JSONObject to String返回null（使用有效的JSON）

Question

我面临以下困难： 我创建了一个JSONObject。 使用System.out.println (finalJson)我得到下面的输出。根据{{​​3}}，它是有效的JSON。但是当我尝试通过调用finalJson.toString ()来获取String表示时，它只返回null。怎么可能？我在这里缺少什么？

{
    "id": "cell: chris-VirtualBoxCell01",
    "name": "cell: chris-VirtualBoxCell01",
    "data": {},
    "children": [
        {
            "id": "node: chris-VirtualBoxNode01",
            "name": "node: chris-VirtualBoxNode01",
            "data": {},
            "children": [
                {
                    "id": "applicationServer: C4Cluster_server1",
                    "name": "applicationServer: C4Cluster_server1",
                    "data": {},
                    "children": []
                },
                {
                    "id": "nodeAgent: nodeAgent",
                    "name": "nodeAgent: nodeAgent",
                    "data": {},
                    "children": []
                }
            ]
        },
        {
            "id": "deploymentManagerNode: chris-VirtualBoxCellManager01",
            "name": "deploymentManagerNode: chris-VirtualBoxCellManager01",
            "data": {},
            "children": [
                {
                    "id": "deploymentManager: deploymentManager",
                    "name": "deploymentManager: deploymentManager",
                    "data": {},
                    "children": []
                }
            ]
        }
    ]
}

以下是应该感兴趣的片段：

//Get JSON from Grails domain model
JSON json = ServerNode.findAll ("from ServerNode as s where s.root=true") as JSON;
//check result
System.out.println (json);
//turn into a JSONObject by chopping off surrounding []
String jsonString = json.toString ().substring (1, json.toString ().length ()-1);
//check result
System.out.println (jsonString);
//render to JSONObject
JSONObject jObject = new JSONObject (jsonString);

//modify structure a little more        
JSONObject finalJson = modifyJson (jObject);

//check the final form (produces the above output)  
System.out.println (finalJson);

//turn into string and print --> returns null       
String jString = finalJson.toString ();
System.out.println (jString);