I have a Map containing a mixture of types like in this simple example
final Map<String, Object> map = new LinkedHashMap<String, Object>();
map.put("a", 1);
map.put("b", "a");
map.put("c", 2);
final Gson gson = new Gson();
final String string = gson.toJson(map);
final Type type = new TypeToken<LinkedHashMap<String, Object>>(){}.getType();
final Map<Object, Object> map2 = gson.fromJson(string, type);
for (final Entry<Object, Object> entry : map2.entrySet()) {
System.out.println(entry.getKey() + " : " + entry.getValue());
}
What I get back are plain Object
s, no Integer
s, no String
s. The output looks like
a : 开发者_JAVA百科java.lang.Object@48d19bc8
b : java.lang.Object@394a8cd1
c : java.lang.Object@4d630ab9
Can I fix it somehow? I'd expect that such simple cases will be handled correctly by default.
I know that the information about the type can't always be preserved, and possibly 1
and "1"
means exactly the same in JSON. However, returning plain content-less objects just makes no sense to me.
Update: The serialized version (i.e. the string
above) looks fine:
{"a":1,"b":"a","c":2}
Gson isn't that smart. Rather provide a clear and static data structure in flavor of a Javabean class so that Gson understands what type the separate properties are supposed to be deserialized to.
E.g.
public class Data {
private Integer a;
private String b;
private Integer c;
// ...
}
in combination with
Data data1 = new Data(1, "a", 2);
String json = gson.toJson(data1);
Data data2 = gson.fromJson(json, Data.class);
Update: as per the comments, the keyset seems to be not fixed (although you seem to be able to convert it manually afterwards without knowing the structure beforehand). You could create a custom deserializer. Here's a quick'n'dirty example.
public class ObjectDeserializer implements JsonDeserializer<Object> {
@Override
public Object deserialize(JsonElement element, Type type, JsonDeserializationContext context) throws JsonParseException {
String value = element.getAsString();
try {
return Long.valueOf(value);
} catch (NumberFormatException e) {
return value;
}
}
}
which you use as follows:
final Gson gson = new GsonBuilder().registerTypeAdapter(Object.class, new ObjectDeserializer()).create();
// ...
Gson gson = new GsonBuilder()
.registerTypeAdapter(Object.class, new JsonDeserializer<Object>() {
@Override
public Object deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
JsonPrimitive value = json.getAsJsonPrimitive();
if (value.isBoolean()) {
return value.getAsBoolean();
} else if (value.isNumber()) {
return value.getAsNumber();
} else {
return value.getAsString();
}
}
}).create();
Upgrade to Gson 2.1. It prints this:
a : 1.0
b : a
c : 2.0
You are storing the data in a Map. It looks like you need to cast the object to the type you need.
If you want a JSON string from Map<Object, Object>
, I think json-simple
is better choice than Gson
.
This is a brief example from http://code.google.com/p/json-simple/wiki/EncodingExamples :
//import java.util.LinkedHashMap;
//import java.util.Map;
//import org.json.simple.JSONValue;
Map obj=new LinkedHashMap();
obj.put("name","foo");
obj.put("num",new Integer(100));
obj.put("balance",new Double(1000.21));
obj.put("is_vip",new Boolean(true));
obj.put("nickname",null);
String jsonText = JSONValue.toJSONString(obj);
System.out.print(jsonText);
Result: {"name":"foo","num":100,"balance":1000.21,"is_vip":true,"nickname":null}
For decoding, refer to http://code.google.com/p/json-simple/wiki/DecodingExamples .
精彩评论