Convert JSON to Java Object

by Hussein Terek · July 20, 2018

This tutorials shows 2 ways for converting a JSON string to a Java object.

Gson is the most popular library for converting JSON string to Java objects.

With Gson, you can do the conversion in one line as the following:

public static Student convertUsingGson(String jsonStr)

{

Gson gson = new Gson();

Student student = gson.fromJson(jsonStr, Student.class);

return student;

}

Some points to be considered when using Gson:

If the JSON string holds an invalid object attribute, then Gson implicitly ignores it.
If the JSON string misses some object attributes, then only the attributes included in the JSON are converted, other attributes take their default values.

Another common library used for converting JSON String to Java object is Jackson.

With Jackson, you do the conversion as the following:

public static Student convertUsingJackson(String jsonStr)

{

Student student = null;

try

{

ObjectMapper mapper = new ObjectMapper();

student = mapper.readValue(jsonStr, Student.class);

}

catch(Exception ex)

{

System.out.println("Error while converting JSON string to Student object.");

ex.printStackTrace();

}

return student;

}

Some points to be considered when using Jackson:

If the POJO doesn’t contain a default constructor, then the conversion fails.
If the JSON string holds any invalid object attribute, then the conversion fails unless you add explicitly at the POJO level this annotation:
1
@JsonIgnoreProperties(ignoreUnknown = true)
If the JSON string misses some object attributes, then only the attributes included in the JSON are converted, other attributes take their default values.
To convert a JSON array to a List of objects use:
1
List<Student> students = mapper.readValue(jsonStr, new TypeReference<List<Student>>(){});
To convert a JSON String to a HashMap use:
1
Map<String, Object> studentsMap = mapper.readValue(jsonStr, new TypeReference<Map<String,Object>>(){});