BootApplication.java

package com.verydapeng;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.rest.core.annotation.RepositoryRestResource;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.validation.Valid;
import javax.validation.constraints.Size;
import java.util.stream.Stream;

@SpringBootApplication
@RestController
public class BootApplication {

    public static void main(String[] args) {
        SpringApplication.run(BootApplication.class, args);
    }

    @RequestMapping("/")
    String hello(@RequestBody @Valid User user) {
        return "hello " + user;
    }

}

@Entity
class User {

    @Id
    @GeneratedValue
    private long id;
    @Size(min = 6)
    private String username;
    @Size(min = 6)
    private String password;

    public User() {
    }

    @JsonCreator
    public User(@JsonProperty("username") String username,
                @JsonProperty("password") String password) {
        this.username = username;
        this.password = password;
    }

    public long getId() {
        return id;
    }

    public String getUsername() {
        return username;
    }

    public String getPassword() {
        return password;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                '}';
    }
}

@RepositoryRestResource
interface UserRepo extends JpaRepository<User, Long> {
}