Java - 用 Jackson 写入 Json

Question

I'm currently doing a project for school and I'm trying to write an object at the end of my json file (in the cards attribute) without rewriting everything using the Jackson library.

The problem is that when I try to do this my object is written correctly, but it is written at the end of the file, and I try to put it in the list of cards [ THERE ].

someone have an idea? Thanks.

EDIT: I already know how to write to a file by rewriting it, for answer to my teacher's instructions I need to do it without rewriting the file.

My Json File:

{

  "cards": [
    {
      "subject": "The Earth",
      "questions": [
        {
          "challenge": "What is the highest mountain of the world?",
          "answer": "Everest"
        },
        {
          "challenge": "What is the largest ocean in the world?",
          "answer": "Pacific Ocean"
        }
      ],
      "author": "Roger",
      "theme": "IMPROBABLE"
    },

    {
      "subject": "Holidays",
      "questions": [
        {
          "challenge": "What is the most touristic country in the world?",
          "answer": "France"
        },
        {
          "challenge": "In 2019, how many pictures did vacationers take per day?",
          "answer": "55"
        }
      ],
      "author": "Roger",
      "theme": "PLEASURE"
    }
    
  ]

}

My class Main:

public class Main {
  
  public static void main(String[] args) {
    Question question1 = new Question("Roger", Theme.SCHOOL, "Test", "c1", "a1");
    Question question2 = new Question("Roger", Theme.SCHOOL, "Test", "c2", "a2");

    BasicCard bc = new BasicCard("Roger", Theme.SCHOOL, "Test", Arrays.asList(question1,question2));
    
    try {
          File file = new File(Constants.DECK_PATH);
          FileWriter fileWriter = new FileWriter(file, true);

          ObjectMapper mapper = new ObjectMapper();
          SequenceWriter seqWriter = mapper.writerWithDefaultPrettyPrinter().writeValues(fileWriter);
          seqWriter.write(bc);
          seqWriter.close();
      } catch (IOException e) {
          e.printStackTrace();
      }

  }

}

Result

{

  "cards": [
    {
      "subject": "The Earth",
      "questions": [
        {
          "challenge": "What is the highest mountain of the world?",
          "answer": "Everest"
        },
        {
          "challenge": "What is the largest ocean in the world?",
          "answer": "Pacific Ocean"
        }
      ],
      "author": "Roger",
      "theme": "IMPROBABLE"
    },

    {
      "subject": "Holidays",
      "questions": [
        {
          "challenge": "What is the most touristic country in the world?",
          "answer": "France"
        },
        {
          "challenge": "In 2019, how many pictures did vacationers take per day?",
          "answer": "55"
        }
      ],
      "author": "Roger",
      "theme": "PLEASURE"
    }
  ]
}{
  "subject" : "Test",
  "questions" : [ {
    "challenge" : "c1",
    "answer" : "a1"
  }, {
    "challenge" : "c2",
    "answer" : "a2"
  }, {
    "challenge" : "c3",
    "answer" : "a3"
  }, {
    "challenge" : "c4",
    "answer" : "a4"
  } ],
  "author" : "Damien",
  "theme" : "SCHOOL"
}

Answer 1

The problem here is the separation between two layers:

1. The filesystem, in your case provided through FileWriter
2. The logical layer (json), in your case provided through Jackson ( ObjectMapper )

You currently append on the lower layer ( 1. The filesystem ). This layer doesn't know that you're going to write json. It doesn't even know what json is. It simply let's you append more bytes to the end of an existing file.

What you want is to append to a existing json structure. To do so, you have to read in the existing object, append the desired object to it and replace the file completely by writing the updated object to the filesystem.

    public static void main(String[] args) {
        Question question1 = new Question("Roger", Theme.SCHOOL, "Test", "c1", "a1");
        Question question2 = new Question("Roger", Theme.SCHOOL, "Test", "c2", "a2");

        BasicCard bc = new BasicCard("Roger", Theme.SCHOOL, "Test", Arrays.asList(question1,question2));
        
        ObjectMapper mapper = new ObjectMapper();
        File file = new File(Constants.DECK_PATH);

        try {
            // read the existing object from the file
            Map<String, List<BasicCard>> object = mapper.readValue(file, new TypeReference<Map<String, List<BasicCard>>>(){});
            // append your BasicCard bc to the list
            object.computeIfAbsent("cards", k -> new ArrayList<>()).add(bc);
            
            // override the contents of the file by writing the object entirely again
            mapper.writeValue(file, object);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

Answer 2

You can use the following to append the json to existing file.

void write() {
    ObjectMapper om = new ObjectMapper()
    ObjectNode original = om.readValue(new File("oldFile.json"), ObjectNode)

    Subject s = new Subject(List.of(new Question("c1", "a1"),
                             new Question("c2", "a2")), "Test")

    original.set("questions", om.valueToTree(s))

    om.writeValue(new File("newFile.json"), original)
}