Cannot convert JSON HTTP Response to Java Object using Jackson

Question

I have created a REST service which sends GET requests to retrieve a JSON response from a specified URL.

I have got it working to the point where I retrieve the JSON message successfully although my next issue is converting the JSON message to JAVA Objects.

Below is my Rest Client:

public class RestADClient {


// HTTP GET request
public String sendGet(String url) throws Exception {

    URL obj = new URL(url);
    HttpURLConnection con = (HttpURLConnection) obj.openConnection();

    // optional default is GET
    con.setRequestMethod("GET");

    //add request header

    con.setRequestProperty("Authorization", "myAuthorizationProp");

    int responseCode = con.getResponseCode();
    System.out.println("\nSending 'GET' request to URL : " + url);
    System.out.println("Response Code : " + responseCode);

    BufferedReader in = new BufferedReader(
            new InputStreamReader(con.getInputStream()));
    String inputLine;
    StringBuffer response = new StringBuffer();

    while ((inputLine = in.readLine()) != null) {
        response.append(inputLine);
    }
    in.close();

    System.out.println("Response Message: " + response.toString());

    ObjectMapper mapper = new ObjectMapper();

    try {
        // Convert JSON string to Object
        String jsonInString = "[{  \"frequency\": \"ONE_MIN\",  \"metricId\": 2253538,  \"metricName\": \"DB|Server:1|IO|Data File Average Read Size\",  \"metricPath\": \"Databases|X|IO|Data File Average Read Size\",  \"metricValues\": [  {    \"count\": 14,    \"current\": 65323,    \"max\": 65536,    \"min\": 0,    \"occurrences\": 0,    \"standardDeviation\": 0,    \"startTimeInMillis\": 1447927800000,    \"sum\": 251408,    \"useRange\": true,    \"value\": 17958  }]}]";
        ADMetrics adMetrics = mapper.readValue(jsonInString, ADMetrics.class);
        System.out.println(adMetrics);

    } catch (JsonGenerationException e) {
        e.printStackTrace();
    } catch (JsonMappingException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

    return response.toString();
}

}

Below is my ADMetrics POJO class:

import java.util.List;
public class ADMetrics {

private String frequency;
private int metricId;
private String metricName;
private String metricPath;
private List<String> metricValues;
private int count;
private int current;
private int max;
private int min;
private int occurences;
private int standardDeviation;
private int startTimeInMillis;
private int sum;
private boolean useRange;
private int value;

public String getFrequency() {
    return frequency;
}

public void setFrequency(String frequency) {
    this.frequency = frequency;
}

public int getMetricId() {
    return metricId;
}

public void setMetricId(int metricId) {
    this.metricId = metricId;
}
public String getMetricName() {
    return metricName;
}

public void setMetricName(String metricName) {
    this.metricName = metricName;
}

public String getMetricPath() {
    return metricPath;
}

public void setMetricPath(String metricPath) {
    this.metricPath = metricPath;
}

public List<String> getMetricValues() {
    return metricValues;
}

public void setMetricValues(List<String> metricValues) {
    this.metricValues = metricValues;
}

public int getCount() {
    return count;
}

public void setCount(int count) {
    this.count = count;
}

public int getCurrent() {
    return current;
}

public void setCurrent(int current) {
    this.current = current;
}

public int getMax() {
    return max;
}

public void setMax(int max) {
    this.max = max;
}

public int getMin() {
    return min;
}

public void setMin(int min) {
    this.min = min;
}

public int getOccurences() {
    return occurences;
}

public void setOccurences(int occurences) {
    this.occurences = occurences;
}

public int getStandardDeviation() {
    return standardDeviation;
}

public void setStandardDeviation(int standardDeviation) {
    this.standardDeviation = standardDeviation;
}

public int getStartTimeInMillis() {
    return startTimeInMillis;
}

public void setStartTimeInMillis(int startTimeInMillis) {
    this.startTimeInMillis = startTimeInMillis;
}

public int getSum() {
    return sum;
}

public void setSum(int sum) {
    this.sum = sum;
}

public boolean isUseRange() {
    return useRange;
}

public void setUseRange(boolean useRange) {
    this.useRange = useRange;
}

public int getValue() {
    return value;
}

public void setValue(int value) {
    this.value = value;
}

Below is my JUnit test:

public class ADTest {

String url = "myURL";

@Test
public void testResponseContainsMetricFrequency() throws Exception {

    RestADClient restADClient = new RestADClient();


    assertTrue(restADClient.sendGet(url).contains("frequency"));
}}

When I run the test, I get the following:

Response Code : 200

Response Message: [{ "frequency": "ONE_MIN", "metricId": 2253538, "metricName": "DB|Server:169|IO|Data File Average Read Size", "metricPath": "Databases|QA-CARS|IO|Data File Average Read Size", "metricValues": [ { "count": 13, "current": 66013, "max": 72003, "min": 0, "occurrences": 0, "standardDeviation": 0, "startTimeInMillis": 1447929360000, "sum": 198688, "useRange": true, "value": 15284 }]}]

org.codehaus.jackson.map.JsonMappingException: Can not deserialize instance of rest.client.ADMetrics out of START_ARRAY token at [Source: java.io.StringReader@78b729e6; line: 1, column: 1]

Have a look at this https://stackoverflow.com/questions/1688099/converting-json-to-java?rq=1 — MBK, Nov 20 '15 at 00:28
Please don't use StringBuffer as it was replaced by StringBuilder more than ten years ago. — Peter Lawrey, Nov 22 '15 at 20:11

score 0 · Answer 1 · answered Nov 19 '15 at 12:16

"[{  \"frequency\": \"ONE_MIN\",  \"metricId\": 2253538,  \"metricName\": \"DB|Server:1|IO|Data File Average Read Size\",  \"metricPath\": \"Databases|X|IO|Data File Average Read Size\",  \"metricValues\": [  {    \"count\": 14,    \"current\": 65323,    \"max\": 65536,    \"min\": 0,    \"occurrences\": 0,    \"standardDeviation\": 0,    \"startTimeInMillis\": 1447927800000,    \"sum\": 251408,    \"useRange\": true,    \"value\": 17958  }]}]"

Incorrect the form of JSON object.

Try this:

"{  \"frequency\": \"ONE_MIN\",  \"metricId\": 2253538,  \"metricName\": \"DB|Server:1|IO|Data File Average Read Size\",  \"metricPath\": \"Databases|X|IO|Data File Average Read Size\",  \"metricValues\": [  {    \"count\": 14,    \"current\": 65323,    \"max\": 65536,    \"min\": 0,    \"occurrences\": 0,    \"standardDeviation\": 0,    \"startTimeInMillis\": 1447927800000,    \"sum\": 251408,    \"useRange\": true,    \"value\": 17958  }]}"

score 0 · Answer 2 · answered Nov 19 '15 at 12:17

0

Remove the outer "[]" from your JSON. You are passing an array of ADMetrics rather than a single ADMetrics which the readValue deserializer is expecting

answered Nov 19 '15 at 12:17

IanB

449
4
11

score 0 · Answer 3 · 2015-11-19T12:26:12.957

Can you try implementing serialisation in your pojo class as below

     public class ADMetrics implements Serializable{

private static final long serialVersionUID = 1L;
//All your variables and getter and setter method
}

Method One:

Also your JSON is not a JSONObject it is an JSONARRAY, use the below code to convert your above String to JSONARRAY

JsonArray jArray = new JsonParser().parse(json).getAsJsonArray();

The jArray is JSONArray from GSON , hence work with GSON to convert how u need or convert GSON to JSON.

OR

Method Two:

Directly convert the above input json from

Gson gson = new Gson();
            String output = gson.toJson(input);
            //System.out.println("output from gson "+output);
        JSONArray js = new JSONArray(output);

Then work with json as usual.

in Method One: use this line JsonArray jArray = new JsonParser().parse(jsonInString).getAsJsonArray(); — , Nov 19 '15 at 12:31

score 0 · Answer 4 · answered Nov 20 '15 at 00:14

Since the JSON you're receiving has an array, you need to deserialize to a list of objects. Presumably just grab the first one in the list:

    List<ADMetrics> adMetricsList = mapper.readValue(jsonInString, new TypeReference<List<ADMetrics>>(){});
    ADMetrics adMetrics = adMetricsList.get(0);

Or if you're not sure what else may be in the list and just want to get the first element, you can do the conversion in two steps:

    JsonNode tree = mapper.readTree(jsonInString);
    ADMetrics adMetrics = mapper.convertValue(tree.get(0), ADMetrics.class);

Cannot convert JSON HTTP Response to Java Object using Jackson

4 Answers4

Method One:

Method Two: