Many to many mapping attribute value type should not be persistent entity

Question

I am using hibernate with annotations and I have made two entities, one name User and other is Chatroom. I want to define bidirectional many to many mapping. But in one entity its not allowing me to apply many to many annotation and gives error "Many to many mapping attribute value type should not be persistent entity" here are the entities

package com.ciit.onlineexam.model;

import com.ciit.onlineexam.util.Mailer;
import org.apache.commons.io.FileUtils;

import javax.jws.soap.SOAPBinding;
import javax.persistence.*;
import java.io.File;
import java.io.IOException;
import java.io.Serializable;
import java.util.*;

@Entity
@Table(name="users")

public class User implements Serializable{



    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="id_users")
    public int getId_user() {
        return id_user;
    }
    private int id_user;
    public void setId_user(int id_user) {
        this.id_user = id_user;
    }


    @Column(name = "email")
    private String email;

    @Column(name="password")
    private String password;

    @Column(name = "name")
    private String name;

    private String reg_id;
    private String contact;

    @Column(name="isInformed")
    private boolean informed;

//Giving error on this list
@ManyToMany(cascade = CascadeType.ALL)
    private List<Chatroom> chatrooms = new ArrayList<Chatroom>(0);

  public User(){}


    public List<Chatroom> getChatrooms() {
        return chatrooms;
    }

    public void setChatrooms(List<Chatroom> chatrooms) {
        this.chatrooms = chatrooms;
    }


    public boolean isInformed() {
        return informed;
    }


    public void setInformed(boolean informed) {
        this.informed = informed;
    }



    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getReg_id() {
        return reg_id;
    }



    //public void setClas(Set<Clas> clas) {
      //  this.clas=clas;
    //}

    public void setReg_id(String reg_id) {
        this.reg_id = reg_id;
    }

    public String getContact() {
        return contact;
    }

    public void setContact(String contact) {
        this.contact = contact;
    }


    @OneToMany(mappedBy = "primaryKey.user",cascade = CascadeType.ALL)
    public Set<ClassesStudents> getClassesStudents() {
        return  classesStudents;
    }
    private Set<ClassesStudents> classesStudents;




    public void setClassesStudents(Set<ClassesStudents> classesStudents) {
        this.classesStudents = classesStudents;
    }



    public List<User> sendMailForAccountDetails(List<User> users, String path) throws Exception {
        List<User> users1 = new ArrayList<User>();
        for (User user : users) {
            if (!user.isInformed()) {
                Mailer mailer = new Mailer();
                String subject = "You have been added to AcademicsTOOLS ";
                String emailContents = FileUtils.readFileToString(new File(path + "emailTemplates" + "/" + "SendAccountDetails.html"));
                emailContents = emailContents.replaceAll("-AddedStudentName-", user.getName());
                emailContents = emailContents.replaceAll("-emailId-", user.email);
                emailContents = emailContents.replaceAll("-accountPassword-", user.getPassword());
              //  emailContents = emailContents.replaceAll("-ProfileLink-", path);
                mailer.postMail(user.email, subject, emailContents);
                user.setInformed(true);
                users1.add(user);
                System.out.println("Reached in email AD ==================================================");
            }
        }
        return users1;
    }



    public User sendMailForAccountDetails(User user, String path) throws Exception {
        List<User> users = new ArrayList<User>();
        users.add(user);
        sendMailForAccountDetails(users, path);
        user.setInformed(true);
        return user;
    }


    @Override
    public String toString() {
        return "User{" +
                "id_user=" + id_user +
                ", email='" + email + '\'' +
                ", password='" + password + '\'' +
                ", name='" + name + '\'' +
                ", reg_id='" + reg_id + '\'' +
                ", contact=" + contact +
                '}';
    }


}

Other class is of Chatroom

package com.ciit.onlineexam.model;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

@Entity
@Table(name = "Chatroom")
public class Chatroom {

    @Column(name="Creator",nullable = false)
    private int creator;

    @Id
    @GeneratedValue(strategy= GenerationType.AUTO)
    @Column(name="id_chatroom")
    private int id;

    @Column(name="Name",nullable = false)
    private String name;

    @Column(name="Description")
    private String description;

    @Column(name = "Category")
    private String category;

    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(name = "userschatrooms",
            joinColumns = { @JoinColumn(name = "id_chatroom",nullable = false)},
            inverseJoinColumns = { @JoinColumn(name = "id_users")})
    private List<User> listOfUsers = new ArrayList<User>(0);

    public Chatroom(){}

    public String getName() {
        return name;
    }

    public void setName(String group_name) {
        this.name = group_name;
    }


    public int getCreator() {
        return creator;
    }

    public void setCreator(int group_creator) {
        this.creator = group_creator;
    }


    public int getId() {
        return id;
    }

    public void setId(int group_id) {
        this.id = group_id;
    }


    public String getDescription() {
        return description;
    }

    public void setDescription(String group_description) {
        this.description = group_description;
    }

    public String getCategory() {
        return category;
    }

    public void setCategory(String category) {
        this.category = category;
    }

    public void setListOfUsers(List<User> listOfStudents) {
        this.listOfUsers = listOfStudents;
    }

    public List<User> getListOfUsers() {
        return listOfUsers;
    }

}