Auto generate unknown fallback

[habara-k]

Resolve #1537.
#1462

Summary

Previously, in the line-bot-sdk-java, the Unknown class was manually implemented and used as the defaultImpl (fallback) in @JsonTypeInfo. However, with the transition to generating code from OpenAPI definitions, generating the Unknown class directly from the OpenAPI spec became challenging. Consequently, the fallback option was disabled.

This change created an issue when decoding webhooks. The type field in webhooks may receive new values as future features are developed. If an older SDK version encounters these new values, it could crash while parsing the webhook.

This PR aims to re-enable the fallback option by adding code that automatically generates the Unknown class from the Java code produced by OpenAPI, even if the method is a bit rough or not entirely elegant . This ensures that we can maintain the fallback mechanism under OpenAPI management.

[Yang-33]

what if an interface requires implementation class has to have some fields like MessageContent and UnknownMessageContent ?

line-bot-sdk-java/line-bot-webhook/src/main/java/com/linecorp/bot/webhook/model/UnknownMessageContent.java

Lines 19 to 20 in f79e256

	public record UnknownMessageContent(String id) implements MessageContent {
	}

[Yang-33]

It shouldn't compile. though the current patch seems working as of now, but given cases like this, modifying the generator's code might be a better choice.

In the generator's #postProcessModels, it can handle all models, so wouldn't it be better to create an unknown* class (like UnknownMessageContent ) that implements the interface here?

line-bot-sdk-java/generator/src/main/java/com/linecorp/bot/codegen/LineJavaCodegenGenerator.java

Lines 111 to 138 in f79e256

	public ModelsMap postProcessModels(ModelsMap objs) {
	ModelsMap modelsMap = super.postProcessModels(objs);
	for (ModelMap model : modelsMap.getModels()) {
	CodegenModel codegenModel = model.getModel();
	// remove the `type` field from child classes due to remove duplicated type field with `@JsonTypeName`.
	if (codegenModel.parent != null) {
	codegenModel.setAllVars(codegenModel.getAllVars().stream()
	.filter(codegenProperty -> !codegenProperty.name.equals("type"))
	.collect(Collectors.toList()));
	}
	// if the class have a parent, set ` implements ${parent}`.
	// and put @JsonTypeName annotation.
	if (codegenModel.parent != null) {
	addImplements(codegenModel, codegenModel.parent);
	}
	// Implements ReplyEvent if the class has "replyToken" field.
	if (codegenModel.parent != null && codegenModel.parent.equals("Event")) {
	if (codegenModel.vars.stream().anyMatch(codegenProperty -> codegenProperty.name.equals("replyToken"))) {
	addImplements(codegenModel, "ReplyEvent");
	}
	}
	}
	return modelsMap;
	}

(just an idea, probably this doesn't work without modification)

    @Override
    public ModelsMap postProcessModels(ModelsMap objs) {
        ModelsMap modelsMap = super.postProcessModels(objs);

+       List<ModelMap> additionalModels = new ArrayList<>();
        for (ModelMap model : modelsMap.getModels()) {
            CodegenModel codegenModel = model.getModel();

            // remove the `type` field from child classes due to remove duplicated type field with `@JsonTypeName`.
            if (codegenModel.parent != null) {
                codegenModel.setAllVars(codegenModel.getAllVars().stream()
                        .filter(codegenProperty -> !codegenProperty.name.equals("type"))
                        .collect(Collectors.toList()));
            }

            // if the class have a parent, set ` implements ${parent}`.
            // and put @JsonTypeName annotation.
            if (codegenModel.parent != null) {
                addImplements(codegenModel, codegenModel.parent);
            }

            // Implements ReplyEvent if the class has "replyToken" field.
            if (codegenModel.parent != null && codegenModel.parent.equals("Event")) {
                if (codegenModel.vars.stream().anyMatch(codegenProperty -> codegenProperty.name.equals("replyToken"))) {
                    addImplements(codegenModel, "ReplyEvent");
                }
            }

+          // Set additional unknown* class for jackson's defaultImpl to have unknwon* class as fallback
+          if (codegenModel.discriminator != null) {
+              String fallbackModelName = "Unknown" + codegenModel.classname;
+              CodegenModel fallbackModel = new CodegenModel();
+              fallbackModel.name = fallbackModelName;
+              // set some fields 
+              // ...
+              addImplements(fallbackModel, codegenModel.classname);
+              ModelMap fallbackModelMap = new ModelMap();
+              fallbackModelMap.put("model", fallbackModel);
+              additionalModels.add(fallbackModelMap);
            }
        }
+       modelsMap.getModels().addAll(additionalModels);
        return modelsMap;
    }

[habara-k]

@Yang-33
Thank you for the code idea. As I mentioned in the PR description, the previous approach was rough and inelegant, lacking flexibility. I have revised the code based on the draft you provided.