I created a test project to try and reproduce the issue.
Used Mx 7.23.5 and 8.0.0 with AdoptOpenJDK\jdk-126.96.36.199-hotspot and Eclipse Version: Kepler Service Release 2 Build id: 20140224-0627.
Created and entity with 1 string attribute.
Created a java action with 2 input parameters:
Created a simple JSON structure:
From the structure created the import mapping
And with the following code imported the data from the string via the mapping:
// BEGIN USER CODE final InputStream inputStream = new ByteArrayInputStream(jsonStr.getBytes(StandardCharsets.UTF_8)); List<IMendixObject> mendixObjects = Core.integration().importStream(getContext(), inputStream, importmap, null, false); return true; // END USER CODE
This works fine, so I think the issue that you are experiencing should have something to do with the json being supplied to the java action rather than the jdk or Eclipse version. I would investigate this via a simple microflow in which you create a string variable with your json and perform a import activity with the import_mapping and see if that will also fail or not.