io.swagger.v3.core.util.Json public static method mapper() is not thread safe

[walkingbear]

The public static method mapper() like below is not thread safe. It appears that the "mapper" field is designed to be lazily initialized and only initialize once. But current implementation does not guarantee the mapper field is initialized only once. If we want the lazy and only once initialization semantic, we need to use double checked locking or simply synchronize the mapper() method, otherwise, the mapper has chance to be initialized multiple times.

current implementation:

    private static ObjectMapper mapper;

    public static ObjectMapper mapper() {
        if (mapper == null) {
            mapper = ObjectMapperFactory.createJson();
        }
        return mapper;
    }

suggested implementation using double checked locking

public static ObjectMapper mapper() {
        if (mapper == null) {
            synchronized(Json.class) {
                if (mapper == null) {
                    mapper = ObjectMapperFactory.createJson();
                }
        }
}
        

[walkingbear]

Closing this and opened swagger-api/swagger-core#4672 instead.