Converting Java objects to JSON with Jackson

I want my JSON to look like this:


"information": [
"timestamp": "xxxx",
"feature": "xxxx",
"ean": 1234,
"data": "xxxx"
,
"timestamp": "yyy",
"feature": "yyy",
"ean": 12345,
"data": "yyy"
]



Code so far:

import java.util.List;

public class ValueData

private List<ValueItems> information;

public ValueData()



public List<ValueItems> getInformation()
return information;


public void setInformation(List<ValueItems> information)
this.information = information;


@Override
public String toString()
return String.format("information:%s", information);





and

public class ValueItems

private String timestamp;
private String feature;
private int ean;
private String data;


public ValueItems()



public ValueItems(String timestamp, String feature, int ean, String data)
this.timestamp = timestamp;
this.feature = feature;
this.ean = ean;
this.data = data;


public String getTimestamp()
return timestamp;


public void setTimestamp(String timestamp)
this.timestamp = timestamp;


public String getFeature()
return feature;


public void setFeature(String feature)
this.feature = feature;


public int getEan()
return ean;


public void setEan(int ean)
this.ean = ean;


public String getData()
return data;


public void setData(String data)
this.data = data;


@Override
public String toString()
return String.format("timestamp:%s,feature:%s,ean:%s,data:%s", timestamp, feature, ean, data);




I just missing the part how I can convert the Java object to JSON with Jackson:

public static void main(String args)
// CONVERT THE JAVA OBJECT TO JSON HERE
System.out.println(json);



My Question is: Are my classes correct? Which instance do I have to call and how that I can achieve this JSON output?

8 Answers
8

To convert your object in JSON with Jackson:

object

ObjectWriter ow = new ObjectMapper().writer().withDefaultPrettyPrinter();
String json = ow.writeValueAsString(object);


I know this is old (and I am new to java), but I ran into the same problem. And the answers were not as clear to me as a newbie... so I thought I would add what I learned.

I used a third-party library to aid in the endeavor: org.codehaus.jackson
All of the downloads for this can be found here.

org.codehaus.jackson

For base JSON functionality, you need to add the following jars to your project's libraries:
jackson-mapper-asl
and
jackson-core-asl

Choose the version your project needs. (Typically you can go with the latest stable build).

Once they are imported in to your project's libraries, add the following import lines to your code:

import

import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;


With the java object defined and assigned values that you wish to convert to JSON and return as part of a RESTful web service

User u = new User();
u.firstName = "Sample";
u.lastName = "User";
u.email = "sampleU@example.com";

ObjectMapper mapper = new ObjectMapper();

try
// convert user object to json string and return it
return mapper.writeValueAsString(u);

catch (JsonGenerationException | JsonMappingException e)
// catch various errors
e.printStackTrace();



The result should looks like this:
"firstName":"Sample","lastName":"User","email":"sampleU@example.com"

"firstName":"Sample","lastName":"User","email":"sampleU@example.com"

This might be useful:

objectMapper.writeValue(new File("c:\employee.json"), employee);

// display to console
Object json = objectMapper.readValue(
objectMapper.writeValueAsString(employee), Object.class);

System.out.println(objectMapper.writerWithDefaultPrettyPrinter()
.writeValueAsString(json));


just do this

Gson gson = new Gson();
return Response.ok(gson.toJson(yourClass)).build();


jackson

Gson

Well, even the accepted answer does not exactly output what op has asked for. It outputs the JSON string but with " characters escaped. So, although might be a little late, I am answering hopeing it will help people! Here is how I do it:

"

StringWriter writer = new StringWriter();
JsonGenerator jgen = new JsonFactory().createGenerator(writer);
jgen.setCodec(new ObjectMapper());
jgen.writeObject(object);
jgen.close();
System.out.println(writer.toString());


Note: To make the most voted solution work, attributes in the POJO have to be public or have a public getter/setter:

public

getter

setter

By default, Jackson 2 will only work with fields that are either
public, or have a public getter method – serializing an entity that
has all fields private or package private will fail.

Not tested yet, but I believe that this rule also applies for other JSON libs like google Gson.

You could do this:

String json = new ObjectMapper().writeValueAsString(yourObjectHere);


public class JSONConvector

public static String toJSON(Object object) throws JSONException, IllegalAccessException
String str = "";
Class c = object.getClass();
JSONObject jsonObject = new JSONObject();
for (Field field : c.getDeclaredFields())
field.setAccessible(true);
String name = field.getName();
String value = String.valueOf(field.get(object));
jsonObject.put(name, value);

System.out.println(jsonObject.toString());
return jsonObject.toString();



public static String toJSON(List list ) throws JSONException, IllegalAccessException
JSONArray jsonArray = new JSONArray();
for (Object i : list)
String jstr = toJSON(i);
JSONObject jsonObject = new JSONObject(jstr);
jsonArray.put(jsonArray);

return jsonArray.toString();




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