[BUG] [Java] [java-inflector] Crash happen when generating java-inflector code on Windows

Created by: chenqping

Description

when generating java server stub code with java-inflector on Windows platform, both NPM package and the jar crashed with the following exception:

	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:715)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:1008)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:428)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)
Caused by: java.lang.StringIndexOutOfBoundsException: String index out of range: -1
	at java.lang.String.substring(String.java:1967)
	at org.openapitools.codegen.languages.JavaInflectorServerCodegen.apiFilename(JavaInflectorServerCodegen.java:214)
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:635)
	... 4 more

Process finished with exit code 1

openapi-generator version

4.3.1 and master branch

OpenAPI declaration file content or url

openapi: "3.0.0"
info:
  version: 1.0.0
  title: Swagger Petstore
  license:
    name: MIT
servers:
  - url: http://petstore.swagger.io/v1
paths:
  /pets:
    get:
      summary: List all pets
      operationId: listPets
      tags:
        - pets
      parameters:
        - name: limit
          in: query
          description: How many items to return at one time (max 100)
          required: false
          schema:
            type: integer
            format: int32
      responses:
        '200':
          description: A paged array of pets
          headers:
            x-next:
              description: A link to the next page of responses
              schema:
                type: string
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Pets"
        default:
          description: unexpected error
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
    post:
      summary: Create a pet
      operationId: createPets
      tags:
        - pets
      responses:
        '201':
          description: Null response
        default:
          description: unexpected error
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
  /pets/{petId}:
    get:
      summary: Info for a specific pet
      operationId: showPetById
      tags:
        - pets
      parameters:
        - name: petId
          in: path
          required: true
          description: The id of the pet to retrieve
          schema:
            type: string
      responses:
        '200':
          description: Expected response to a valid request
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Pets"
        default:
          description: unexpected error
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
components:
  schemas:
    Pet:
      required:
        - id
        - name
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
        tag:
          type: string
    Pets:
      type: array
      items:
        $ref: "#/components/schemas/Pet"
    Error:
      required:
        - code
        - message
      properties:
        code:
          type: integer
          format: int32
        message:
          type: string

Command line used for generation

java -jar openapi-generator-cli.jar generate -g java-inflector -i petstore-v3.0.yaml -o java-inflector

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

After investigating the code, I found this was a problem of the file separator differece between Linux and Windows, so I substituted File.separator for "/" in some files.