@thorben NICE CATCH! I started to look at scanner
but had never tried a implementation.
Working code:
var Scanner= Java.type("java.util.Scanner");
scannerResource = new Scanner(resource, "UTF-8");
var content = scannerResource.useDelimiter("\\Z").next();
scannerResource.close();
So complete working code looks like:
var processDefinitionId = execution.getProcessDefinitionId();
var deploymentId = execution.getProcessEngineServices().getRepositoryService().getProcessDefinition(processDefinitionId).getDeploymentId();
var resource = execution.getProcessEngineServices().getRepositoryService().getResourceAsStream(deploymentId, 'content.txt');
var Scanner= Java.type("java.util.Scanner");
scannerResource = new Scanner(resource, "UTF-8");
var content = scannerResource.useDelimiter("\\Z").next();
scannerResource.close();
Tested with Sync and Async and it works.