[ Java 文章收集 ] How to convert Java object to / from JSON (Gson)

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Preface
In this tutorial, we will show you how to use Gson to convert Java object to / from JSON. Below is the basic description of Gson:

Gson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Gson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of.

There are a few open-source projects that can convert Java objects to JSON. However, most of them require that you place Java annotations in your classes; something that you can not do if you do not have access to the source-code. Most also do not fully support the use of Java Generics. Gson considers both of these as very important design goals.

Note.

JSON stands for JavaScript Object Notation, it is a lightweight data-interchange format. You can see many Java applications started to throw away XML format and start using JSON as a new data-interchange format. Java is all about object, often times, you need to convert an object into JSON format for data-interchange or vice verse.

Jackson is another high performance JSON processor, try this Jackson 2 – Java object to / from JSON

1. Quick Reference

1.1 toJson() – Convert Java object to JSON

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1. package demo.gson;  
2.   
3. import java.io.FileWriter;  
4. import java.util.ArrayList;  
5. import java.util.List;  
6.   
7. import com.google.gson.Gson;  
8.   
9. public class People {  
10.     public String       name = null;  
11.     public int          age = 0;  
12.     public List   addr = new ArrayList();  
13.     public boolean      isSingle=false;  
14.   
15.     @Override  
16.     public String toString()  
17.     {  
18.         StringBuffer strBuf = new StringBuffer(String.format("Name: %s\n", name));  
19.         strBuf.append(String.format("Age: %d\n", age));  
20.         strBuf.append(String.format("IsSingle? %s\n", isSingle));  
21.         strBuf.append("Addr:\n");  
22.         for(String a:addr) strBuf.append(String.format("\t%s\n", a));  
23.         return strBuf.toString();  
24.     }  
25.       
26.     public static void ToJson() throws Exception  
27.     {  
28.         People p = new People();  
29.         p.name = "John"; p.age = 36; p.addr.add("abc"); p.addr.add("123"); p.isSingle=true;  
30.         Gson gson = new Gson();  
31.           
32.         // 1. Java object to JSON, and save into a file  
33.         FileWriter fw = new FileWriter("john.json");  
34.         gson.toJson(p, fw);  
35.         fw.flush(); fw.close();  
36.   
37.         // 2. Java object to JSON, and assign to a String  
38.         String jsonInString = gson.toJson(p);  
39.         System.out.printf("%s\n", jsonInString);  
40.     }  
41. }  

Output:

{"name":"John","age":36,"addr":["abc","123"],"isSingle":true}

1.2 fromJson() – Convert JSON to Java object

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1. public static void FromJson() throws Exception{  
2.     Gson gson = new Gson();  
3.   
4.     // 1. JSON to Java object, read it from a file.  
5.     FileReader fw = new FileReader("john.json");  
6.     People p1 = gson.fromJson(fw, People.class);  
7.     System.out.printf("P1:\n%s\n", p1);  
8.     fw.close();  
9.   
10.     // 2. JSON to Java object, read it from a Json String.  
11.     String jsonInString = "{'name':'John','age':36,'addr':['abc','123'],'isSingle':true}";  
12.     People p2 = gson.fromJson(jsonInString, People.class);  
13.     System.out.printf("P2:\n%s\n", p2);  
14.   
15.     // JSON to JsonElement, convert to String later.  
16.     JsonElement json = gson.fromJson(new FileReader("john.json"), JsonElement.class);  
17.     String result = gson.toJson(json);  
18.     System.out.printf("P3:\n%s\n", result);  
19. }  

2. FAQs
Some commonly ask questions.

2.1 Convert a JSON Array to a List, using TypeToken

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1. package demo.gson;  
2.   
3. import java.util.List;  
4.   
5. import com.google.gson.Gson;  
6. import com.google.gson.reflect.TypeToken;  
7.   
8. public class Movie {  
9.     public String   name = "";  
10.     public int      cost = 0;  
11.     public Movie(String n, int c){this.name = n; this.cost = c;}  
12.       
13.     @Override  
14.     public String toString()  
15.     {  
16.         return String.format("%s/%d", name, cost);  
17.     }  
18.       
19.     public static void main(String args[])  
20.     {  
21.         String json = "[{\"name\":\"Scary Movie\",\"cost\":50}, {\"name\":\"Action Movie\",\"cost\":50}]";  
22.         Gson gson = new Gson();  
23.         List list = gson.fromJson(json, new TypeToken>(){}.getType());  
24.         for(Movie m:list) {System.out.printf("%s\n", m);}  
25.     }  
26. }  

Output:

Scary Movie/50
Action Movie/50

2.2 Convert a JSON to a Map

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1. public static void CovJson2Map()  
2. {  
3.     String json = "{\"name\":\"Comedy Movie\",\"cost\":30}";  
4.     Gson gson = new Gson();  
5.     Map map = gson.fromJson(json, new TypeToken
6.     map.forEach((x,y)->System.out.printf("%s|%s\n", x, y));  
7. }  

Output:

name|Comedy Movie
cost|30.0

2.3 Enable JSON pretty print feature in Gson
1. By default, Gson display the JSON output like the following :

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1. Gson gson = new Gson();  
2. People p = new People();  
3. p.name = "John"; p.age = 36; p.addr.add("abc"); p.addr.add("123"); p.isSingle=true;  
4. String json = gson.toJson(p);  
5. System.out.println(json);  

Output:

{"name":"John","age":36,"addr":["abc","123"],"isSingle":true}

2. To enable the pretty-print, create the Gson object with GsonBuilder

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1. Gson gson = new GsonBuilder().setPrettyPrinting().create();  
2. People p = new People();  
3. p.name = "John"; p.age = 36; p.addr.add("abc"); p.addr.add("123"); p.isSingle=true;  
4. String json = gson.toJson(p);  
5. System.out.println(json);  

Output:

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1. {  
2.   "name": "John",  
3.   "age": 36,  
4.   "addr": [  
5.     "abc",  
6.     "123"  
7.   ],  
8.   "isSingle": true  
9. }