How to fix jpa one to many - spring

Question

Need to fetch data from one table to another.I performed jpa one to many mapping. But id cannot fetched. Where is my mistake?
I have tried mapping using one to many and many to one concepts but can't able to fetch data from one table to another
User.java
#Entity
#Table(name = "users")
public class User implements Serializable{
private static final long serialVersionUID = 1L;
#Id
#Column(name = "User_ID")
#GeneratedValue(generator="system-uuid")
#GenericGenerator(name="system-uuid", strategy = "uuid")
private String id;
private String firstName;
private String lastName;
private Long phoneNumber;
#NotNull(message="Password is compulsory")
#Email(message = "Email is invalid")
private String email;
private String password;
#OneToMany(mappedBy="user", cascade = CascadeType.ALL)
Set<Data> data = new HashSet<Data>();
public Set<Data> getData() {
return data;
}
public void setData(Set<Data> data) {
this.data = data;
}
public User() {
super();
}
Data.java
#Entity
#Table(name = "data")
public class Data implements Serializable{
private static final long serialVersionUID = 1L;
#Id
#Column(name = "DataID")
#GeneratedValue(generator="system-uuid")
#GenericGenerator(name="system-uuid", strategy = "uuid")
private String id;
#ManyToOne(fetch = FetchType.EAGER,cascade= CascadeType.ALL)
#JoinColumn(name = "User_ID")
private User user;
public Data() {
super();
}
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
DataController.java
#PostMapping("/data/{userId}")
public Data createData(
#PathVariable(value= "userId") String userId,
#Valid #RequestBody Data data) {
return userRepository.findById(userId).map(user -> {
data.setUser(user);
return dataRepository.save(data);
}).orElseThrow(() -> new ResourceNotFoundException("userId" + userId +
"not found"));
}
Results in no error but can't able to fetch user id