如何在java中将列表数据转换为json

Question

我有一个函数在 java 类中以 List 的形式返回数据。现在根据我的需要，我必须将其转换为Json 格式。

下面是我的函数代码sn-p：

public static List<Product> getCartList() {
    List<Product> cartList = new Vector<Product>(cartMap.keySet().size());
    for(Product p : cartMap.keySet()) {
        cartList.add(p);
    }
    return cartList;
}

我尝试使用此代码转换为 json，但由于函数属于 List 类型，因此出现类型不匹配错误...

public static List<Product> getCartList() {
    List<Product> cartList = new Vector<Product>(cartMap.keySet().size());
    for(Product p : cartMap.keySet()) {
        cartList.add(p);
    }

    Gson gson = new Gson();
     // convert your list to json
     String jsonCartList = gson.toJson(cartList);
     // print your generated json
     System.out.println("jsonCartList: " + jsonCartList);

     return jsonCartList;

        }

请帮我解决这个问题。

Answer 1

您可以使用以下使用 Jackson 库的方法

public static <T> List<T> convertToList(String jsonString, Class<T> target) {
    if(StringUtils.isEmpty(jsonString)) return List.of();

        return new ObjectMapper().readValue(jsonString, new ObjectMapper().getTypeFactory().
                constructCollectionType(List.class, target));
    } catch ( JsonProcessingException | JSONException e) {
        e.printStackTrace();
        return List.of();
    }
}

Answer 2

JSONObject responseDetailsJson = new JSONObject();
JSONArray jsonArray = new JSONArray();

List<String> ls =new  ArrayList<String>();

for(product cj:cities.getList()) {
    ls.add(cj);
    JSONObject formDetailsJson = new JSONObject();
    formDetailsJson.put("id", cj.id);
    formDetailsJson.put("name", cj.name);
    jsonArray.put(formDetailsJson);
}

responseDetailsJson.put("Cities", jsonArray);

return responseDetailsJson;

Answer 3

试试这些简单的步骤：

ObjectMapper mapper = new ObjectMapper();
String newJsonData = mapper.writeValueAsString(cartList);
return newJsonData;
ObjectMapper() is com.fasterxml.jackson.databind.ObjectMapper.ObjectMapper();

Answer 4

使用 Gson 库尝试如下所示。

早期的转换列表格式为：

[Product [Id=1, City=Bengalore, Category=TV, Brand=Samsung, Name=Samsung LED, Type=LED, Size=32 inches, Price=33500.5, Stock=17.0], Product [Id=2, City=Bengalore, Category=TV, Brand=Samsung, Name=Samsung LED, Type=LED, Size=42 inches, Price=41850.0, Stock=9.0]]

转换源从这里开始。

//** Note I have created the method toString() in Product class.

//Creating and initializing a java.util.List of Product objects
List<Product> productList = (List<Product>)productRepository.findAll();

//Creating a blank List of Gson library JsonObject
List<JsonObject> entities = new ArrayList<JsonObject>();

//Simply printing productList size
System.out.println("Size of productList is : " + productList.size());

//Creating a Iterator for productList
Iterator<Product> iterator = productList.iterator();

//Run while loop till Product Object exists.
while(iterator.hasNext()){

    //Creating a fresh Gson Object
    Gson gs = new Gson();

    //Converting our Product Object to JsonElement 
    //Object by passing the Product Object String value (iterator.next())
    JsonElement element = gs.fromJson (gs.toJson(iterator.next()), JsonElement.class);

    //Creating JsonObject from JsonElement
    JsonObject jsonObject = element.getAsJsonObject();

    //Collecting the JsonObject to List
    entities.add(jsonObject);

}

//Do what you want to do with Array of JsonObject
System.out.println(entities);

转换后的 Json 结果是：

[{"Id":1,"City":"Bengalore","Category":"TV","Brand":"Samsung","Name":"Samsung LED","Type":"LED","Size":"32 inches","Price":33500.5,"Stock":17.0}, {"Id":2,"City":"Bengalore","Category":"TV","Brand":"Samsung","Name":"Samsung LED","Type":"LED","Size":"42 inches","Price":41850.0,"Stock":9.0}]

希望这能帮助很多人！

Answer 5

public static List<Product> getCartList() {

    JSONObject responseDetailsJson = new JSONObject();
    JSONArray jsonArray = new JSONArray();

    List<Product> cartList = new Vector<Product>(cartMap.keySet().size());
    for(Product p : cartMap.keySet()) {
        cartList.add(p);
        JSONObject formDetailsJson = new JSONObject();
        formDetailsJson.put("id", "1");
        formDetailsJson.put("name", "name1");
       jsonArray.add(formDetailsJson);
    }
    responseDetailsJson.put("forms", jsonArray);//Here you can see the data in json format

    return cartList;

}

可以通过以下形式获取数据

{
    "forms": [
        { "id": "1", "name": "name1" },
        { "id": "2", "name": "name2" } 
    ]
}

Answer 6

使用gson 会简单得多。使用以下代码 sn -p：

 // create a new Gson instance
 Gson gson = new Gson();
 // convert your list to json
 String jsonCartList = gson.toJson(cartList);
 // print your generated json
 System.out.println("jsonCartList: " + jsonCartList);

从 JSON 字符串转换回您的 Java 对象

 // Converts JSON string into a List of Product object
 Type type = new TypeToken<List<Product>>(){}.getType();
 List<Product> prodList = gson.fromJson(jsonCartList, type);

 // print your List<Product>
 System.out.println("prodList: " + prodList);

Answer 7

我编写了自己的函数来返回填充组合框的对象列表：

public static String getJSONList(java.util.List<Object> list,String kelas,String name, String label) {
        try {
            Object[] args={};
            Class cl = Class.forName(kelas);
            Method getName = cl.getMethod(name, null);
            Method getLabel = cl.getMethod(label, null);
            String json="[";
            for (int i = 0; i < list.size(); i++) {
            Object o = list.get(i);
            if(i>0){
                json+=",";
            }
            json+="{\"label\":\""+getLabel.invoke(o,args)+"\",\"name\":\""+getName.invoke(o,args)+"\"}";
            //System.out.println("Object = " + i+" -> "+o.getNumber());
            }
            json+="]";
            return json;
        } catch (ClassNotFoundException ex) {
            Logger.getLogger(JSONHelper.class.getName()).log(Level.SEVERE, null, ex);
        } catch (Exception ex) {
            System.out.println("Error in get JSON List");
            ex.printStackTrace();
        }
        return "";
    }

并从任何地方调用它：

String toreturn=JSONHelper.getJSONList(list, "com.bean.Contact", "getContactID", "getNumber");