What is the best backend-id-conversion for composite primary key

Question

I'm trying to use BackendIdConverter for my entities and repositories.

For those entities with compound primary keys, I'm using JSON encoding/decoding.

The uri is ugly.

{
  "_embedded": {
    "...": [
      {
        "id": {
          "x": 34,
          "y": "B"
        },
        "_links": {
          "self": {
            "href": "http://localhost:8080/some/%7B%22x%22:34,%22y%22:%22B%22%7D"
          },
          ...
        }
      },
      ...
    ]
  },
  ...
}

The %7B%22x%22:34,%22y%22:%22B%22%7D is encoded value of {"x":34,"y":"B"}.

What is the best(simplest/readable) encoding/decoding method for a POJO, safe for a path segment?