[REQ] Java Records (JEP 395) support

[lpandzic]

Option to generate java models as records.

[spacether]

JEP 39 is defined here:
https://openjdk.java.net/jeps/395

[hgjd]

It could even be a sensible default when using the Spring generator with useSpringBoot3, as that requires Java 17.
The model cannot be an enum or extend another class. Am I missing any other limitations?

[fabiofranco85]

It would be amazing if we got this improvement - specially when we set useSpringBoot3
One question though is: will the records DTOs cope well with swagger annotations?

[Kinae]

Plus one on this

[Aryesia]

Plus one

[grubeninspekteur]

Please don't write "plus one", it clutters the issue and generates notifications for subscribers. You can "thumbs up" the initial description.

---

I would really like to help implementing this feature, but I am overwhelmed after looking into the pojo mustache template. There seem to be so many special cases to consider (Jackson nullables for example).

[GlobeDaBoarder]

Intro

So, this issue caught my attention a few months ago. And recently I started working on setting up a new project with OpenAPI generator and decided to give generating records as models a try.

I'm excited to share that this endeavor has been largely successful, and our project now leverages the immutable nature of records for its DTOs.

I'm here to share my findings, solutions, and templates with you. It's important to note, however, that while this approach has worked well for us, it might not be universally suitable, particularly due to some inherent limitations of records. In my experience, this strategy shows the most promise for simpler APIs, where the benefits of records truly shine.

Before diving into the solutions and templates, let's first unpack the limitations of records. Understanding these constraints is crucial for evaluating whether records are the right fit for your API development needs

Limitations:

1. Records do not allow inheritance:

OpenAPI spec allows inheritance in the API and so does the default template used by openapi generator for spring for example. Since records are technically a very special type of class that is final and immutable, it also extends the java. lang. Record class. Due to Java's nature of "single inheritance" (it can extend only one class), creating inheritance models with records can not be achieved.

I believe there is no good workaround for that in records. Of course, records can still implement interfaces and we could do some logical coupling using interfaces but it is not really the same as using inheritance. Additionally, from what I tested out SpringDoc doesn't really work well with picking up interface getter methods as fields of models, so as a result your API will end up having a model that looks like '{}' in SwaggerUI and I don't think this is what you would want.

Another possible workaround is using composition. This workaround is also not ideal since you will have some sort of nested parent object in your model and JSON as well.

So, if inheritance in your API is what you need, I'm afraid records are not the way to go and there is no workaround. Use classes, be it a default template provided by the generator out of the box or some other one that supports inheritance generation.

2. Records do not allow default values (there is a workaround though):

OpenAPI spec has a default: keyword which poses certain implementation challenges in records. It's not as straightforward as in classes and requires a certain workaround with a constructor. An example of that could be something like this:

public record RecordWithDefault(
        String withDefaultValue,
        String normalRecordField
) {

    @Builder
    public RecordWithDefault(String withDefaultValue, String normalRecordField) {
        this.withDefaultValue = Objects.requireNonNullElse(withDefaultValue, "default");
        this.normalRecordField = normalRecordField;
    }
}

This would technically work and although perhaps a bit messy, some could consider it still a good solution due to records advantage of immutability over classes.

But, remember that records are meant to be simple and immutable data carriers. It is kind of the core philosophy behind them. Of course, we can do stuff like this. But, perhaps at that point, you could be better off using just simple classes if default values are what you need in your API. A class solution like this looks much cleaner and more intuitive:

@Builder
@Data
public class ClassWithDefault {
    @Builder.Default
    private String withDefaultValue = "default";
    private String normalRecordField;
}

Generally, it all boils down to whether you want to sacrifice the simplicity of records and pollute them with this "default through constructor" approach but retain immutability, or go for a simpler class approach, although it potentially being more problematic due to its mutable nature

So far, due to the complexity of this workaround, I have not gotten to implement it in my template file. Although it seems like it is technically possible I will leave it as "an idea for future improvement" type of thing.

3. JsonNullable (jackson-databind-nullable) wrapper:

As mentioned by @grubeninspekteur, is indeed a tough case to consider.

Before we dive deeper into implementation details though, I feel like it is very important to mention and to fully understand how JsonNullable works. The underlying idea behind JsonNullable is that unlike Optional or just null it can represent 3 states:

1) Defined with a Non-Null Value:

• This state occurs when JsonNullable wraps a non-null value. It represents a field that is explicitly set with a specific value. For instance, if you have a JSON field age and it is set to 30, JsonNullable would wrap this value as JsonNullable.of(30).
• In code: JsonNullable<String> jsonValue = JsonNullable.of("Hello");
• In JSON: { "jsonValue": "Hello" }

2) Defined with a Null Value:

• In this state, JsonNullable explicitly wraps a null value. This is different from a field not being present at all; it's a clear indication that the field is set but its value is null. This is particularly important in APIs where setting a field to null might have specific implications, such as clearing a value.
• In code: JsonNullable<String> jsonValue = JsonNullable.of(null);
  In JSON: { "jsonValue": null }

3) Undefined:

• The third state is when JsonNullable represents an undefined or non-existent field. This is used to indicate that a field is not included in the JSON payload at all. It's neither null nor set to any value. This state is crucial for partial updates in APIs, where the absence of a field means "leave unchanged", while null might mean "clear the value".
• In code: JsonNullable<String> jsonValue = JsonNullable.undefined();
• In JSON: {} (the field jsonValue does not appear in the JSON)

Here's a snipped from the documentation describing that:

The JsonNullable wrapper shall be used to wrap Java bean fields for which it is important to distinguish between an explicit "null" and the field not being present. A typical usage is when implementing Json Merge Patch where an explicit "null"has the meaning "set this field to null / remove this field" whereas a non-present field has the meaning "don't change the value of this field".

The default generation template approaches this 3-state scenario in a smart way. If the field is said to be nullable: in the OpenAPI spec, here's what get's generated:

Field:

private JsonNullable<String> nullableField = JsonNullable.<String>undefined();

Fluent setter:

public VisibilityObject nullableField(String nullableField) {
    this.nullableField = JsonNullable.of(nullableField);
    return this;
}

Getter:

@Schema(name = "nullableField", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
@JsonProperty("nullableField")
public JsonNullable<String> getNullableField() {
    return nullableField;
}

This is a smart way to stick to this 3-way logic because:

• If we NEVER call a fluent setter: the field remains as JsonNullable.undefined(). This sets isPresent to false and the underlying object is null. This means that the field doesn't even really exist and can be omitted from JSON
• If we call a fluent setter with null : isPresent set to true. But the underlying object will be null.
• If we call a fluent setter with some actual string text: the underlying String object will contain text supplied and isPresent will be set to true.

From what I understand, achieving the 3-states of JsonNullable in records is impossible..

In records, it's impossible to achieve the three distinct states of JsonNullable effectively. This limitation arises because you can't set a field's default value in a record to JsonNullable.undefined() directly. Moreover, using constructors to create default values typically involves null checks, which doesn't allow for distinguishing between JsonNullable being explicitly set to null, having a non-null value, or being undefined.

If we try something like this:

public record RecordWithJsonNullable(
        String normalValue,
        JsonNullable<String> jsonNullableValue
) {
    @Builder
    public RecordWithJsonNullable(String normalRecordField, String JsonNullableValue) {
        this(
                normalRecordField,
                Objects.requireNonNullElse(JsonNullable.of(JsonNullableValue), JsonNullable.undefined())
        );
    }
}

Or this:

public record RecordWithJsonNullable(
        String normalValue,
        JsonNullable<String> jsonNullableValue
) {
    @Builder
    public RecordWithJsonNullable(String normalRecordField, String JsonNullableValue) {
        this(
                normalRecordField,
                JsonNullable.of(JsonNullableValue)
        );
    }
}

We are sacrificing one of the states with such construction trickery. In the first case, our jsonNullableValue can never be .of(null) and in the second case it can never be undefined().

So, if you want to be using JsonNullable you are better off using the default template or some other templates that are based on classes.

To conclude the limitations of using records as models for OpenAPI Springn generator:

• inheritance can not be achieved
• default values can be achieved but involve some not-very-pretty-looking constructor trickery
• JsonNullable can not be implemented due to records' immutable nature and no default values for fields

Why Records could still be a reasonable approach for some APIs

At this point after reading through all of the limitations you might be wondering: "Why even try to use records at this point?"

Well, I want you to think about a few things in regard to your API:

• Do I need inheritance in my API?
• Do I need default values in the DTOs of my API?
• Do I really need JsonNullable?

While the first two questions usually have clear answers based on your business requirements, the necessity of JsonNullable might not be as intuitive. As previously mentioned, JsonNullable is crucial for JSON Merge Patch to differentiate between three states. However, consider if JSON Merge Patch is necessary for your API. Alternative RESTful solutions for updating a resource include:

• PUT for Full Updates: Supplying a full DTO in the request body to update every field.
• JSON PATCH (RFC6902): Using special operations syntax for patch updates.
• PATCH with Full Body for Partial Updates: Submitting the whole body but changing only altered fields on the server side.
• Field-Selective Updates/PATCH with Special DTO: Updating only the fields specified in a DTO containing a subset of fields.
• Separate PATCH Request for Each Field: Suitable for highly granular updates on not overly large resources.
• Query Parameter Updates: Ideal for simple scenarios with straightforward update logic.
• Use of gRPC or GraphQL: These architectures often simplify handling partial updates.

If any of these alternatives align with your API's requirements, you might not need JsonNullable and could consider using records as DTOs, leveraging their immutability and reduced boilerplate for simpler API structures."

How to generate records:

1. template files:

In order to generate models/DTOs as records we will have to place 2 custom template files in our /resources/templates folder:

• pojo.mustache -- this is an actual model generation template. In our case, it will be a template file responsible for generating records as models.

/**
 * {{description}}{{^description}}{{classname}}{{/description}}{{#isDeprecated}}
 * @deprecated{{/isDeprecated}}
 */
{{>additionalModelTypeAnnotations}}
{{#description}}
{{#isDeprecated}}
@Deprecated
{{/isDeprecated}}
{{#swagger2AnnotationLibrary}}
@Schema({{#name}}name = "{{name}}", {{/name}}description = "{{{description}}}"{{#deprecated}}, deprecated = true{{/deprecated}})
{{/swagger2AnnotationLibrary}}
{{/description}}
{{#discriminator}}
{{>typeInfoAnnotation}}
{{/discriminator}}
{{#jackson}}
{{#isClassnameSanitized}}
{{^hasDiscriminatorWithNonEmptyMapping}}
@JsonTypeName("{{name}}")
{{/hasDiscriminatorWithNonEmptyMapping}}
{{/isClassnameSanitized}}
{{/jackson}}
{{>generatedAnnotation}}
{{#vendorExtensions.x-class-extra-annotation}}
{{{vendorExtensions.x-class-extra-annotation}}}
{{/vendorExtensions.x-class-extra-annotation}}
@Builder(toBuilder = true)
public record {{classname}} (
{{#vars}}
	{{#isNullable}}
	@Nullable()
	{{/isNullable}}
        {{#useBeanValidation}}
        {{>beanValidation}}
        {{/useBeanValidation}}
        {{^useBeanValidation}}
        {{#required}}@NotNull{{/required}}
        {{/useBeanValidation}}
        {{#deprecated}}
        @Deprecated
        {{/deprecated}}
	{{#swagger2AnnotationLibrary}}
	@Schema(name = "{{{baseName}}}"{{#isReadOnly}}, accessMode = Schema.AccessMode.READ_ONLY{{/isReadOnly}}{{#example}}, example = "{{{.}}}"{{/example}}{{#description}}, description = "{{{.}}}"{{/description}}{{#deprecated}}, deprecated = true{{/deprecated}}, requiredMode = {{#required}}Schema.RequiredMode.REQUIRED{{/required}}{{^required}}Schema.RequiredMode.NOT_REQUIRED{{/required}})
        {{/swagger2AnnotationLibrary}}
        {{#jackson}}
        @JsonProperty("{{baseName}}")
        {{/jackson}}
        {{#isDate}}
        @DateTimeFormat(iso = DateTimeFormat.ISO.DATE)
        {{/isDate}}
        {{#isDateTime}}
        @DateTimeFormat(iso = DateTimeFormat.ISO.DATE_TIME)
        {{/isDateTime}}
        {{#vendorExtensions.x-extra-annotation}}
        {{{vendorExtensions.x-extra-annotation}}}
        {{/vendorExtensions.x-extra-annotation}}
        {{{datatypeWithEnum}}} {{name}}{{#-last}}{{/-last}}{{^-last}},{{/-last}}
{{/vars}}
) {
}

You can also find it here, in my little demo GitHub repo I made

• model.mustahce primarily includes imports and references pojo.mustache. In our case, this removes a few imports from the standard model template file and adds Lombok's @Builder annotation for the sake of convenience.

package {{package}};

import lombok.Builder;
{{#imports}}import {{import}};
{{/imports}}
{{#openApiNullable}}
import jakarta.annotation.Nullable;
{{/openApiNullable}}
{{#useBeanValidation}}
import {{javaxPackage}}.validation.Valid;
import {{javaxPackage}}.validation.constraints.*;
{{/useBeanValidation}}
{{^useBeanValidation}}
import {{javaxPackage}}.validation.constraints.NotNull;
{{/useBeanValidation}}
{{#performBeanValidation}}
import org.hibernate.validator.constraints.*;
{{/performBeanValidation}}
{{#jackson}}
{{/jackson}}
{{#swagger2AnnotationLibrary}}
import io.swagger.v3.oas.annotations.media.Schema;
{{/swagger2AnnotationLibrary}}

{{^parent}}
{{#hateoas}}
import org.springframework.hateoas.RepresentationModel;
{{/hateoas}}
{{/parent}}

import java.util.*;
import {{javaxPackage}}.annotation.Generated;

{{#models}}
{{#model}}
{{#additionalPropertiesType}}
import java.util.Map;
import java.util.HashMap;
import com.fasterxml.jackson.annotation.JsonAnyGetter;
import com.fasterxml.jackson.annotation.JsonAnySetter;
{{/additionalPropertiesType}}
{{#isEnum}}
{{>enumOuterClass}}
{{/isEnum}}
{{^isEnum}}
{{#vendorExtensions.x-is-one-of-interface}}{{>oneof_interface}}{{/vendorExtensions.x-is-one-of-interface}}{{^vendorExtensions.x-is-one-of-interface}}{{>pojo}}{{/vendorExtensions.x-is-one-of-interface}}
{{/isEnum}}
{{/model}}
{{/models}}

You can find this one here:

2. Known issues with the template (as of post date):

Functionally, there are no issues I have discovered yet, there are however a few aesthetic/formatting issues:

• So far there is a little issue that bugs me a lot but that I can't wrap my head around yet and it is in the pojo.mustache template. It has to do purely with formatting. I can not figure out how to consistently have a single line break after each row. So far the generated records can have 0-2 rows in between fields which is a bit annoying and aesthetically unpleasing. If you know where the corresponding issue in the template is, please let me know.
• I don't like that bean validation annotations like @Pattern or @Email or @Size or whatever are all generated into one line. Would like to have them generated each on a new line.

Those little issues I will try to get to someday later.

3. Plugin configuration:**

Make sure to include <templateDirectory>src/main/resources/templates</templateDirectory> and you're pretty much good to go

If you want to see a more detailed configuration of the plugin, feel free to look at the pom.xml file in my repository.

That's it!

Conclusions

To sum up, while records are not a one-size-fits-all solution for every API and use case, they can be quite useful, particularly for certain scenarios and simpler APIs, thanks to their immutable nature. This was the motivation behind my work on these templates. As always, I encourage you to consider your specific needs, feel free to experiment, and don't hesitate to reach out with any questions about the templates I've created.

If you think I've overlooked certain limitations or made an error in this discussion, please let me know—let's make this a collaborative effort.

For future improvements, I'm considering exploring the implementation of default values through constructors, as it seems technically feasible, as well as trying to figure out line formatting issues. Your input or interest in this feature would be greatly appreciated, so do let me know if this is something you'd find useful.

I apologize for the length of this comment, but I hope it proves helpful in your decision-making process regarding the use of records in API development. Thank you for reading, and I look forward to your thoughts and contributions!

[spacether]

Another reason that Java Records would not fully support json schema object payloads is that they do not support properties where invalid java names are used. If one built the model by implementing the Map interface, those invalidly named keys would be preserved and one could write getters for them.
That's what I do in my Java implementation.

[Chrimle]

This is a matter of overriding .mustache template files to generate Java records instead of Java classes.

Here are some templates which generates Java records (which does not use Lombok):
openapi-to-java-records-mustache-templates. It is continuously being updated to support more configurations.

[lmebonia]

Plus one

[datoodvali]

+1