[BUG][Java][Spring] Syntax error in generated API/Controller code when using example definition

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

We are using OpenAPI and openapi-generator-maven-plugin with Spring Boot. We're trying to create an example object in our response. When the API/Controller code given is generated, it contains the syntax error shown below.

openapi-generator version

4.0.0

OpenAPI declaration file content or url

openapi: 3.0.2
info:
  title: Flex PLM ACL API
  description: MDM color app to call the Flex PLM ACL API.
  version: 1.0.0
servers:
  - url: https://flexplm-acl-dev.apps.an01.pcf.dcsg.com/api/v1
paths:
  '/colors/{season}':
    parameters:
      - name: season
        description: season ID
        required: true
        in: path
        schema:
          type: string
    patch:
      tags:
        - Color
      operationId: testing
      description: test
      responses:
        '200':
          description: Return existing colors in a season
          content:
            application/json:
              schema:
                type: object
                properties:
                  testName:
                    type: string
                example:               # <--- Lines leading to error
                  testName: amy

Command line used for generation

A Maven Plugin was used:

<plugin>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <version>4.0.0</version>
    <executions>
        <execution>
            <id>FlexPLM-ACL</id>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <inputSpec>${project.basedir}/apis/flexplm-acl.yml</inputSpec>
                <generatorName>spring</generatorName>
                <apiPackage>com.dcsg.pdcomm.flexplmacl.api</apiPackage>
                <!-- Both of these APIs use the exact same DTOs, so we have put them into the same package -->
                <modelPackage>com.dcsg.pdcomm.flexplm.dto</modelPackage>
                <modelNameSuffix>DTO</modelNameSuffix>
                <supportingFilesToGenerate>ApiUtil.java</supportingFilesToGenerate>
                <configOptions>
                    <library>spring-boot</library>
                    <dateLibrary>java8</dateLibrary>
                    <java8>true</java8>
                    <interfaceOnly>true</interfaceOnly>
                </configOptions>
                <output>${project.build.directory}/generated-sources</output>
            </configuration>
        </execution>
    </executions>
</plugin>

Steps to reproduce

The following is the generated code in the Controller/API. Notice the escaped backslash rather than the expected escaped quotation output.

// Given output
ApiUtil.setExampleResponse(request, "application/json", "\"{\\"testname\\":\\"amy\\"}\"");

// Expected output
ApiUtil.setExampleResponse(request, "application/json", "\"{\"testname\":\"amy\"}\"");

Related issues/PRs

None that we are aware of.

Suggest a fix

We will investigate further and come back to edit this issue.

Start by reproducing the issue with the provided OpenAPI declaration and the openapi-generator Maven Plugin using the Spring generator and interfaceOnly option. Inspect how the example response is converted into generated API/Controller Java code, then verify the generated source compiles and uses the expected escaping shown in the report.