give a default value for an attribute if the value is null in json by jackson

You should be able to override the setter. Add the @JsonProperty(value="x") annotations to the getter and setter to let Jackson know to use them:

private class Student {
    private static final Integer DEFAULT_X = 1000;
    private Integer x = DEFAULT_X;

    @JsonProperty(value="x")
    public Integer getX() {
        return x;
    }

    @JsonProperty(value="x")
    public void setX(Integer x) {
        this.x = x == null ? DEFAULT_X : x;
    }
}

public class Student {
    private Integer x = Integer.valueOf(1000);

    public Integer getX() {
        return x;
    }

    public void setX(Integer x) {
        if(x != null) {
           this.x = x;
        }
    }
}

This works for me........

Test code 1:

public static void main(String[] args) throws IOException {
        String s = "{\"x\":null}";
        ObjectMapper mapper = new ObjectMapper();
        Student ss = mapper.readValue(s, Student.class);
        System.out.println(ss.getX());
    }

output:

1000

Test code 2:

public static void main(String[] args) throws IOException {
        String s = "{}";
        ObjectMapper mapper = new ObjectMapper();
        Student ss = mapper.readValue(s, Student.class);
        System.out.println(ss.getX());
    }

output:

1000

Consider extending JsonDeserializer

custom deserializer:

public class StudentDeserializer extends JsonDeserializer<Student> {
    @Override
    public Student deserialize(JsonParser p, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        JsonNode node = p.getCodec().readTree(p);
        // if JSON is "{}" or "{"x":null}" then create Student with default X
        if (node == null || node.get("x").isNull()) {
            return new Student();
        }
        // otherwise create Student with a parsed X value
        int x = (Integer) ((IntNode) node.get("x")).numberValue();
        Student student = new Student();
        student.setX(x);
        return student;
    }
}

and it's use:

ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(Student.class, new StudentDeserializer());
mapper.registerModule(module);     
Student readValue = mapper.readValue(<your json string goes here>", Student.class);