Mapping JSON to Entity

Let’s say that you have a json file which you want to map to a table in the database in your application with Spring Boot

languages.json

First, in order to map this to a class, we need to create an entity.

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.validation.constraints.Size;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
@Data
@AllArgsConstructor
@NoArgsConstructor
@Entity
public class Language {
 
 @Id
 @Size(min = 3, max = 3)
 private String languageCode;
 
 private String languageName;
public Language(String languageCode) {
  super();
  this.languageCode = languageCode;
 }

All annotations except for Entity above class definition comes from Lombok library. Data annotation provides getters, setters, equals function, the other two is pretty self-explanatory. If you decide to use Lombok in your project and have problems, I suggest you to take a look at this post.

I wanted to use language code field as the primary key, since it’s internationally accepted and unique for each spoken language. You can also have a Long field and let javax.persistence library to auto generate an id for you, it is entirely up to your design choices.

After that, I created a command line runner to load this json into my database at the beginning of the application.

package com.countries.mehmet;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

import com.countries.mehmet.domain.Language;
import com.countries.mehmet.service.ILanguageService;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.InputStream;
import java.util.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
@Component
public class LanguagesCommandLineRunner implements CommandLineRunner {
private static final Logger log =
   LoggerFactory.getLogger(LanguagesCommandLineRunner.class);
 
 
 @Autowired
 private ILanguageService languageService;
 
 @Override
 public void run(String... args) throws Exception {
  
  loadLanguages();
  
 }
 
 private void loadLanguages() {
  ObjectMapper mapper = new ObjectMapper();
  TypeReference<List<Language>> typeReference = new TypeReference<List<Language>>(){};
  
  InputStream inputStream = TypeReference.class.getResourceAsStream("/json/languages.json");
  
  try {
   List<Language> languages = mapper.readValue(inputStream, typeReference);
   languageService.save(languages);
   log.info(String.format("%d languages saved to H2 database", languages.size()));
   
  } catch (Exception e) {
   log.error("Failed to save languages to db :" + e.getMessage());
  }
 }
}

CommandLineRunner interface have one abstract method that needs to be implemented, i.e. run.

import java.util.Iterator;
import java.util.List;
import com.countries.mehmet.domain.Language;
public interface ILanguageService {
  List<Language> list();
  Iterator<Language> save(List<Language> languages);
  Language findById(String id);
}

Language service is an interface which needs to be implemented in order to connect to your database in the back. While implementing, I used JPA repository and an H2 in-memory database.