[JAVA] Gson exception when deserializing type=string format=byte to byte[]

Description

When processing a JSON response body containing a field with base64-encoded data (type=string, format=byte), the Gson deserializer throws an the following exception:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 13 path $.content] with root cause

java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 13 path $.content
	at com.google.gson.stream.JsonReader.beginArray(JsonReader.java:351)
	at com.google.gson.internal.bind.ArrayTypeAdapter.read(ArrayTypeAdapter.java:70)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:131)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:222)
	at com.google.gson.Gson.fromJson(Gson.java:932)
	at com.google.gson.Gson.fromJson(Gson.java:897)
	at com.google.gson.Gson.fromJson(Gson.java:846)
	at nl.pompkracht.portal.port.adapter.out.rest.ats.swagger.JSON.deserialize(JSON.java:134)
	at nl.pompkracht.portal.port.adapter.out.rest.ats.swagger.ApiClient.deserialize(ApiClient.java:710)
	at nl.pompkracht.portal.port.adapter.out.rest.ats.swagger.ApiClient.handleResponse(ApiClient.java:913)
	at nl.pompkracht.portal.port.adapter.out.rest.ats.swagger.ApiClient.execute(ApiClient.java:840)
...

Swagger-codegen version

3.0.23

Swagger declaration file content or url

The following schema (part of the declaration file) corresponds to the object that cannot be deserialized. More, specifically, content is the offending field here:

{
	"BinaryContentDTO": {
		"required"  : [
			"content",
			"mediaType",
			"suggestedFileName"
		],
		"type"      : "object",
		"properties": {
			"content"          : {
				"type"  : "string",
				"format": "byte"
			},
			"suggestedFileName": {
				"type": "string"
			},
			"mediaType"        : {
				"type": "string"
			}
		}
	}
}

Command line used for generation

<plugin>
	<groupId>io.swagger.codegen.v3</groupId>
	<artifactId>swagger-codegen-maven-plugin</artifactId>
	<version>3.0.23</version>
	<executions>
		<execution>
			<id>swagger-ats-java</id>
			<goals>
				<goal>generate</goal>
			</goals>
			<configuration>
				<inputSpec>../swagger-docs/ats-api-docs.json</inputSpec>
				<language>java</language>
				<invokerPackage>nl.pompkracht.portal.port.adapter.out.rest.ats.swagger</invokerPackage>
				<apiPackage>nl.pompkracht.portal.port.adapter.out.rest.ats.swagger.api</apiPackage>
				<modelPackage>nl.pompkracht.portal.port.adapter.out.rest.ats.swagger.model</modelPackage>
				<output>target/generated-sources/swagger-ats</output>
				<ignoreFileOverride>/dev/null</ignoreFileOverride>
				<generateApiTests>false</generateApiTests>
				<generateModelTests>false</generateModelTests>
				<configOptions>
					<dateLibrary>java8</dateLibrary>
				</configOptions>
			</configuration>
		</execution>
	</executions>
</plugin>

Steps to reproduce

• Generate Java client from OpenAPI spec with an operation that contains base-64 encoded data in the response body
• Call the relevant method in the generated code
• Verify that the Gson exception occurs

Related issues/PRs

Found an issue with serializing byte[] array types from generated server code, but this applies to consuming base64 data from generated client code, so not really related I think

Suggest a fix/enhancement

Was able to work around the issue by adding a JsonDeserializer type adapter for byte arrays to gson, like this:

ApiClient apiClient = new ApiClient();
var serializer = apiClient.getJSON();
serializer.setGson(serializer.getGson().newBuilder()
		.registerTypeAdapter(byte[].class, new ByteArrayDeserializer())
		.create()
);

....

private static class ByteArrayDeserializer implements JsonDeserializer<byte[]> {
	@Override
	public byte[] deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
		return Base64.getDecoder().decode(json.getAsString());
	}
}

Start by reproducing the response deserialization path through JSON.deserialize and ApiClient.deserialize, then inspect how Gson's ArrayTypeAdapter handles the byte[] field. Done means a generated Java client can deserialize a type=string, format=byte response without the reported exception, while preserving normal byte-array handling.