I am trying to create a report from a JSON file with multiple subreports included (subreports are using different connections than the masterreport and also each other). I can create the subreports properly individually but I can't create the master report (it is rendering the subreport but not getting the data, showing me nulls instead of the data). I guess I couldn't configure the dataSources' of the subreports, my question is how can I add the dataSource that points to a json file? (subreports' dataSource is different than the masterReport's dataSource)
You can specify a data adapter for an individual data set by setting the net.sf.jasperreports.data.adapter property on each data set.
For JasperReports Server, you would deploy each data adapter as a file, and set up the Report Unit as per https://community.jaspersoft.com/wiki/how-deploy-and-use-data-adapter-jasperreports-server-v56 which still works in JasperReports Server 6.X and 7.X.