code: protected void initJsonWriter() {
  		String writerList = config.getJsonWriters();
  		if (writerList != null) {
  			String[] writers = writerList.split(",");
  			Set<String> supportedWriters = TreeWriterRegistry.getWritersByFormat("json");
  			TreeWriter selectedWriter = null;
  			for (String writer : writers) {
  				writer = writer.trim().toLowerCase();
  				if (!writer.isEmpty()) {
  					for (String supportedWriter : supportedWriters) {
  						int i = supportedWriter.lastIndexOf('.');
  						if (i > -1) {
  							supportedWriter = supportedWriter.substring(i + 1);
  						}
  						if (supportedWriter.toLowerCase().contains(writer)) {
  							selectedWriter = TreeWriterRegistry.getWriter(supportedWriter);
  							logger.info("Default JSON serializer/writer is \"" + selectedWriter.getClass() + "\".");
  							TreeWriterRegistry.setWriter("json", selectedWriter);
  							break;
  						}
  					}
  					if (selectedWriter != null) {
  						break;
  					}
  				}
  			}
  		}
  	}
docs: Set global JSON writer API (Jackson, Gson, Boon, FastJson, etc.)