Javaのflexjsonライブラリを使用してマップをシリアル化するにはどうすればよいですか？

import flexjson.JSONSerializer;
import java.util.*;
public class JsonSerializeMapTest {
   public static void main(String[] args) {
      JSONSerializer serializer = new JSONSerializer().prettyPrint(true);
      Student student = new Student("Adithya", "Sai", 28, "Hyderabad");
      Map<String, Object> map = new HashMap<String, Object>();
      map.put("Student1", "Raja");
      map.put("Student2", "Ravi");
      map.put("my_student", student);
      String jsonStr = serializer.serialize(map);
      System.out.println(jsonStr);
   }
}
// Student class
class Student {
   private String firstName;
   private String lastName;
   private int age;
   private String address;
   public Student() {}
   public Student(String firstName, String lastName, int age, String address) {
      super();
      this.firstName = firstName;
      this.lastName = lastName;
      this.age = age;
      this.address = address;
   }
   public String getFirstName() {
      return firstName;
   }
   public String getLastName() {
      return lastName;
   }
   public int getAge() {
      return age;
   }
   public String getAddress() {
      return address;
   }
   public String toString() {
      return "Student[ " +
             "firstName = " + firstName +
             ", lastName = " + lastName +
            ", age = " + age +
         ", address = " + address +" ]";
   }  
}

{
   "my_student": {
      "address": "Hyderabad",
      "age": 28,
      "class": "Student",
      "firstName": "Adithya",
      "lastName": "Sai"
 },
 "Student1": "Raja",
 "Student2": "Ravi"
}