[Java] How can we set List and Map to null by default in models?

[yehor-masalitin]

Description

For example we have model in crud-service:

ModelDto {
 private List<String> list = null;
 private SomeOtherModelDto object = null;
 //get set
}

in swagger spec it will appear as:

"list": {
  "type": "array",
  "items": {
    "type": "string"
  }
}

and then swagger-codegen will generate such dto:

ModelDto {
 private List<String> list = new ArrayList<>();
 private SomeOtherModelDto object = null;

 //get set
}

How can we prevent this?

It is important for us because there is a difference between null list and empty list.

Thank you.

Swagger-codegen version

io.swagger:swagger-codegen-cli:2.2.2

Command line used for generation

generate -i http://SOME_URL -l java -o bin/sdk --artifact-id sdk-name

[ePaul]

The OpenAPI Specification 2.0 (= Swagger Specification 2.0) doesn't allow defining anything as JSON null.
What it allows is to not make it mandatory to include a property in the containing object. This would be done by not listing it in the required list of the containing object.

When mapping this to Java, I would expect a receiver (client for response, server for request) to have the property as null when the value is not passed. A sender would simply not include the null value in the sent object.

I guess the initialization of the list was done to prevent null pointer exceptions when trying to access the object. I would argue that allowing a nullable list property (for non-required properties) would be the better way to go.

[yehor-masalitin]

@ePaul Yes, it is main point, we don't need nulls in json, but this list(in my example) is not required, and json for deserialization just do not contain this field (with list) at all, but after deserialization it is (because of default empty array value) and for us it would be perfect to have nullable list (and Map) property.

[yehor-masalitin]

Any suggestions? @wing328 maybe you can help me?

[yehor-masalitin]

@wing328 I created solution for this: #5341

[yehor-masalitin]

@fehguy Can you please review my pull?

[cbornet]

I think it's not a good idea to add yet another option. I would set null by default.
An empty list could be generated when the field is required though.

[yehor-masalitin]

@wing328 @fehguy @cbornet @ePaul
Hello, I’ll try to explain why this patch is so important for us.

We have RESTfull API service, which supports partial update. For us it means that user can update only 1 field, if he send us only 1 field in JSON.

For example we have model in API

BookDto {
 List<String> autors;
 String name;
}

And if user send JSON:

{
    "name" : "new name"
}

To our API, we will update only name.

But, if we perform this operation via SDK, what will happen:

//User creates dto using sdk library:
BookDto dto = new BookDto();
//(In this dto autors will be set to new ArrayList<String>() by default)

//Then user set name to "new name"
dto.setName("new name")

It means that our API will get following JSON:

{
    
    "autors" : [],
    "name": "new name"
}

And in this case our API will override real values to empty list, and we will lose data.

We cannot implement rules to check null or empty, because in this case it means that we cannot set autors to empty array (if we want so).

In conclusion: we cannot use swagger codgen to generate sdk for our users without this patch.

[wing328]

@egormasalitin thanks for the patch.

I think it's not a good idea to add yet another option. I would set null by default.
An empty list could be generated when the field is required though.

Would that meet your requirement? I think we can easily update the template to use null for optional property and an empty list for required property.

[yehor-masalitin]

@wing328 Hello, thank you a lot for your answer. This solution is better than nothing, but still not exactly what we want, because in case of partial update the problems remains the same.

For example we have same dto, but autors is required field:

BookDto {
 //required
 List<String> autors;
 //optional
 String name;
}

It means that when we will decide to change name only, we will override autors

Can you please tell me why required List or Map cannot be null?

[wing328]

It means that when we will decide to change name only, we will override authors

Let's step back and try to review required and optional. If a field is required, it must be present in the JSON/XML payload (and the RESTful backend is expecting that field in the payload). For your use case, you only want to update name (optional). Can you mark autors as optional instead?

I just want to avoid omitting a required field in the payload, which does not conform to the spec.

[yehor-masalitin]

@wing328 yes, I get it, thank you, I thought about required in another way. Sorry about that. In this case solution:

I think we can easily update the template to use null for optional property and an empty list for required property.

Fits perfectly