch does. Otherwise date-time fields serialize as"
+            + " epoch numbers instead of RFC 3339.")
+    public void testJackson3DefaultMapperFallsBackToDefaultDateFormat_issue_24588() {
+        for (String library : new String[]{JavaClientCodegen.RESTCLIENT, JavaClientCodegen.WEBCLIENT}) {
+            final Path output = newTempFolder();
+            final CodegenConfigurator configurator = new CodegenConfigurator()
+                    .setGeneratorName(JAVA_GENERATOR)
+                    .setLibrary(library)
+                    .setAdditionalProperties(Map.of(
+                            CodegenConstants.API_PACKAGE, "xyz.abcdef.api",
+                            JavaClientCodegen.USE_JACKSON_3, true,
+                            JavaClientCodegen.USE_SPRING_BOOT4, true,
+                            JavaClientCodegen.OPENAPI_NULLABLE, false
+                    ))
+                    .setInputSpec("src/test/resources/3_1/java/petstore.yaml")
+                    .setOutputDir(output.toString().replace("\\", "/"));
+
+            List<File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
+
+            validateJavaSourceFiles(files);
+            assertFileContains(
+                    output.resolve("src/main/java/xyz/abcdef/ApiClient.java"),
+                    "public static JsonMapper createDefaultMapper(@Nullable DateFormat dateFormat) {",
+                    "if (null == dateFormat) {",
+                    "dateFormat = createDefaultDateFormat();"
+            );
+        }
+    }
+
 
     @Test
     public void testRestClientWithUseSingleRequestParameter_issue_19406() {

samples/client/petstore/java/restclient-springBoot4-jackson3-jspecify/src/main/java/org/openapitools/client/ApiClient.java

@@ -134,6 +134,9 @@ public static DateFormat createDefaultDateFormat() {
     }
 
     public static JsonMapper createDefaultMapper(@Nullable DateFormat dateFormat) {
+        if (null == dateFormat) {
+            dateFormat = createDefaultDateFormat();
+        }
         return JsonMapper.builder()
             .defaultDateFormat(dateFormat)
             .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)

samples/client/petstore/java/restclient-springBoot4-jackson3/src/main/java/org/openapitools/client/ApiClient.java

@@ -135,6 +135,9 @@ public static DateFormat createDefaultDateFormat() {
     }
 
     public static JsonMapper createDefaultMapper(@Nullable DateFormat dateFormat) {
+        if (null == dateFormat) {
+            dateFormat = createDefaultDateFormat();
+        }
         return JsonMapper.builder()
             .defaultDateFormat(dateFormat)
             .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)

samples/client/petstore/java/webclient-springBoot4-jackson3-jspecify/src/main/java/org/openapitools/client/ApiClient.java

@@ -148,6 +148,9 @@ public static DateFormat createDefaultDateFormat() {
     }
 
     public static JsonMapper createDefaultMapper(@Nullable DateFormat dateFormat) {
+        if (null == dateFormat) {
+            dateFormat = createDefaultDateFormat();
+        }
         return JsonMapper.builder()
             .defaultDateFormat(dateFormat)
             .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)

samples/client/petstore/java/webclient-springBoot4-jackson3/src/main/java/org/openapitools/client/ApiClient.java

@@ -81,8 +81,8 @@
 import org.openapitools.client.auth.OAuth;
 
 @jakarta.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.25.0-SNAPSHOT")
 public class ApiClient extends JavaTimeFormatter {
     public enum CollectionFormat {
         CSV(","), TSV("\t"), SSV(" "), PIPES("|"), MULTI(null);
 
         protected final String separator;
@@ -100,13 +100,13 @@
     protected HttpHeaders defaultHeaders = new HttpHeaders();
     protected MultiValueMap<String, String> defaultCookies = new LinkedMultiValueMap<String, String>();
 
     protected String basePath = "http://petstore.swagger.io/v2";
 
     protected final WebClient webClient;
     protected final DateFormat dateFormat;
     protected final JsonMapper mapper;
 
     protected Map<String, Authentication> authentications;
 
     /**
      * The {@link ExceptionProvider} used to create exceptions thrown by this client.
@@ -149,6 +149,9 @@
     }
 
     public static JsonMapper createDefaultMapper(@Nullable DateFormat dateFormat) {
+        if (null == dateFormat) {
+            dateFormat = createDefaultDateFormat();
+        }
         return JsonMapper.builder()
             .defaultDateFormat(dateFormat)
             .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
@@ -392,7 +395,7 @@
     /**
      * Parse the given string into Date object.
      */
     public Date parseDate(String str) {
         try {
             return dateFormat.parse(str);
         } catch (ParseException e) {
@@ -403,7 +406,7 @@
     /**
      * Format the given Date object into string.
      */
     public String formatDate(Date date) {
         return dateFormat.format(date);
     }
 
@@ -667,7 +670,7 @@
      * @param returnType The return type into which to deserialize the response
      * @return The response body in chosen type
      */
     public <T> ResponseSpec invokeAPI(String path, HttpMethod method, Map<String, Object> pathParams, MultiValueMap<String, String> queryParams, Object body, HttpHeaders headerParams, MultiValueMap<String, String> cookieParams, MultiValueMap<String, Object> formParams, List<MediaType> accept, MediaType contentType, String[] authNames, ParameterizedTypeReference<T> returnType) throws RestClientException {
         final WebClient.RequestBodySpec requestBuilder = prepareRequest(path, method, pathParams, queryParams, body, headerParams, cookieParams, formParams, accept, contentType, authNames);
         return requestBuilder.retrieve();
     }
@@ -678,7 +681,7 @@
      * @param uriParams The path parameters
      * return templatized query string
      */
     protected String generateQueryUri(MultiValueMap<String, String> queryParams, Map<String, Object> uriParams) {
         StringBuilder queryBuilder = new StringBuilder();
         queryParams.forEach((name, values) -> {
             if (CollectionUtils.isEmpty(values)) {