Retrieve object returned by JavaScript in Java

Question

By using JavaScript APIs in Java 7, I am able to compile and invoke JavaScript functions. The issue is with value returned by the JavaScript function. Simple types can be type-cast easily. However, if a JavaScript function returns an object. How to convert the returned object into json string?

    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine se = mgr.getEngineByName("JavaScript");
    if (se instanceof Compilable) {
        Compilable compiler = (Compilable) se;
        CompiledScript script = compiler
                .compile("function test() { return { x:100, y:200, z:150 } }; test();");
        Object o = script.eval();
        System.out.println(o);
    } else {
        System.out.println("Engine cann't compile code.");
    }

How to convert object returned by JavaScript to JSON string?

Pablo Francisco Pérez Hidalgo · Accepted Answer · 2014-02-28 12:23:32Z

1

Maybe Google JSON libraries for Java (GSON) will be useful for you:

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

You can use them to seriazlize/deserialize to objects as long as you deffine their classes with the proper getters and setters which have to match with the Bindings you shoud specify at eval call.

If you need to inspect the returned object attributes before serializing, deffine a class similar to this one.

public class Serializer {
    static public Map<String, Object> object2Map(NativeObject o)
    {
        Map<String, Object> ret = new HashMap<>();
        for(Object keyField: o.getAllIds())
        {
            try {
                Object valObject = o.get(keyField.toString());
                ret.put(keyField.toString(), valObject);
            } catch (Exception e) {
                continue; 
            }
        }
        return ret;
    }
}

And use it to map the object attributes in a map.

The idea is to iterate over the object attributes and then generate an object of a specific class which can be used by GSON or generate the JSON string by yourself.

edited Feb 28, 2014 at 12:23

answered Feb 28, 2014 at 10:48

Pablo Francisco Pérez Hidalgo

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8 Comments

Chir Over a year ago

GSON can work only if the returned object has attributes defined in them, which is not the case here. The script.eval() method returns NativeObject. GSON will not be useful here!

Pablo Francisco Pérez Hidalgo Over a year ago

@Chir You can try then to use code reflection (getDeclaredFields) to ask the object for its attributes.

Chir Over a year ago

Unfortunately the object of type NativeObject doesn't give me correct information about attributes it has.

Pablo Francisco Pérez Hidalgo Over a year ago

@Chir Doesn't object.getClass() list the JSON fields? If the answer is it doesn't then my approach is useless. Did you try to specify the Bindings at eval call.

Pablo Francisco Pérez Hidalgo Over a year ago

@Chir I have just copied your code into an editor and played with it: NativeObject o = (NativeObject)script.eval(); for(Object cf: o.getAllIds()) System.out.println("F: " + cf.toString() + "-----" + o.get(cf.toString()).toString()); Why Don't you use this code to build your JSON, the only difficulty is to manage nested objects but it can be done easily using a recursive algorithm

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