How to create a Java API to pass-through a stream of json lines from another source

I would like to create a Java Spring Boot project which offers a REST API to a client. A request to the API will be forwarded (with some changes) to a database. I would like to receive the response from the db and forward it to the client.

I know how to do this for a simple request where I get a single result from the database. But the db returns a stream of json lines and I would like to pass it through to the client directly, as a stream. So I do not want to collect the stream until it ends (storing it in memory) and then sent it to the client. I may also want to apply some conversion or filtering to every single json line. A buffer that stores a few lines as a cache would be ok.

If I understand correctly, you want to return one row at a time from a ResultSet - Spring Boot offers several mechanisms to do this, see - https://www.baeldung.com/spring-mvc-sse-streams

A simple example, using StreamingResponseBody to write the database rows as a JSON array, might look something like:

@Controller
public class SomeController
{
  @Autowired
  private final DataSource dataSource;

  @GetMapping
  public ResponseEntity<StreamingResponseBody> handleGet(@RequestParam("someParam") int someParam) 
  {
    return new ResponseEntity(query(someParam), HttpStatus.OK);
  }

  private StreamingResponseBody query(int someParam)
  {
    return out -> 
    {
      out.write((byte)'[');

      try (
        var connection = dataSource.getConnection();
        var preparedStatement = prepareStatement(connection, someParam);
        var resultSet = preparedStatement.executeQuery()
      )
      {
         while (resultSet.next())
         {
            out.write(resultSet.getBytes("some_column_containing_json")); 
            if (!resultSet.isLast()) out.write((byte)',');
         }
      }
      finally
      {
        out.write((byte)']');
      }
    };
  }

  private PreparedStatement prepareStatement(Connection connection, int someParam)
  {
     var preparedStatement = connection.prepareStatement("SELECT some_column_containing_json FROM some_table WHERE some_other_column = ?");
     preparedStatement.setInt(1, someParam);

     return preparedStatement;
  }
}