[BUG] [JAVA] Broken code generation when using `format: enum`

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator? N/A I guess?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

https://github.com/OpenAPITools/openapi-generator/pull/24812 breaks code generation in my project (confirmed via git bisect). Noted tangentially in another issue https://github.com/OpenAPITools/openapi-generator/issues/24942 (and was reproducible in that context) but filing here explicitly with a smaller test case as requested by @Mattias-Sehlstedt + @jpfinne.

openapi-generator version

7.26.0-SNAPSHOT as built from 993bfc9aafdf434c4682fcd863fb19530a3549b8; a regression since 7.25.0

OpenAPI declaration file content or url

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>test</groupId>
  <artifactId>enum-format-repro</artifactId>
  <version>1.0-SNAPSHOT</version>
  <packaging>jar</packaging>

  <dependencies>
    <dependency>
      <groupId>org.openapitools</groupId>
      <artifactId>jackson-databind-nullable</artifactId>
      <version>0.2.6</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-databind</artifactId>
      <version>2.17.0</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.datatype</groupId>
      <artifactId>jackson-datatype-jsr310</artifactId>
      <version>2.17.0</version>
    </dependency>
    <dependency>
      <groupId>com.google.code.findbugs</groupId>
      <artifactId>jsr305</artifactId>
      <version>3.0.2</version>
    </dependency>
    <dependency>
      <groupId>javax.annotation</groupId>
      <artifactId>javax.annotation-api</artifactId>
      <version>1.3.2</version>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>org.openapitools</groupId>
        <artifactId>openapi-generator-maven-plugin</artifactId>
        <!-- Switch between 7.25.0 (passes) and 7.26.0-SNAPSHOT (fails) -->
        <version>7.25.0</version>
        <!--
        <version>7.26.0-SNAPSHOT</version>
        -->
        <executions>
          <execution>
            <goals>
              <goal>generate</goal>
            </goals>
            <configuration>
              <inputSpec>${project.basedir}/src/main/openapi/test.yaml</inputSpec>
              <generatorName>java</generatorName>
              <library>native</library>
              <generateApiTests>false</generateApiTests>
              <generateModelTests>false</generateModelTests>
              <generateApiDocumentation>false</generateApiDocumentation>
              <generateModelDocumentation>false</generateModelDocumentation>
              <apiPackage>test.api</apiPackage>
              <modelPackage>test.model</modelPackage>
              <configOptions>
                <sourceFolder>src/main/java</sourceFolder>
              </configOptions>
            </configuration>
          </execution>
        </executions>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.13.0</version>
        <configuration>
          <source>11</source>
          <target>11</target>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

openapi: 3.0.3
info:
  title: Enum format test
  version: 1.0.0
paths: {}
components:
  schemas:
    Ruleset:
      type: object
      properties:
        bypassMode:
          type: string
          format: enum
          enum:
            - RULESET_BYPASS_MODE_UNSPECIFIED
            - RULESET_BYPASS_MODE_ALLOWED
            - RULESET_BYPASS_MODE_DISABLED

Generation Details

as above

Steps to reproduce

mvn clean compile with 7.25.0: no error

mvn clean install -Dmaven.test.skip=true -Dmaven.javadoc.skip=true -am -pl modules/openapi-generator-maven-plugin on trunk sources of the generator, then building test project with 7.26.0:

[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] /tmp/enum-repro/target/generated-sources/openapi/src/main/java/test/model/Ruleset.java:[28,18] cannot find symbol
  symbol:   class Enum
  location: package test.model
[ERROR] /tmp/enum-repro/target/generated-sources/openapi/src/main/java/test/model/Ruleset.java:[45,41] method valueOf in class java.lang.Enum<E> cannot be applied to given types;
  required: java.lang.Class<T>,java.lang.String
  found:    java.lang.String
  reason: cannot infer type-variable(s) T
    (actual and formal argument lists differ in length)
[ERROR] /tmp/enum-repro/target/generated-sources/openapi/src/main/java/test/model/Ruleset.java:[47,37] method valueOf in class java.lang.Enum<E> cannot be applied to given types;
  required: java.lang.Class<T>,java.lang.String
  found:    java.lang.String
  reason: cannot infer type-variable(s) T
    (actual and formal argument lists differ in length)
[ERROR] /tmp/enum-repro/target/generated-sources/openapi/src/main/java/test/model/Ruleset.java:[49,38] method valueOf in class java.lang.Enum<E> cannot be applied to given types;
  required: java.lang.Class<T>,java.lang.String
  found:    java.lang.String
  reason: cannot infer type-variable(s) T
    (actual and formal argument lists differ in length)
[INFO] 4 errors 
[INFO] -------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE

The diff to the generated source file:

27a28
> import test.model.Enum;
44c45
<     RULESET_BYPASS_MODE_UNSPECIFIED(String.valueOf("RULESET_BYPASS_MODE_UNSPECIFIED")),
---
>     RULESET_BYPASS_MODE_UNSPECIFIED(Enum.valueOf("RULESET_BYPASS_MODE_UNSPECIFIED")),
46c47
<     RULESET_BYPASS_MODE_ALLOWED(String.valueOf("RULESET_BYPASS_MODE_ALLOWED")),
---
>     RULESET_BYPASS_MODE_ALLOWED(Enum.valueOf("RULESET_BYPASS_MODE_ALLOWED")),
48c49
<     RULESET_BYPASS_MODE_DISABLED(String.valueOf("RULESET_BYPASS_MODE_DISABLED"));
---
>     RULESET_BYPASS_MODE_DISABLED(Enum.valueOf("RULESET_BYPASS_MODE_DISABLED"));
50c51
<     private String value;
---
>     private Enum value;
52c53
<     BypassModeEnum(String value) {
---
>     BypassModeEnum(Enum value) {
57c58
<     public String getValue() {
---
>     public Enum getValue() {
67c68
<     public static BypassModeEnum fromValue(String value) {
---
>     public static BypassModeEnum fromValue(Enum value) {

Related issues/PRs

not that I could see

Suggest a fix

FWIW Claude claims

The intent of typeMapping.put("enum", "Enum") was to support the new getCommonSchemaType discriminator-resolution path. The fix should not pollute the global typeMapping; instead, handle it inline inside getCommonSchemaType / getCommonTypeMapping without registering "enum" as a general-purpose format mapping — or, at minimum, add importMapping.put("Enum", "java.lang.Enum") and fix the Mustache template so the generated code uses the correct BypassModeEnum.valueOf(String) call rather than Enum.valueOf(String) (the two-argument java.lang.Enum.valueOf(Class<T>, String) is unrelated to what the template emits).

Reproduce the failure with the Maven setup in pom.xml and the schema in src/main/openapi/test.yaml, comparing Java generation on 7.25.0 with the current version. Inspect the regression introduced by PR 24812 and the generated Ruleset.java; done means the generated project compiles and the enum-format case has regression coverage.