Java 无法在 JSON 对象中放置空值
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Unable to put null values in JSON object
提问by Aishwarya
I am trying to pass parameter to api using JSON.
我正在尝试使用 JSON 将参数传递给 api。
class Sample
{ ...
String token;
...
void method()
{ ...
JSONObject params = new JSONObject();
params.put(KEY_TOKEN,token);
params.put(KEY_DATE,date);
Log.e("params ",params+"");
... }
I get the value of params as {"date":"2017-06-19"}but token is seen nowhere.
I have not initialized token and its value is null as its an instance variable. So is it something that uninitialized value are not included?
我得到了 params 的值，{"date":"2017-06-19"}但是在任何地方都看不到令牌。我还没有初始化令牌，它的值为 null 作为它的一个实例变量。那么是否不包括未初始化的值？
采纳答案by T.J. Crowder
Right there in the documentation, in the first paragraph:
就在文档中，在第一段中：
Values may not be
null,NaNs, infinities, or of any type not listed here.
值可能不是
null、NaNs、无穷大或此处未列出的任何类型。
So yes, it is "...something that nullvalues are not included..."(edit: that was a quote from your original question; your updated question changes it to "uninitialized values" but the default value of an object reference is null, so...)
所以是的，它是“......null不包括值的东西......” （编辑：这是你原始问题的引用；你更新的问题将其更改为“未初始化的值”，但对象引用的默认值是null， 所以...）
It's a "feature" of that class, though; JSONitself understands nulljust fine. Further down in the documentation it says you use a "sentinal value," NULL, to represent null. Which seems...odd. There's a note about it:
不过，这是该课程的“特征”；JSON本身null就很好理解。在文档的进一步下方，它说您使用“信号值”NULL来表示null. 这似乎……很奇怪。有一个关于它的注释：
Warning: this class represents
nullin two incompatible ways: the standard Javanullreference, and the sentinel valueNULL. In particular, callingput(name, null)removes the named entry from the object butput(name, JSONObject.NULL)stores an entry whose value isJSONObject.NULL.
警告：此类
null以两种不兼容的方式表示：标准 Javanull引用和标记值NULL。特别是，调用put(name, null)会从对象中删除命名条目，但会put(name, JSONObject.NULL)存储值为 的条目JSONObject.NULL。
So:
所以：
params.put(KEY_TOKEN, token == null ? JSONObject.NULL : token);
回答by Priya Jain
Json itself accpets null but JSONOBJECT class does not.Hence, you are not able to do that. Also Try using Hymanson/gson for json and object mapping instead. If you need help with it, let me know.
Json 本身接受 null 但 JSONOBJECT 类不接受。因此，您不能这样做。也可以尝试使用 Hymanson/gson 来代替 json 和对象映射。如果您需要帮助，请告诉我。
回答by Stephen C
According to RFC 4627, JSON treats the nullsymbol as a valid value.
根据RFC 4627，JSON 将null符号视为有效值。
The catch is that JSON nullrepresents the Javascript nullvalue. By contrast, the Java version of nullis (according to the experts) more closely aligned with Javascript's undefinedvalue.
问题在于 JSONnull表示 Javascriptnull值。相比之下，Java 版本null（根据专家的说法）更接近于 Javascript 的undefined价值。
The original author of the org.jsonlibrary decided that JSONObjectshould treat JSON nullin a way that is consistent with the Javascript semantics. Hence it is represented (in Java) as JSONObject.NULL.
该org.json库的原作者决定JSONObject应该null以与 Javascript 语义一致的方式处理 JSON 。因此，它表示（在 Java 中）为JSONObject.NULL.
回答by Lars
I solved this problem by creating the null JSON value using a string representation to stuff the null value into the object. Clearly a hack, but it works.
我通过使用字符串表示创建空 JSON 值来将空值填充到对象中来解决这个问题。显然是一个黑客，但它的工作原理。
JSONObject obj = new JSONObject("{\"keyword\": null}");
System.out.println(obj); // {"keyword": null}
