[Bug]: Valid YAML doc fails to parse when starting with a curly brace

[RafeArnold]

Description

When a valid OpenAPI YAML document starts with an opening curly brace {, parsing returns a result with a null OpenAPI and error messages. This affects Swagger 2.0 and OpenAPI 3.0/3.1.

The error is caused by the parser checking to see if the first non-whitespace character of the document is a { and, if it is, assuming the rest of the document is valid JSON and using a JSON specific deserialiser.

Affected Version

2.1.33

Steps to Reproduce

The error can be reproduced by running the following Java code

import io.swagger.parser.OpenAPIParser;
import io.swagger.v3.parser.core.models.ParseOptions;
import io.swagger.v3.parser.core.models.SwaggerParseResult;

import java.util.List;

class Parse {
    public static void main(String[] args) {
        String doc = """
                {
                  openapi: "2.0.0",
                  "info": {
                    "title": "Test API spec",
                    "version": "1.0.0"
                  },
                  "paths": {}
                }
                """;
        var parser = new OpenAPIParser();
        SwaggerParseResult result = parser.readContents(doc, List.of(), new ParseOptions());
        assert result.getOpenAPI() != null;
        assert result.getMessages().isEmpty();
    }
}

Expected Behavior

The assertions in the provided code example succeed.

Actual Behavior

The assertions in the provided code example fail.

Environment

• Java version: Corretto 17
• Build tool: 8.14
• OS: macOS Sequoia 15.6.1

Checklist

• I have searched the existing issues and this is not a duplicate.
• I have provided sufficient information for maintainers to reproduce the issue.

[RafeArnold]

I'm somewhat confused as to why the parser uses a different ObjectMapper depending on which format it thinks the document is in. Maybe this is because YAML 1.1 is not a strict superset of JSON so a JSON specific ObjectMapper is required for some very niche cases?