Custom JSON Serialization With GSON

Posted 23 January 2017. 2 min read.

Intro

Here's a curious one, in which I found myself with a limitation of not being able to output JSON with scientific notation values.

If you're wondering why that is, since both JSON and JavaScript allow scientific notation of number values, you are absolutely correct and that's a great question. The strange thing was that I found myself outputting perfectly valid JSON to be consumed by something specific which didn't allow scientific notation. I'm not entirely sure why that was a bridge I had to cross, but the path of least resistance involved my forcing the output values to be serialized in standard notation.

Serializing JSON With GSON

Normally, serialization of JSON via GSON is so simple, a developer with merely a basic understanding of Java could pull it off 😜. Here's a sample for baseline comparison.

	...
	/**
	* @return String -ified (JSON) version of Object ob, via GSON
	*/
	public static getSomeJsonStr(Object ob){
	...
	Gson g = new Gson();
	return g.toString(ob);
	}
	...

view raw Generic.java hosted with ❤ by GitHub

So, since the formatting of the numeric values, which I was passing into a Map as Doubles for serialization, we'll be registering a new TypeAdapter with the JsonSerializer and instruct it how to handle the value in place of the default. Here's the result, wrapped into a method in a utility class.

	...
	/**
	* Makes use of a custom type adapter to convert Double values without
	* using scientific notation.
	*
	* @return Gson instance with custom handling of Double values
	*/
	public static String getCustomGsonInstance() {
	GsonBuilder gb = new GsonBuilder();
	gb.registerTypeAdapter(Double.class, new JsonSerializer<Double>() {
	public JsonElement serialize(final Double src, final Type typeOfSrc, final JsonSerializationContext context) {
	BigDecimal value = BigDecimal.valueOf(src);

	return new JsonPrimitive(value);
	}
	});
	Gson g = gb.create();
	return g;
	}
	...

view raw Utils.java hosted with ❤ by GitHub

Now that we have a new way of handling our Gson instantiation, we can use it as such:

	...
	/**
	* @return String -ified (JSON) version of Object ob, via custom GSON instance
	*/
	public static getSomeJsonStr(Object ob){
	...
	Gson g = Utils.getCustomGsonInstance();
	return g.toString(ob);
	}
	...

view raw New.java hosted with ❤ by GitHub

Summary

I'm still left questioning why I had to do this, but the exercise is an intersting one anyway. Should someone need to customize their responses even further, this is a great place to start from. I'm still a pretty big fan of GSON and while it may not be for everyone, or even the best option in OSGi plugin development with options like JAX-RS and others, it's certainly a heavily used library that is both useful and generally easy to implement.

🍻

software development, operations, tooling, and musings on technology by Eric McCormick

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