How to update a nested JSON object in JSON file using Java?

Question

I have a JSON file containing an array of User objects. Each User object contains a Goals array containing objects and within each Goal object is a Milestone array containing 1 to many milestone objects.

I was able to utilize JSON-Simple to parse them with ease and store the information in the form of Java objects, but I'm finding it challenging to re-write certain blocks in the JSON file or append to one of the arrays. The examples I found online are too simplistic and didn't account for this much nesting.

If I wanted to rewrite a Goal object in my JSON file or a particular Milestone in the array of milestones, how would I go about updating the file based on the attributes assigned in their corresponding Java object models? Is it possible to modify/append a particular block in the JSON text file within one of the arrays?

[
{       
    "username": "example",
    "password": "pwd",  
    "Goals":[
        {   
            "goalId": "G0001",
            "goalName": "COEN 275 Midterm",
            "startDate": "02/14/2017",
            "endDate": "02/28/2017",
            "numMilestones": 5,
            "color": "#4286f4",
            "hoursPerDay": 4,
            "milestones":[
                {   
                    "milestoneName": "Chapter 1",
                    "Difficulty": "E",
                    "dueDate": "02/18/2018",
                    "completed": true
                },
                {   "milestoneName": "Chapter 2",
                    "Difficulty": "E",
                    "dueDate": "02/22/2018",
                    "completed": true
                },
                {   "milestoneName": "Chapter 3",
                    "Difficulty": "M",
                    "dueDate": "02/23/2018",
                    "completed": true
                },
                {   "milestoneName": "Chapter 4",
                    "Difficulty": "M",
                    "dueDate": "02/25/2018",
                    "completed": false
                },
                {   "milestoneName": "Chapter 5",   
                    "Difficulty": "H",
                    "dueDate": "02/28/2018",
                    "completed": false
                }
            ]
        }
    ]
},
{       
    "username": "username",
    "password": "pwd",  
    "Goals":[
        {   
            "goalId": "G0001",
            "goalName": "COEN 279 Midterm",
            "startDate": "02/14/2017",
            "endDate": "02/28/2017",
            "numMilestones": 5,
            "color": "#4286f4",
            "hoursPerDay": 4,
            "milestones":[
                {   
                    "milestoneName": "Chapter 5",
                    "Difficulty": "E",
                    "dueDate": "02/18/2018",
                    "completed": true
                },
                {   "milestoneName": "Chapter 6",
                    "Difficulty": "E",
                    "dueDate": "02/22/2018",
                    "completed": true
                },
                {   "milestoneName": "Chapter 7",
                    "Difficulty": "M",
                    "dueDate": "02/23/2018",
                    "completed": true
                },
                {   "milestoneName": "Chapter 11",
                    "Difficulty": "M",
                    "dueDate": "02/25/2018",
                    "completed": false
                },
                {   "milestoneName": "Chapter 12",  
                    "Difficulty": "H",
                    "dueDate": "02/28/2018",
                    "completed": false
                }
            ]
        }
    ]
}
]

Jose da Silva Gomes · Accepted Answer · 2018-03-02 13:37:28Z

5

Arrays in JSON simple are classes of the type JSONArray (this is just a java util ArrayList), so once you get the array and the object (this is a java util HashMap) you can modify it.

JSONArray users = (JSONArray) JSONValue.parse(json);
JSONObject firstUser = (JSONObject) users.get(0);

JSONArray firstUserGoals = (JSONArray) firstUser.get("Goals");
JSONObject firstUserFirstGoal = (JSONObject) firstUserGoals.get(0);

JSONArray firstUserFirstGoalMilestones = (JSONArray) firstUserFirstGoal.get("milestones");
JSONObject firstUserFirstGoalFirstMilestone = (JSONObject) firstUserFirstGoalMilestones.get(0);

So you can edit a milestone

firstUserFirstGoalFirstMilestone.put("milestoneName", "Chapter 0");

You can create a new milestone

JSONObject milestone1 = new JSONObject();
milestone1.put("milestoneName", "Chapter 1");
milestone1.put("Difficulty", "H");
milestone1.put("dueDate", "02/18/2018");
milestone1.put("completed", true);

Then add it to the milestones list, at position 1. If no index is specified is added at last.

firstUserFirstGoalMilestones.add(1, milestone1);

Finally you can get the JSON as string.

String finalJson = users.toJSONString();

Or write to file (exceptions should be handled)

Writer writer = new FileWriter(path);
users.writeJSONString(writer);
writer.close();

edited Mar 2, 2018 at 13:37

answered Mar 2, 2018 at 4:39

Jose da Silva Gomes

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7 Comments

btrballin Over a year ago

Bro, I am not worried about modifying the JSONObject itself, but having those updates be reflected in the JSON text file

Jose da Silva Gomes Over a year ago

How is the JSON stored, in your file system?

Jose da Silva Gomes Over a year ago

In case it is a text file in your file system you just write to the file again. I've just update de answer.

btrballin Over a year ago

Hi, this worked great. On a side note, how would I modify a specific attribute rather than appending to a JSON array? If you are able to provide a solution for that, that would answer all my questions and I can mark it as the accepted answer. Also the JSON file is simply stored in a different package.

Jose da Silva Gomes Over a year ago

Yes that's correct, if the key does not exists, it is created with the specified value, if the key already exists then the value is updated. You can also delete keys with the method remove(String name)

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