Java - 与 Jackson 一起编写 Json

Question

我目前正在为学校做一个项目，我正在尝试在我的 json 文件末尾（在卡片属性中）编写一个对象，而无需使用 Jackson 库重写所有内容。

问题是，当我尝试执行此操作时，我的对象写入正确，但它写在文件末尾，我尝试将其放入卡片列表 [ THERE ]。

有人有想法吗？谢谢。

编辑：我已经知道如何通过重写来写入文件，为了回答我老师的指示，我需要在不重写文件的情况下这样做。

我的 Json 文件：

{

  "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"
    }
    
  ]

}

我的主班：

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();
      }

  }

}

结果

{

  "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

这里的问题是两层之间的分离：

1. 文件系统，在您的情况下是通过FileWriter 提供的
2. 逻辑层 (json)，在您的情况下是通过 Jackson (ObjectMapper) 提供的

您当前附加在较低层 (1. The filesystem)。这一层不知道你要写json。它甚至不知道 json 是什么。它只是让您将更多字节附加到现有文件的末尾。

您想要的是附加到现有的 json 结构。为此，您必须读入现有对象，将所需对象附加到其中，并通过将更新后的对象写入文件系统来完全替换文件。

    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

您可以使用以下内容将 json 附加到现有文件中。

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)
}