Is composite-primary key the best solution - Hibernate Spring

Question

I am building a chat application with Spring, Hibernate and JPA and I want two tables - chats and messages. Chat will have many messages, also id and user_first and user_second. Right now I am doing it with a composite primary key where the composite is user_first and user_second and the id is unique auto-increment:

public class ChatPK implements Serializable {
    protected int firstUser;

    protected int secondUser;
... 

@Entity
@IdClass(ChatPK.class)
@Table(name = "chats1")
public class Chat implements Serializable {
    @Id
    private int firstUser;

    @Id
    private int secondUser;

    @Column(name = "id", unique = true, nullable = false, insertable = false, updatable = false)
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    private int id;
...

is there any better way to do it and why?

Update: Chat is a uniqe row between two people there cant be two chats with the same two users. I want to take all messages that I need along with the two users that had those messages, so I can put the sender id into a hashmap key and the messages into a value. To an answer bellow SessionId is a different thing than what my table chat is doing not a different way to implement what I want.

Answer 1

No it is not the best. The main role of a primary key is to bring uniqueness, secondary would be a fast access. Keys are used for caching as well, so the size of your key would detirmine how much you can cache as well. Making composite ckey would mean you need to override hashcode and equals possibly. You are overcomplicating it.

Obviously firstUsier Id and second user Id is not enough to bring uniqueness to your chat , because you can have many chats sessions between same users.

I think you should rename your id to "sessionID" use it as primary key and make the two relationships between yourtwo users outside of the primary key.

@Entity
@Table(name = "chats1")
public class Chat implements Serializable {
    @ManyToOne
    private User firstUser;

    @ManyToOne
    private User secondUser;

    @Id
    @Column(name = "sessionId")
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    private int sessionId;
...

UPDATE: The question turned into Pros and Cons of Composite keys which has been discussed in details here Why are composite keys discouraged in hibernate?