please post the code you persist through it and your entity's properties

and also the full stack trace of the exception  you've got

I am not expert in JPA field,and maybe this is not related to your problem,but i do not think so.
Because this about Hibernate(JPA) ,another implementation of JPA and i think it maybe solves your problem.
Please refer to javanotepad.blogspot.com/.../...-in-hibernate.html
Please let me know whether this helps you or not.

it is possibly because you have set the primary  key of the object,
while in your definition you have defined as auto generated.

but i know that my database  (MySQL) supports Auto Increment so i didnt use @GeneratedValue
as you see the code my persistence object  is very simple but i dont y does it work just for first time :D

my persistence  file

@Entity
@Table(name="user_table",schema="myschema")
public class user  {

@Id
@Column(name="ID",nullable=false)
private int id;

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

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

@Column(name="EMAIL",nullable=true)
private String email;

public User() { }

public User(int id) { this.id = id; }

public int getId() { return this.id; }

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

public String getUsername() { return this.username; }

public void setUsername(String username) { this.username = username; }

public String getPassword() { return this.password; }

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

public String getEmail() { return this.email; }

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

my stateless EJB

try {
//this.emf = null;
this.emf = Persistence.createEntityManagerFactory("test");
this.manager = emf.createEntityManager();
this.manager.getTransaction().begin();
this.manager.persist(user);
this.manager.getTransaction().commit();
} catch (EntityExistsException ex1) {
System.out.println(ex1 + " AZ TARAFE INITIAL SERVICE BEAN");
}
finally {
try {
this.manager.close();
} catch (Exception ee) {
System.out.println(ee );
return false;
}

my persistence.xml file
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistencehttp://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
<persistence-unit name="test" transaction-type="JTA">
<jta-data-source>ref</jta-data-source>
<class>persistobject.User</class>
<exclude-unlisted-classes>true</exclude-unlisted-classes>
<properties/>
</persistence-unit>

</persistence>

first before closing EM try to flush it by calling this.manager.flush()
second, where do you create your entity User? in other words, if you
create the user  entity once, this is considered a already attached
entity.
I think you should create User each time you are going to persist

em.beginTransaction()
User user = new User() //this line is between begin transaction and commit
user.setId(10L);
...
em.persist(user);

em.commit();

if you're not sure whether this user is a new user or an already
persisted user then I recommend you to use user = em.merge(user);
instead, this is usually happens when you pass your entity through a
method argument.

thanks for everything. i could solve  my problem  by myself. but it's so funny i just added this annotation @GeneratedValue(strategy=GenerationType.SEQUENCE)
and my problem is solved.
i red somewhere if your database  supports Auto Increment it's not necessary to add this annotation @GeneratedValue.
but It seems i had to add this annotation.
anyways thanks again and again

It was not funny! a small change may end up in a big problem
regardless of that, the code you were posted was different and there
was no @GeneratedValue !

congratulations for solving your problem!
the spec says if you have a auto generated id you should set the id field value manually, and if you have the auto generated id you should not set it your self.

Look at your database  table description  (in MySQL use command "desc TABLE_NAME") and see if its "id" column is "auto_increment"?
If not, modify (alter) your table and make id column auto_increment.

Then in your entity add this annotation to id property:
@GeneratedValue(strategy=GenerationType.AUTO)