How to save array of object in sprig boot with crudrepository?

I am very new to Spring boot and this is my first spring dummy project where I’m trying to learn to save an array of objects in mysql-db.

Actually, I am trying to use the saveAll method from the crudRepository in my controller. But it’s giving me an error no instance(s) of type variable(s) S exist so that Iterable<S> conforms to List<User>

My JSON looks like this:

[
    {
        "first_name": "Jack",
        "last_name": "Jill",
        "email": "[e-mail]",
        "password": "passq3623"
    },
    {
        "first_name": "John",
        "last_name": "Doe",
        "email": "[e-mail]",
        "password": "pass23"
    }
]

This is my entity class

package com.example.testbatch.Entities;

import javax.persistence.*;

@Entity
@Table(name = "user")
public class UserEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public int id;
    public String first_name;
    public String last_name;
    public String email;
    public String password;
}

Here is my User Model Class

package com.example.testbatch.Models;

public class User {
    public int id;
    public String firstName;
    public String lastName;
    public String email;
    public String password;
}

Here is my repository

package com.example.testbatch.Repositories;

import com.example.testbatch.Entities.UserEntity;
import org.springframework.data.repository.CrudRepository;

import java.util.List;

public interface UserRepository extends CrudRepository<UserEntity, Integer> {
    @Override
    List<UserEntity> findAll();

}

and here is my rest controller

package com.example.testbatch.Controllers;

import com.example.testbatch.Entities.UserEntity;
import com.example.testbatch.Models.User;
import com.example.testbatch.Repositories.UserRepository;
import org.apache.poi.ss.formula.functions.T;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import javax.transaction.Transactional;
import java.util.ArrayList;
import java.util.List;

@RestController
@RequestMapping("/api/v1/user")
public class UserController {

    @Autowired
    UserRepository userRepository;



    @PostMapping("/register")
    public void saveUsers(@RequestBody UserEntity userEntity) {
        List<User> persistedUser = userRepository.saveAll(List.of(userEntity));
    }


}

Can anyone please help me? This is ery first time I am usig spring boot

Answer

You have some problems about your code:

At first you have a DTO (User class), so, you should use this DTO inside your controller in saveUsers method and more importantly you pass an array of objects in your JSON, so you have to use a List not an Object as the parameter:

public void saveUsers(@RequestBody List<User> users) {...}

Another problem is you don’t have any getters and setters inside you DTO (public class User) and Entity (public class UserEntity) classes and also you have to add a method inside your DTO class for mapping DTO to Entity:

public class User {
    ...
    public static UserEntity dtoToEntity(User user){
        UserEntity userEntity = new UserEntity();
        userEntity.setEmail(user.getEmail());
        userEntity.setPassword(user.getPassword());
        userEntity.setFirst_name(user.getFirstName());
        userEntity.setLast_name(user.getLastName());
        return userEntity;
    }
}

Also you must change your JSON properties to match your DTO’s properties:

[
    {
        "firstName": "Jack",
        "lastName": "Jill",
        ...
    },
    ...
]

As the last one, change your controller saveUsers method as (saveAll method return Iterable not List):

@PostMapping("/register")
public void saveUsers(@RequestBody List<User> users) {

    List<UserEntity> userEntities = new ArrayList<>();

    for (User user : users) {
        userEntities.add(dtoToEntity(user));
    }

    Iterable<UserEntity> persistedUser = userRepository.saveAll(userEntities);
}

Notice: As a point remember that because you have auto-generated id inside your Entity, so you don’t need id property for your DTO, obviously because it’s auto-generated and you don’t need to get it from input;