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Strange jackson ObjectMapper behavior

I'm having a trouble with my Java application receiving Spotify API auth tokens and mapping them to POJO with Jackson.
Every time my app requests data from Spotify API, I start with getting a new access token from this link: https://accounts.spotify.com/api/token?grant_type=client_credentials
and the answer is JSON, that looks like this:
{
"access_token":"BQAJmzZOdh2egvWEOEwy4wv-VKdhTUc4eZYJrIfAibjWLR4MPfrbV6KBNIiomPwJKsQN-3vmrGmG7lOXFaI",
"token_type":"Bearer",
"expires_in":3600,
"scope":""
}
Every time I launch my app, the first time it works nice, but then it crashes with:
Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `com.slowedandthrowed.darkjazzbot.mapping.spotify.TokenRequest` (although at least
one Creator exists): no String-argument constructor/factory method to deserialize from String value ('BQCcDMOKUiVPscDrrBH77b2QbN9FuqjAJHuM3_1QD39MO9L20XzXneZUlJeIyukBVhPpaCWnKWRjUdggaCM') at [Source: (String)"{"access_token":"BQCcDMOKUiVPscDrrBH77b2QbN9FuqjAJHuM3_1QD39MO9L20XzXneZUlJeIyukBVhPpaCWnKWRjUdggaCM","token_type":"Bearer","expires_in":3600,"scope":""}"; line: 1, column: 17]
I kinda duct taped this problem by creating a new ObjectMapper every time I need to map access token JSON to POJO, but if it would be a production app, it would harm the performance, so I need to find out whats the problem in using one ObjectMapper instance for all the time.
I also tried to map this JSON to Map<String,String> instead of mapping it to TokenRequest.class and the result was the same, so I don't think that this is the reason why mapping fails.
private String requestAccessToken() throws IOException {
TokenRequest tokenRequest = null;
URL accessTokenUrl = new URL(SPOTIFY_TOKEN_LINK);
HttpURLConnection tokenConnection = (HttpURLConnection) accessTokenUrl.openConnection();
tokenConnection.setRequestProperty("Authorization", authString);
tokenConnection.setDoOutput(true);
tokenConnection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
tokenConnection.setRequestMethod("POST");
OutputStreamWriter wr = new OutputStreamWriter(tokenConnection.getOutputStream());
wr.write(TOKEN_REQUEST_PARAMETERS);
wr.flush();
System.out.println("Wow! Posted!");
InputStream inputStream = tokenConnection.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"));
StringBuilder inputBuilder = new StringBuilder();
String input = null;
while ((input = reader.readLine()) != null) inputBuilder.append(input);
System.out.println("================================= TOKEN INPUT ======================================");
System.out.println(inputBuilder.toString());
System.out.println("================================= TOKEN INPUT ======================================");
tokenRequest = spotifyObjectMapper.readValue(inputBuilder.toString(), TokenRequest.class);
inputStream.close();
reader.close();
tokenConnection.disconnect();
return tokenRequest.getAccessToken();
}
TokenRequest.java:
package com.slowedandthrowed.darkjazzbot.mapping.spotify;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonPropertyOrder;
import com.fasterxml.jackson.annotation.JsonRootName;
#JsonInclude(JsonInclude.Include.NON_NULL)
#JsonPropertyOrder({
"access_token",
"token_type",
"expires_in",
"scope"
})
#JsonRootName("access_token")
public class TokenRequest {
#JsonProperty("access_token")
private String accessToken;
#JsonProperty("token_type")
private String tokenType;
#JsonProperty("expires_in")
private Long expiresIn;
#JsonProperty("scope")
private String scope;
#JsonProperty("access_token")
public String getAccessToken() {
return accessToken;
}
#JsonProperty("access_token")
public void setAccessToken(String accessToken) {
this.accessToken = accessToken;
}
#JsonProperty("token_type")
public String getTokenType() {
return tokenType;
}
#JsonProperty("token_type")
public void setTokenType(String tokenType) {
this.tokenType = tokenType;
}
#JsonProperty("expires_in")
public Long getExpiresIn() {
return expiresIn;
}
#JsonProperty("expires_in")
public void setExpiresIn(Long expiresIn) {
this.expiresIn = expiresIn;
}
#JsonProperty("scope")
public String getScope() {
return scope;
}
#JsonProperty("scope")
public void setScope(String scope) {
this.scope = scope;
}
}

I fixed my program because i created the problems for myself.
I recieved different data thru different APIs and mapped it to a POJOs via single instance of Jackson ObjectMapper. Some JSON object contain data by themself:
{
"property1":value1,
"property2":value2
}
While other had single nested object with data inside:
{
"object":{"property1":value1,
"property2":value2
}}
So i decided to turn UNWRAP_ROOT_VALUE to TRUE and then my object mapper changed its state so its behavior became different between my first and second attempt to recieve access token.

Passing json as a String

For a Json body like
{
"firstName": "hello",
"lastName": "abc"
}
I am writing as
JSONObject body = new JSONObject();
body.put("firstName", "hello");
body.put("lastName", "abc");
and then converting body to string to pass it as string parameter
How can I write the same for body with response like
{
"class": {
"firstName": "hello",
"lastName": "abc"
}
}
I need to convert json to string afterwards

I think this should do the trick
JSONObject innerBody = new JSONObject();
innerBody.put("firstName", "hello");
innerBody.put("lastName", "abc");
JSONObject outerBody = new JSONObject();
outerBody.put("class",innerBody);

Create a class:
public class DataSource {
private String firstName;
private String lastName;
//Constructor, getter, setter
}
And then:
JSONObject body = new JSONObject();
DataSource data = new DataSource();
data.setFirstName("bla");
data.setLastName("bla bla");
body.put("class", data );

From JSON to object without table name

I'm trying to Parse very simply JSON to object (with GSON)
My JSON:
[{"username":"admin","password":"admin","name":"admin","email":"admin#admin.com"},{"username":"mark20","password":"mark123","name":"mark","email":"mark#steew.com"}]
is there 2 users, so I create 2 class, Users with list of users and User :
public class Users {
ArrayList<User> users;
Users(ArrayList<User> users){
this.users = users;
}
}
.
public class User {
String userame;
String password;
String name;
String email;
}
and here is my parsing code:
public void onResponse(Call call, Response response) throws IOException {
String body = response.body().string();
Gson gson = new GsonBuilder().create();
Users users = gson.fromJson(body, Users.class);
}
of course in variable body I have got correct JSON, but in last lane I got:
JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY
What can be wrong here ? How to fix it ?

Your json is an array of User not a wrapper Users of array User.
Read your json like this:
User[] users = gson.fromJson(body, User[].class);
If you want an ArrayList<>:
List<User> userList = Arrays.asList(users);
Another way is to use TypeToken:
Type listType = new TypeToken<ArrayList<User>>(){}.getType();
List<User> userList = gson.fromJson(body, listType);

If I'm reading from what I read elsewhere correctly try:
public void onResponse(Call call, Response response) throws IOException {
String body = response.body().string();
Gson gson = new GsonBuilder().create();
User[] users = gson.fromJson(body, User[].class);
}

How to ask gson to avoid escaping json in a json response?

I have a response object like this:
public class TestResponse {
private final String response;
private final ErrorCodeEnum error;
private final StatusCodeEnum status;
// .. constructors and getters here
}
I am serializing above class using Gson library as shown below:
Gson gson = new GsonBuilder().setPrettyPrinting().serializeNulls().create();
System.out.println(gson.toJson(testResponseOutput));
And the response I am getting back is shown below:
{
"response": "{\"hello\":0,\"world\":\"0\"}",
"error": "OK",
"status": "SUCCESS"
}
As you can see, my json string in "response" field is getting escaped. Is there any way I can ask gson not to do that and instead return a full response like this:
{
"response": {"hello":0,"world":"0"},
"error": "OK",
"status": "SUCCESS"
}
And also - Is there any problem if I do it above way?
NOTE: My "response" string will always be JSON string or it will be null so only these two values will be there in my "response" string. In "response" field, I can have any json string since this library is calling a rest service which can return back any json string so I am storing that in a string "response" field.

If your response field can be arbitrary JSON, then you need to:
Define it as an arbitrary JSON field (leveraging the JSON type system already built into GSON by defining it as the root of the JSON hierarchy - JsonElement)
public class TestResponse {
private final JsonElement response;
}
Convert the String field to an appropriate JSON object representation. For this, you can use GSON's JsonParser class:
final JsonParser parser = new JsonParser();
String responseJson = "{\"hello\":0,\"world\":\"0\"}";
JsonElement json = parser.parse(responseJson); // Omits error checking, what if responseJson is invalid JSON?
System.out.println(gson.toJson(new TestResponse(json)));
This should print:
{
"response": {
"hello": 0,
"world": "0"
}
}
It should also work for any valid JSON:
String responseJson = "{\"arbitrary\":\"fields\",\"can-be\":{\"in\":[\"here\",\"!\"]}}";
JsonElement json = parser.parse(responseJson);
System.out.println(gson.toJson(new TestResponse(json)));
Output:
{
"response": {
"arbitrary": "fields",
"can-be": {
"in": [
"here",
"!"
]
}
}
}

I know this is old but just adding an potential answer in case it is needed.
Sounds like you just want to return the response without escaping. Escaping is a good thing, it will help to prevent security issues and prevent your JS application from crashing with errors.
However, if you still want to ignore escaping, try:
Gson gson = new GsonBuilder().setPrettyPrinting().disableHtmlEscaping().serializeNulls().create();

add simple TypeAdapter and use jsonValue(value)
gson 2.8.0
version 1:
#Test
public void correctlyShow() {
TestResponse2 src = new TestResponse2("{\"arbitrary\":\"fields\",\"can-be\":{\"in\":[\"here\",\"!\"]}}");
Gson create = new GsonBuilder().registerTypeAdapter(String.class, ADAPTER).create();
Stopwatch createStarted = Stopwatch.createStarted();
String json2 = create.toJson(src);
System.out.println(json2 + " correctlyShow4 " + createStarted.stop());
}
public class TestResponse2 {
private final String response;
public TestResponse2(String response) {
this.response = response;
}
public String getResponse() {
return response;
}
}
private static final TypeAdapter<String> ADAPTER = new TypeAdapter<String>() {
#Override
public String read(JsonReader in) throws IOException {
throw new UnsupportedOperationException("Unsupported Operation !!!");
}
#Override
public void write(JsonWriter out, String value) throws IOException {
out.jsonValue(value);
}
};
...
vesrion 2
#Test
public void correctlyShow() {
TestResponse2 src = new TestResponse2("{\"arbitrary\":\"fields\",\"can-be\":{\"in\":[\"here\",\"!\"]}}");
String json2 = new Gson().toJson(src);
System.out.println(json2 + " correctlyShow4 ");
}
public class TestResponse2 {
#JsonAdapter(value = AdapterStringJson.class)
private final String response;
public TestResponse2(String response) {
this.response = response;
}
public String getResponse() {
return response;
}
}
private class AdapterStringJson extends TypeAdapter<String> {
#Override
public String read(JsonReader in) throws IOException {
throw new UnsupportedOperationException("Unsupported Operation !!!");
}
#Override
public void write(JsonWriter out, String value) throws IOException {
out.jsonValue(value);
}
}

You should have a nested object.
public class Response {
private final Integer hello;
private final String world;
}
public class TestResponse {
private final Response response;
private final ErrorCodeEnum error;
private final StatusCodeEnum status;
// .. constructors and getters here
}

Instead of a String, depending on your needs, you could use a Map (or similar) or a nested Object. There should not be a problem representing it this way but in your example, if it were a String, there would be a problem if you didn't escape characters such as the double-quote.

How to send json ouput from servlet to jsp?

I'm making an inventory control. I was trying to check if the entered quantity of item is less than the stock quantity or not. I'm getting the servet json output. But I can't send it to jsp back.
Jsp JQuery code.
<script>
$('document').ready(function() {
$('#submit_btn').click((function() {
var $name = $("select#item_name").val();
var $qty = $("input#qty").val();
$.post('BuyItem', {item_name: $name, item_qty: $qty}, function(data) {
if (data !== null) {
alert(text(data));
$("input#qty").val("");
} else {
alert("Invalid Item!");
}
}
);
}));
});
</script>
And this is servlet query.
while (rs.next()) {
if (rs.getInt("qty") > qty) {
int id = rs.getInt("item_id");
Gson gson = new Gson();
String json = gson.toJson(id);
// System.out.println("one" + json);
response.setContentType("application/json");
// response.setCharacterEncoding("UTF-8");
response.getWriter().print(json);
} else {
Gson gson = new Gson();
String json = gson.toJson("Stock doesn\'t have enough item quantity.");
// System.out.println("two" + json);
response.setContentType("application/json");
// response.setCharacterEncoding("UTF-8");
response.getWriter().print(json);
}
}
From both System.out.println() s output is always coming correctly. But not sending to jsp back. Please help me with this.

what you have to do and also a best practice is to create a DTO class with the properties that you need to pass to gson library.
public class ResponseDTO implements Serializable{
private Integer id;
//more properties ...
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id= id;
}
// other getters & setters
}
and inside your loop, set the values to the dto object, then pass it to gson.
Gson gson = new Gson();
ResponseDTO dto = null;
String json = "";
response.setContentType("application/json");
......
if (rs.getInt("qty") > qty) {
dto = new ResponseDTO();
int id = rs.getInt("item_id");
dto.setId(id);
......
json = gson.toJson(dto);
} else {
...... // similar
json = gson.toJson("{data: 'Some message'}");
}
response.getWriter().print(json);
gson will give you the proper json structure to the client side.
try and see !

Gson allows strings and numbers to serialize to JSON by themselves (which is probably ok by the definition), but a lot of other libraries would consider this invalid JSON.
Try wrapping your response in an object so that the response is {"id": 5} and not just 5.