Library:
- gson (2.2.4)
<dependency> <groupId>com.google.code.gson</groupId> <artifactId>gson</artifactId> <version>2.2.4</version> </dependency>
( 1 ) Simple Java “POJO”/JSON
package com.hmkcode.vo;
import java.util.LinkedList;
import java.util.List;
public class Article {
private String title;
private String url;
private List<String> categories;
private List<String> tags;
//getters & setters...
}
Convert Java to JSON & JSON to Java
package com.hmkcode;
import com.google.gson.Gson;
import com.hmkcode.vo.Article;
public class App
{
public static void main( String[] args )
{
Gson gson = new Gson();
// Java --> JSON
String json = gson.toJson(createArticle());
System.out.println("toJson: "+json);
// JSON --> Java
Article article = gson.fromJson(json, Article.class);
System.out.println("fromJson: "+article);
}
private static Article createArticle(){
Article article = new Article();
article.setTitle("GSON - Java JSON Library");
article.setUrl("http://hmkcode.com/gson-json-java");
article.addCategory("Java");
article.addTag("Java");
article.addTag("GSON");
article.addTag("JSON");
return article;
}
}
( 2 ) Dealing with Collection<E>
Gson gson = new Gson();
List<Article> articles = new LinkedList<Article>();
articles.add(createArticle());
articles.add(createArticle());
// Java --> JSON
String json = gson.toJson(articles);
However, when converting this list back “from JSON to Java” we need to be a little bit careful. Converting JSON to Java using the simple way, will work but it will not give us List<Article> it will give us List<Map>.
// JSON --> Java
List list = gson.fromJson(json, List.class);
System.out.println("Class Type: "+list.get(0).getClass());
OutPut:
Class Type: class com.google.gson.internal.LinkedTreeMap
To get List<Article> we need to do one more step.
// JSON --> Java "Get the actual type"
Type type = new TypeToken<List<Article>>(){}.getType();
List list = gson.fromJson(json, type);
System.out.println("Class Type: "+list.get(0).getClass());
Output:
Class Type: class com.hmkcode.vo.Article
complete code:
package com.hmkcode;
import java.lang.reflect.Type;
import java.util.LinkedList;
import java.util.List;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import com.hmkcode.vo.Article;
public class App
{
public static void main( String[] args )
{
Gson gson = new Gson();
List<Article> articles = new LinkedList<Article>();
articles.add(createArticle());
articles.add(createArticle());
// Java --> JSON
String json = gson.toJson(articles);
System.out.println("toJson: "+json);
// JSON --> Java
List list = gson.fromJson(json, List.class);
System.out.println("fromJson: "+list);
System.out.println("Class Type: "+list.get(0).getClass());
// JSON --> Java "Get the actual type"
Type type = new TypeToken<List<Article>>(){}.getType();
list = gson.fromJson(json, type);
System.out.println("fromJson: "+list);
System.out.println("Class Type: "+list.get(0).getClass());
}
private static Article createArticle(){
Article article = new Article();
article.setTitle("GSON - Java JSON Library");
article.setUrl("http://hmkcode.com/gson-json-java");
article.addCategory("Java");
article.addTag("Java");
article.addTag("GSON");
article.addTag("JSON");
return article;
}
}