仅当字段位于嵌套对象中时，Gson才会序列化空值

问题描述 投票：1回答：1

我面临的问题是，当[[仅涉及非顶级属性时，我想对空值进行ser / des，而我不知道如何实现。假设我有一个User类：

Class User { String name; int id; Address address; } 
和一个地址类：
Class Address{
   String street;
   String city;
   String country;
}

现在，我可以在Gson实例下面使用Ser / des空值：
Gson gson = new GsonBuilder().serializeNulls().create();
Address address = new Address(null, "New York", "US");
User user = new User("Adam", 123, address);  
String userJson = gson.toJson(user); 

输出为：
{
  "name": "Adam",
  "id": 123,
  "address": {
      "street": null,
      "city": "New York",
      "country": "US"
  }
}

但是，我做
NOT
想要对用户的顶级属性进行Ser / des空值处理。例如下面的用户：User user = new User("Adam", 123, null); 
我希望具有以下输出，并且没有
address字段：
{
  "name": "Adam",
  "id": 123
}

我现在正尝试使用自定义的序列化器对每个顶级属性进行硬编码，如果它们为空，则将其删除：
public class SerializerForUser implements JsonSerializer<ConfigSnapshot> {

    @Override
    public JsonElement serialize(User user, Type type, JsonSerializationContext jsc) {
        Gson gson = new GsonBuilder().serializeNulls().create();
        JsonObject jsonObject = gson.toJsonTree(user).getAsJsonObject();
        if (user.getAddress() == null) {
            jsonObject.remove("address");
        }
        // if... (same with other top-level attributes)
        return jsonObject;
    }
}

Gson gson = new GsonBuilder().serializeNulls().registerTypeAdapter(User.class, new SerializerForUser()).create();

但是以某种方式它不起作用，例如，当地址为空时，我仍然会得到下面的输出：
{
  "name": "Adam",
  "id": 123,
  "address: null
}

有人可以给我一些提示，指出我在这里错了什么吗？或者，如果有人可以告诉我是否有更直接/通用的方法来实现这一目标，那将是完美的（因为我也想使用相同的gson实例来ser / des其他对象）？任何评论表示赞赏。
java gson
1个回答
0
投票
因为您正在使用

Gson gson = new GsonBuilder().serializeNulls().create(); 

显示空值。
要跳过显示空值，请尝试
Gson gson = new Gson();

您可以在这里测试
public static void main(String[] args) {
    Gson yourGson = new GsonBuilder().serializeNulls().create(); // this is how you create your Gson object, which shows null value
    Address address = new Address(null, "New York", "US");
    User user = new User("Adam", 123, address);
    String userJson = yourGson.toJson(user);
    System.out.println(userJson);

    Gson newGson = new Gson(); // with this one, it doesn't show value
    System.out.println(newGson.toJson(user));
}

更新
我已经尝试过几次重写方法serialize，但直到我尝试＃5时它都失败了>
public class UserCustomSerializer implements JsonSerializer<User> {

    @Override
    public JsonElement serialize(User src, Type typeOfSrc, JsonSerializationContext context) {
        JsonObject obj = new JsonObject();

        if (src.name != null) {
            obj.addProperty("name", src.name);
        }
        obj.addProperty("id", src.id);

        if (src.address != null) {
            // try #1
            //JsonObject addressJsonObj = new JsonObject();
            //addressJsonObj.addProperty("street", src.address.street != null ? src.address.street : null);
            //addressJsonObj.addProperty("city", src.address.city != null ? src.address.city : null);
            //addressJsonObj.addProperty("country", src.address.country != null ? src.address.country : null);
            //obj.add("address", addressJsonObj);

            // try #2
            //Gson gson = new GsonBuilder().serializeNulls().create();
            //JsonElement jsonElement = gson.toJsonTree(src.address);
            //obj.add("address", jsonElement);

            // try #3
            //Gson gson2 = new GsonBuilder().serializeNulls().create();
            //obj.addProperty("address", gson2.toJson(src.address));

            // try #4
            //Gson gson = new GsonBuilder().serializeNulls().create();
            //JsonObject jsonObject = gson.toJsonTree(src.address).getAsJsonObject();
            //obj.add("address", jsonObject);

            // try #5
            JsonObject addressJsonObj = new JsonObject();
            addressJsonObj.addProperty("street", src.address.street != null ? src.address.street : "null");
            addressJsonObj.addProperty("city", src.address.city != null ? src.address.city : "null");
            addressJsonObj.addProperty("country", src.address.country != null ? src.address.country : "null");
            obj.add("address", addressJsonObj);
        }
        return obj;
    }
}

对于尝试＃3，我构建了不正确的字符串。
对于尝试＃1，＃2和＃4，我有空值问题。我搜索并找到了原因以及建议here
在JSON“对象”（又名字典）中，有两种方法来表示缺少的值：根本没有键/值对，或者具有JSON值为null的键。
因此，您可以使用.add带有适当的值，在构建JSON时，它将转换为null，或者您没有.add调用。
我的第5种方法是检查子节点是否为空，我只是从字面上添加字符串“ null”，然后在构建json字符串时将其替换]
private String parseToGson(User user){
    Gson gson = new GsonBuilder().registerTypeAdapter(User.class, new UserCustomSerializer()).create();
    return gson.toJson(user).replace("\"null\"", "null");
}

这里是我定义的一些测试用例
@Test
public void children_attribute_is_null() throws Exception {
    String expected = "{\"name\":\"Adam\"," 
                     + "\"id\":123," 
                     + "\"address\":{" 
                                    + "\""+ "street\":null," 
                                    + "\"city\":\"New York\"," 
                                    + "\"country\":\"US" 
                                + "\"}" 
                     + "}";
    Address address = new Address(null, "New York", "US");
    User user = new User("Adam", 123, address);
    assertEquals(expected, parseToGson(user));

    Gson g = new Gson(); 
    User usr = g.fromJson( parseToGson(user), User.class);
    assertEquals("Adam", usr.name);
    assertEquals(123, usr.id);
    assertEquals(null, usr.address.street);
    assertEquals("New York", usr.address.city);
    assertEquals("US", usr.address.country);
}

@Test
public void parent_attribute_is_null() throws Exception {
    String expected = "{\"name\":\"Adam\"," 
                     + "\"id\":123" + "}";
    User user = new User("Adam", 123, null);
    assertEquals(expected, parseToGson(user));

    Gson g = new Gson(); 
    User usr = g.fromJson( parseToGson(user), User.class);
    assertEquals("Adam", usr.name);
    assertEquals(123, usr.id);
    assertEquals(null, usr.address);
}

@Test
public void parent_attribute_and_children_attribute_are_null() throws Exception {
    String expected = "{\"id\":123," 
                     + "\"address\":{" 
                                + "\"street\":null," 
                                + "\"city\":\"New York\","
                                + "\"country\":\"US" 
                                + "\"}" 
                     + "}";
    Address address = new Address(null, "New York", "US");
    User user = new User(null, 123, address);
    assertEquals(expected, parseToGson(user));

    Gson g = new Gson(); 
    User usr = g.fromJson( parseToGson(user), User.class);
    assertEquals(null, usr.name);
    assertEquals(123, usr.id);
    assertEquals(null, usr.address.street);
    assertEquals("New York", usr.address.city);
    assertEquals("US", usr.address.country);
}
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