[JAVA + okhttp] additionalProperties of type array not properly handled

Description

An additionalProperties of type array is not properly handled by the JavaClientCodegen with okhttp + GSON. See this example:

swagger: '2.0'
info:
  version: 0.0.0
  title: Foo
  
paths:
  /foo1:
    get:
      responses:
        200:
          description: Foo
          schema:
            type: object
            additionalProperties:
              type: array
              items:
                $ref: '#/definitions/Foo'

definitions:
  Foo:
    type: object
    properties:
      foo:
        type: string

This causes the code generator to produce the following class which has no additional field:

public class FooResponse extends HashMap<String, List> {

Note that the type of values in the hashmap is a List where it should have been a List<Foo>. Because of that, GSON doesn't deserialise the values to Foo objects. Instead, it gives just a LinkedHashMap<String, Object>.

If I modify the spec above and create a wrapper definition such as:

swagger: '2.0'
info:
  version: 0.0.0
  title: Foo
  
paths:      
  /foo2:
    get:
      responses:
        200:
          description: Foo
          schema:
            $ref: '#/definitions/FooResponse'

definitions:
  Foo:
    type: object
    properties:
      foo:
        type: string

  FooResponse:
    type: object
    additionalProperties:
      type: array
      items:
        $ref: '#/definitions/Foo'

The generated DefaultApi class now lacks the import of Foo:

package io.swagger.client.api;

import io.swagger.client.ApiCallback;
import io.swagger.client.ApiClient;
import io.swagger.client.ApiException;
import io.swagger.client.ApiResponse;
import io.swagger.client.Configuration;
import io.swagger.client.Pair;
import io.swagger.client.ProgressRequestBody;
import io.swagger.client.ProgressResponseBody;

import com.google.gson.reflect.TypeToken;


import java.io.IOException;


import io.swagger.client.model.FooResponse;

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class DefaultApi {

   // omitted

    /**
     * 
     * 
     * @return Map<String, List<Foo>>
     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
     */
    public Map<String, List<Foo>> foo1Get() throws ApiException {
        ApiResponse<Map<String, List<Foo>>> resp = foo1GetWithHttpInfo();
        return resp.getData();
    }

   // omitted
}

Swagger-codegen version

2.4.5 (latest)

Swagger declaration file content or url

See above.

Command line used for generation

Copy the YAML specs above and paste on https://editor.swagger.io . Then go "Generate Client" > "Java". This is going to produce a Java client that uses the OkHTTP and GSON libraries.

Steps to reproduce

Explained above

Related issues/PRs

Not quite sure if this one is related: https://github.com/swagger-api/swagger-codegen/issues/6884

Suggest a fix/enhancement

Adding import {{modelPackage}}.*; to Java/libraries/okhttp-gson/api.mustache "fixes" the problem with the import.

I managed to get FooResponse to be generated as a HashMap<String, ArrayList<Foo>> by making the following change (hack):

diff --git a/modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultCodegen.java b/modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultCodegen.java
index 096926470..e43fd30ec 100644
--- a/modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultCodegen.java
+++ b/modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultCodegen.java
@@ -994,8 +994,15 @@ public class DefaultCodegen {
                 LOGGER.error("No Type defined for Additional Property " + additionalProperties2 + "\n" //
                         + "\tIn Property: " + p);
             }
-            String inner = getSwaggerType(additionalProperties2);
-            return instantiationTypes.get("map") + "<String, " + inner + ">";
+
+            String additionalPropInstType = toInstantiationType(additionalProperties2);
+
+            if (additionalPropInstType != null) {
+                return instantiationTypes.get("map") + "<String, " + additionalPropInstType + ">";
+            } else {
+                String inner = getSwaggerType(additionalProperties2);
+                return instantiationTypes.get("map") + "<String, " + inner + ">";
+            }
         } else if (p instanceof ArrayProperty) {
             ArrayProperty ap = (ArrayProperty) p;
             String inner = getSwaggerType(ap.getItems());

All tests pass with both changes, but given that I'm not familiar with the codebase I don't feel confident to open a PR for this.

Start in modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultCodegen.java and the Java/libraries/okhttp-gson/api.mustache template, then run the supplied Swagger specs through Java okhttp-gson generation. Done means array-valued additionalProperties preserve the Foo generic type and generated APIs include the required model import.