Json <-> Java serialization that works with GWT

Question

I am looking for a simple Json (de)serializer for Java that might work with GWT. I have googled a bit and found some solutions that either require annotate every member or define useless interfaces. Quite a boring. Why don't we have something really simple like

class MyBean {
    ...
}

new GoodSerializer().makeString(new MyBean());
new GoodSerializer().makeObject("{ ... }", MyBean.class)

Answer 1

OK, I deleted my previous answer because it turned out to be exactly what you didn't want.

I don't know how well it works with GWT, but we use the json-lib library to serialize objects in a normal Java project where I work.

It can create a JSONObject directly from a JavaBean, then use the resulting JSONObject's toString() method to get the actual JSON string back.

Likewise, it can also turn JSON back into a JavaBean.

Answer 2

I seem to be answering this question a lot...

There's a page on code.google.com titled Using GWT for JSON Mashups. It's (unfortunately) way over my head, as I'm not that familiar with GWT, so it may not be helpful.

Answer 3

Not sure if Jackson (http://jackson.codehaus.org/Tutorial) would work for you. I don't know if there's GWT-specific you are looking for; if not it should work.

But its serialization/deserialization works quite well, like:

// read json, create object

ObjectMapper mapper = new ObjectMapper();

MyBean bean = mapper.readValue(jsonAsString, MyBean.class);

// and write out

StringWriter sw = new StringWriter();

mapper.writeValue(sw, user);

String jsonOut = sw.toString();

You do need accessors (getx() to serializer, setX() to deserialize; can annotate methods with other names), but that's about it.

Answer 4

It seems that I found the right answer to my question

I figured out that bean to json and json to bean conversion in GWT isn't a trivial task. Known libraries would not work because GWT would require their full source code and this source code must use only Java classes that are amoung emulated by GWT. Also, you cannot use reflection in GWT. Very tough requirements!

I found the only existing solution named gwt-jsonizer. It uses a custom Generator class and requires a satellite interface for each "jsonable" bean. Unfortunately, it does not work without patching on the latest version of GWT and has not been updated for a long time.

So, I personally decided that it is cheaper and faster to make my beans khow how to convert themselves to and from json. Like this:

public class SmartBean {
 private String name;

 public String getName() { return name; }
 public void setName(String value) { name = value;  }

 public JSONObject toJson() {
  JSONObject result = new JSONObject();
  result.put("name", new JSONString(this.name));
  return result;
 }
 public void fromJson(JSONObject value) {
  this.name = value.get("name").isString().stringValue();
 }

}

JSONxxxx are GWT built-in classes that provide low-level json support.

Answer 5

Take a look at GWT's Overlay Types. I think this is by far the easiest way to work with JSON in GWT. Here's a modified code example from the linked article:

public class Customer extends JavaScriptObject {
    public final native String getFirstName() /*-{ 
        return this.first_name;
    }-*/;
    public final native void setFirstName(String value) /*-{
        this.first_name = value;
    }-*/;
    public final native String getLastName() /*-{
        return this.last_name;
    }-*/;
    public final native void setLastName(String value) /*-{
        this.last_name = value;
    }-*/;
}

Once you have the overlay type defined, it's easy to create a JavaScript object from JSON and access its properties in Java:

public static final native Customer buildCustomer(String json) /*-{
    return eval('(' + json + ')');
}-*/;

If you want the JSON representation of the object again, you can wrap the overlay type in a JSONObject:

Customer customer = buildCustomer("{'Bart', 'Simpson'}");
customer.setFirstName("Lisa");
// Displays {"first_name":"Lisa","last_name":"Simpson"}
Window.alert(new JSONObject(customer).toString());

Answer 6

In Google Web Toolkit Applications, pages 510 to 522, the author, Ryan Dewsbury, shows how to use GWT code generation to do serialization to and from XML and JSON documents.

You can download the code here; you want the chapter 10 code bundles, and then you want to look in the src/com/gwtapps/serialization package. I did not see a license for this code, but have emailed the author to see what he says. I'll update this if he replies.

Issues with this solution:

• You have to add a marker interface on all your objects that you want serialized (he uses java.io.Serializable but I imagine you could use others--if you are using hibernate for your backend, your pojos might already be tagged like this).
• The code only supports string properties; it could be extended.
• The code is only written for 1.4 and 1.5.

So, this is not an out of the box solution, but a great starting point for someone to build a JSON serializer that fits with GWT. Combine that with a JSON serializer on the server side, like json-lib and you're good to go.

I also found this project (again, some marker interface is required).

Answer 7

Have you tried the google json lib itself? Here is the link- http://sites.google.com/site/gson/Home

I dont know if it has a direct support from client GWT or not.

Answer 8

Check this:

GWT Professional JSON Serializer: http://code.google.com/p/gwtprojsonserializer/

!Works with GWT 2.0+!

Answer 9

It is important to say that standard java JSON libraries can not be used for this purpose. It must be compiled for GWT client side into javascript.

This is fully functional GWT client side JSON serializer:

GWT Professional JSON Serializer: http://code.google.com/p/gwtprojsonserializer/

!Works with GWT 2.0+!

Answer 10

The simplest way would be to use GWT's built-in JSON API. Here's the documentation. And here is a great tutorial on how to use it.

It's as simple as this:

String json = //json string
JSONValue value = JSONParser.parse(json);

The JSONValue API is pretty cool. It lets you chain validations as you extract values from the JSON object so that exceptions will be thrown if anything's amiss with the format.

Answer 11

Try this serializer from Google Code: http://code.google.com/p/json-io/

If you need to write or read JSON format in Java, this is the tool to use. No need to create extra classes, etc. Convert a Java object graph to JSON format in one call. Do the opposite - create a JSON String or Stream to Java objects. This is the fastest library I have seen yet to do this. It is faster than ObjectOutputStream and ObjectInputStream in most cases, which use binary format.

Very handy utility.