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?
Popular Answer
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.
Hope this helps!
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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 2020) 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
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.
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.
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("}");
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