Generate Java class from JSON?

Question

In a Java Maven project, how do you generate java source files from JSON? For example we have

{
  "firstName": "John",  
  "lastName": "Smith",  
  "address": {  
    "streetAddress": "21 2nd Street",  
     "city": "New York"
  }
}

When we run mvn generate-sources we want it to generate something like this:

class Address  {
    JSONObject mInternalJSONObject;
     
    Address (JSONObject json){
        mInternalJSONObject = json;
    }
     
    String  getStreetAddress () {
        return mInternalJSONObject.getString("streetAddress");
    }
    
    String  getCity (){
        return mInternalJSONObject.getString("city");
    }
}

class Person {        
    JSONObject mInternalJSONObject;
    
    Person (JSONObject json){
        mInternalJSONObject = json;
    }
    
    String  getFirstName () {
        return mInternalJSONObject.getString("firstName");
    }
    
    String  getLastName (){
        return mInternalJSONObject.getString("lastName");
    }
    
    Address getAddress (){
        return Address(mInternalJSONObject.getString("address"));
    }
}

As a Java developer, what lines of XML do I need to write in my pom.xml in order to make this happen?

I've updated the question and answer to make them on-topic. Please reopen. — Alex R
– Alex R, Commented Jun 8, 2020 at 0:49

starball · Accepted Answer · 2022-12-23 01:52:39Z

299

Try http://www.jsonschema2pojo.org

Or the jsonschema2pojo plug-in for Maven:

<plugin>
    <groupId>org.jsonschema2pojo</groupId>
    <artifactId>jsonschema2pojo-maven-plugin</artifactId>
    <version>1.0.2</version>
    <configuration>
        <sourceDirectory>${basedir}/src/main/resources/schemas</sourceDirectory>
        <targetPackage>com.myproject.jsonschemas</targetPackage>
        <sourceType>json</sourceType>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
        </execution>
    </executions>
</plugin>

The <sourceType>json</sourceType> covers the case where the sources are json (like the OP). If you have actual json schemas, remove this line.

Updated in 2014: Two things have happened since Dec '09 when this question was asked:

The JSON Schema spec has moved on a lot. It's still in draft (not finalised) but it's close to completion and is now a viable tool specifying your structural rules
I've recently started a new open source project specifically intended to solve your problem: jsonschema2pojo. The jsonschema2pojo tool takes a json schema document and generates DTO-style Java classes (in the form of .java source files). The project is not yet mature but already provides coverage of the most useful parts of json schema. I'm looking for more feedback from users to help drive the development. Right now you can use the tool from the command line or as a Maven plugin.

edited Dec 23, 2022 at 1:52

starball♦

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answered Dec 18, 2010 at 23:00

joelittlejohn

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38 Comments

Jeff Axelrod Over a year ago

Wouldn't someone who used your jsonschema2pojo tool have to write their own schema file then? The OP asked to start with a Json file, not a schema. Is there a companion tool to go from Json -> Schema? I assume that such a tool, if it existed, could only provide a guess.

joelittlejohn Over a year ago

As of version 0.3.3, you can use plain old JSON as input :)

joelittlejohn Over a year ago

...and there's now an online generator too: jsonschema2pojo.org

mbmast Over a year ago

Excellent tool. The supplied link contains an online tool where you can paste in sample JSON, click a button and get Java source.

joelittlejohn Over a year ago

@testerjoe If you mean Java source code, then yes jsonschema2pojo does this, and it's available as a maven plugin, ant task, gradle extension, CLI tool, java library, etc...

|

sahinakkaya · Accepted Answer · 2021-08-02 14:20:18Z

22

If you're using Jackson (the most popular library there), try

https://github.com/astav/JsonToJava

Its open source (last updated on Jun 7, 2013 as of year 2021) and anyone should be able to contribute.

Summary

A JsonToJava source class file generator that deduces the schema based on supplied sample json data and generates the necessary java data structures.

It encourages teams to think in Json first, before writing actual code.

Features

Can generate classes for an arbitrarily complex hierarchy (recursively)
Can read your existing Java classes and if it can deserialize into those structures, will do so
Will prompt for user input when ambiguous cases exist

edited Aug 2, 2021 at 14:20

sahinakkaya

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answered Oct 25, 2012 at 14:52

Astav

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Comments

Tim Boudreau · Accepted Answer · 2015-02-07 02:23:07Z

19

Here's an online tool that will take JSON, including nested objects or nested arrays of objects and generate a Java source with Jackson annotations.

answered Feb 7, 2015 at 2:23

Tim Boudreau

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3 Comments

noogrub Over a year ago

This worked very well for me on the first go. I had deeply nested JSON and it worked fine, although I did have to trim redundant portions to get the overall size below 2k. Enabled me to write: MyClass c = new MyClass(); c = gson.fromJson(c.getHTML(someURLthatReturnsJSON), MyClass.class); and the resulting data flowed perfectly. I had to remove all those Jackson notations, but otherwise it worked fine for gson. Thank you.

CoolMind Over a year ago

Thanks, it works. When I feeded a JSON with case-sensitive fields, this site returned a result, while www.jsonschema2pojo.org reported an error.

ka3ak Over a year ago

I was able to generate some java code today, but it stopped working then. The corresponding text area isn't there anymore...

joelittlejohn · Accepted Answer · 2014-06-24 22:40:39Z

7

Answering this old question with recent project ;-).

At the moment the best solution is probably JsonSchema2Pojo :

It does the job from the seldom used Json Schema but also with plain Json. It provides Ant and Maven plugin and an online test application can give you an idea of the tool. I put a Json Tweet and generated all the containing class (Tweet, User, Location, etc..).

We'll use it on Agorava project to generate Social Media mapping and follow the contant evolution in their API.

edited Jun 24, 2014 at 22:40

joelittlejohn

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answered Oct 18, 2012 at 8:16

Antoine Sabot-Durand

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2 Comments

Gregor Over a year ago

That's also my impression, but I didn't try the Maven plugin yet, however the online version is pretty slow and dies for anything other than the usual Person class... So for quick online conversion, @tim-boudreau's tool worked best for me.

AndroidDev Over a year ago

I tried JsonSchema2Pojo but hitting the Preview button pops up the blank preview.

Denis Palnitsky · Accepted Answer · 2009-12-25 09:23:21Z

Thanks all who attempted to help.
For me this script was helpful. It process only flat JSON and don't take care of types, but automate some routine

  String str = 
        "{"
            + "'title': 'Computing and Information systems',"
            + "'id' : 1,"
            + "'children' : 'true',"
            + "'groups' : [{"
                + "'title' : 'Level one CIS',"
                + "'id' : 2,"
                + "'children' : 'true',"
                + "'groups' : [{"
                    + "'title' : 'Intro To Computing and Internet',"
                    + "'id' : 3,"
                    + "'children': 'false',"
                    + "'groups':[]"
                + "}]" 
            + "}]"
        + "}";



    JSONObject json = new JSONObject(str);
    Iterator<String> iterator =  json.keys();

    System.out.println("Fields:");
    while (iterator.hasNext() ){
       System.out.println(String.format("public String %s;", iterator.next()));
    }

    System.out.println("public void Parse (String str){");
    System.out.println("JSONObject json = new JSONObject(str);");

    iterator  = json.keys();
    while (iterator.hasNext() ){
       String key = iterator.next();
       System.out.println(String.format("this.%s = json.getString(\"%s\");",key,key ));

    System.out.println("}");

Chris R · Accepted Answer · 2011-12-21 05:25:20Z

5

I'm aware this is an old question, but I stumbled across it while trying to find an answer myself.

The answer that mentions the online json-pojo generator (jsongen) is good, but I needed something I could run on the command line and tweak more.

So I wrote a very hacky ruby script to take a sample JSON file and generate POJOs from it. It has a number of limitations (for example, it doesn't deal with fields that match java reserved keywords) but it does enough for many cases.

The code generated, by default, annotates for use with Jackson, but this can be turned off with a switch.

You can find the code on github: https://github.com/wotifgroup/json2pojo

answered Dec 21, 2011 at 5:25

Chris R

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Comments

inder · Accepted Answer · 2015-03-14 19:34:03Z

3

I created a github project Json2Java that does this. https://github.com/inder123/json2java

Json2Java provides customizations such as renaming fields, and creating inheritance hierarchies.

I have used the tool to create some relatively complex APIs:

Gracenote's TMS API: https://github.com/inder123/gracenote-java-api

Google Maps Geocoding API: https://github.com/inder123/geocoding

answered Mar 14, 2015 at 19:34

inder

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Comments

xbmono · Accepted Answer · 2021-07-20 02:26:47Z

3

I know there are many answers but of all these I found this one most useful for me. This link below gives you all the POJO classes in a separate file rather than one huge class that some of the mentioned websites do:

https://json2csharp.com/json-to-pojo

It has other converters too. Also, it works online without a limitation in size. My JSON is huge and it worked nicely.

answered Jul 20, 2021 at 2:26

xbmono

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Comments

ehanoc · Accepted Answer · 2012-06-19 12:38:45Z

2

I had the same problem so i decided to start writing a small tool to help me with this. Im gonna share andopen source it.

https://github.com/BrunoAlexandreMendesMartins/CleverModels

It supports, JAVA, C# & Objective-c from JSON .

Feel free to contribute!

edited Jun 19, 2012 at 12:38

answered Apr 20, 2012 at 12:59

ehanoc

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Comments

Foobnix · Accepted Answer · 2014-11-05 12:11:45Z

2

Try my solution

http://htmlpreview.github.io/?https://raw.githubusercontent.com/foobnix/android-universal-utils/master/json/generator.html

{
    "auctionHouse": "sample string 1",
    "bidDate": "2014-05-30T08:20:38.5426521-04:00 ",
    "bidPrice": 3,
    "bidPrice1": 3.1,
    "isYear":true
}

Result Java Class

private String  auctionHouse;
private Date  bidDate;
private int  bidPrice;
private double  bidPrice1;
private boolean  isYear;

JSONObject get

auctionHouse = obj.getString("auctionHouse");
bidDate = obj.opt("bidDate");
bidPrice = obj.getInt("bidPrice");
bidPrice1 = obj.getDouble("bidPrice1");
isYear = obj.getBoolean("isYear");

JSONObject put

obj.put("auctionHouse",auctionHouse);
obj.put("bidDate",bidDate);
obj.put("bidPrice",bidPrice);
obj.put("bidPrice1",bidPrice1);
obj.put("isYear",isYear);

answered Nov 5, 2014 at 12:11

Foobnix

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1 Comment

Rafael Ruiz Tabares Over a year ago

this answer looks so trivial. There is automatic way to generate pojo from json

StaxMan · Accepted Answer · 2010-01-10 23:24:57Z

As far as I know there is no such tool. Yet.

The main reason is, I suspect, that unlike with XML (which has XML Schema, and then tools like 'xjc' to do what you ask, between XML and POJO definitions), there is no fully features schema language. There is JSON Schema, but it has very little support for actual type definitions (focuses on JSON structures), so it would be tricky to generate Java classes. But probably still possible, esp. if some naming conventions were defined and used to support generation.

However: this is something that has been fairly frequently requested (on mailing lists of JSON tool projects I follow), so I think that someone will write such a tool in near future.

So I don't think it is a bad idea per se (also: it is not a good idea for all use cases, depends on what you want to do ).

MB.One · Accepted Answer · 2012-10-18 08:40:20Z

0

You could also try GSON library. Its quite powerful it can create JSON from collections, custom objects and works also vice versa. Its released under Apache Licence 2.0 so you can use it also commercially.

http://code.google.com/p/google-gson/

answered Oct 18, 2012 at 8:40

MB.One

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2 Comments

necromancer Over a year ago

doesn't create java definitions

hop Over a year ago

This is not what the question is about

dgorissen · Accepted Answer · 2011-02-23 14:08:53Z

-3

To add to @japher's post. If you are not particularly tied to JSON, Protocol Buffers is worth checking out.

answered Feb 23, 2011 at 14:08

dgorissen

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1 Comment

james.garriss Over a year ago

Protocol Buffers is not even close to an answer on how to create Java objects from JSON. At the very least you should have recommended a tool for creating Java Objects from Protocol Buffers.

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