使用 GSON 将 json 字符串转换为特定类的问题

Question

I need to send a class to an API with JSON format and then, I need to deserialize it on the destination function. So, I'm using GSON on java. First, I tried to serialize it with the following method:

Gson gson = new Gson();
JsonObject userPpfi = new JsonObject();
userPpfi.addProperty("user", "shahab@gmail.com");
userPpfi.addProperty("ppfi", gson.toJson(oppfi));

and after getting it on the destination, I use the following codes to get these parameters. I can get the user successfully but when I need to deserialize the second method with the GSON, the error will occur.

JsonObject jelement = new JsonParser().parse(message).getAsJsonObject();
user = jelement.get("user").
ppfi = jelement.get("ppfi").toString();
System.out.println("new ppfi : " + gson.fromJson(ppfi,PPFI.class).getId());

And the error is:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 2 path $

So, how can I use the PPFI Class in the destination? Thanks.

sample message

{"user":"shahab@gmail.com","ppfi":"{\"_id\":\"1213\",\"Name\":\"something\",\"item\":5,\"Items\":[{\"Type\":\"pizza\",\"Count\":0},{\"Type\":\"drink\",\"Count\":0},{\"Type\":\"soup\",\"Count\":0}],\"itemtime\":\"Sep 28, 2020, 9:25:31 AM\",\"itemtime2\":\"Sep 28, 2020, 8:25:31 AM\",\"itemtime3\":\"Sep 28, 2020, 10:55:39 AM\",\"itemspec\":[]}"}

and I know this is the main problem (the " of the beginning of the following line):

"{\"_id\":\"1213\",\"Name\":\"something\",\"item\":5,\"Items\":[{\"Type\":\"pizza\",\"Count\":0},{\"Type\":\"drink\",\"Count\":0},{\"Type\":\"soup\",\"Count\":0}],\"itemtime\":\"Sep 28, 2020, 9:25:31 AM\",\"itemtime2\":\"Sep 28, 2020, 8:25:31 AM\",\"itemtime3\":\"Sep 28, 2020, 10:55:39 AM\",\"itemspec\":[]}"

How can I convert the value of ppfi to the PPFI class?? (this value is generated by the GSON as I mentioned before)

Answer 1

Couple of quick things,

1. Sharing the json content would help.
2. Based on exception message, expected is JsonObject which begins with "{", but there is a string.

Again by looking at Json we can confirm what appropriate fix would be.

Answer 2

Finally, I found out about the problem. As I used the JsonObject.addProperty("","") , the value was converting to the string and in the destination, I wasn't successful to convert the string to the final instance of the class. So, there are two solutions.

Changing the source(preferred)

I added the ppfi value as a string using gson.toJson(ppfi) . This function will return the json string. So, I changed it to a JsonTree and I put the class to the JsonTree . So, this is the latest version of my source function:

Gson gson = new Gson();
JsonObject userPpfi = new JsonObject();
userPpfi.addProperty("user", "shahab@gmail.com");
userPpfi.add("ppfi", new Gson().toJsonTree(oppfi));
sqs.sendMessage((gson.toJson(userPpfi)));

and in the destination function, I use it as a simple Json String.

String ppfiFromMessage = jelement.get("ppfi").toString();
currentPPfi = gson.fromJson(ppfiFromMessage,PPFI.class);

And now, I can use the currentPPFI .

Changing only the destination

You can only replace all of the \\ from the PPFI. So, just use

JsonObject jelement = new JsonParser().parse(message).getAsJsonObject();
user = jelement.get("user").
ppfi = jelement.get("ppfi").toString().replace("\\","");
System.out.println("new ppfi : " + gson.fromJson(ppfi,PPFI.class).getId());