如何使用Java中的flexjson库序列化一个map？

public String serialize(Object target)

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"
}