Fast passing of JSON between JS and Java (and vice versa)

Tim Fox timvolpe at
Thu Nov 27 13:11:57 UTC 2014

As you know..

In JS, a JSON Object is represented by a JS object, and in the Java 
world it's often represented by Map<String, Object>.
In JS a JSON array is represented by a JS array, and in the Java world 
it's often represented by a List<Object>.

I'd love to be able to pass JSON between JS and Java and vice versa with 
the minimum of performance overhead. This is particularly important in 
Vert.x as we chuck a lot of JSON around.

Let's say I have a Java interface which expects some JSON:

interface SomeInterface {

     void expectsJSON(Map<String, Object> json);

Right now I am converting from JS-->Java as follows.

var someJson = { foo: "bar"};
String encoded = JSON.stringify(someJson);
Map<String, Object> map = SomeJavaJSONLibrary.decode(encoded);

As you can see it's pretty clunky. The other direction is equally as 
clunky. And it's slow as we're encoding/decoding everything via String.

Then I realised that if I pass a JS object directly into the expectsJSON 
method Nashorn will provide me with a Map<String, Object> that backs the 
original object. I.e. I can do this:

var someJson = { foo: "bar"};

Yay! No encoding overhead. Fast. :)

And it works with nested json:

var someJson = { foo: "bar", nested: { wibble: "blah"}};

Just when I was getting my hopes up that this would be a great super 
fast way of transferring JSON betwen Java and JS, I tried it with a nest 

var someJson = { foo: "bar", nestedArray: [123, 456]};

But in Java, map.get("nestedArray") returns a ScriptObjectMirror not a 
List as I was hoping. :(

So.. passing from JS to Java: JS Object maps to Map, but JS Array maps 
to ScriptObjectMirror. (Seems a bit asymmetric?).

Any reason why we can't map JS Array to Java list when calling JS->Java? 
(Perhaps this is related to my previous question backing a JS Array with 
a List...)

Do you have any other suggestions for transferring JSON between JS and 
Java without too much encoding overhead?

Thanks again!

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