Java JPA writes object to binary

Question

• Database (User)
  ID Name 1 Ali
  2 Abu
  3 Ahmad
  

• WriteBinaryFile.java

  EntityManagerFactory emf = Persistence.createEntityManagerFactory("UserPU");
  EntityManager em = emf.createEntityManager();
  
  Query query1 = em.createNativeQuery("select * from user",User.class);
  List<User> list = query1.getResultList();
  
  for(User userList: list){
      FileOutputStream fos = new FileOutputStream("test.dat");
      ObjectOutputStream oos = new ObjectOutputStream(fos);
  
      oos.writeObject(userList);
      oos.close();
  }
  

• ReadBinaryFile.java

  FileInputStream fis = new FileInputStream("test.dat");
  ObjectInputStream ois = new ObjectInputStream(fis);
  
  User user1 = (User) ois.readObject();
  ois.close();
  System.out.println(user1.getID+" "+user1.getName());
  

• When I read ReadBinaryFile.java, I can get the result as follows:
   3 Ahmad

• My expected result:
  1 Ali
  2 Abu
  3 Ahmad
  

• What am I doing wrong? How can I solve it?

Answer 1

Do you rewrite the file in each loop?

for(User user: list){
    //overwrites the file on each loop
    FileOutputStream fos = new FileOutputStream("test.dat");
    ObjectOutputStream oos = new ObjectOutputStream(fos);

    oos.writeObject(user);
    oos.close();
}

You need to either create a new file for each user, or write the entire list and read it back.

FileOutputStream fos = new FileOutputStream("test.dat");
ObjectOutputStream oos = new ObjectOutputStream(fos);

oos.writeObject(list);
oos.flush();
oos.close(); 

FileInputStream fis = new FileInputStream("test.dat");
ObjectInputStream ois = new ObjectInputStream(fis);

List<User> users = (List<User>) ois.readObject();
ois.close();

for(User user : users){
    System.out.println(user.getID() +" " + user.getName());
}