package db;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class User {
@Id
@GeneratedValue(strategy= GenerationType.AUTO)
private Long id;

private String userName;

private String email;

private String password;

protected User() {}

public Long getId() {
    return id;
}

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

public String getUserName() {
    return userName;
}

public void setUserName(String userName) {
    this.userName = userName;
}

public String getEmail() {
    return email;
}

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

public String getPassword() {
    return password;
}

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

package db;

import java.util.List;

import org.springframework.data.repository.CrudRepository;

public interface UserRepository extends CrudRepository<User, Long> {

    List<User> findByUserName(String userName);
}

package db;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.stereotype.Repository;
import org.springframework.stereotype.Service;

@Repository
public class UserService {

    private static final Logger log = LoggerFactory.getLogger(Application.class);

UserRepository repository;

public User start() {
    User user = new User();
    user.setEmail("alovela@uk.com");
    user.setUserName("Ada");
    user.setPassword("Password");

    repository.save(user);

    return user;
}

2017-07-11 20:31:10.027 ERROR 7604 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause