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Dragome JSNI (Javascript Native Interface)

JSNI is the mechanism used by Dragome to execute javascript code from Java (bytecode).
DOM manipulations and several JS-JRE classes make intensive use of this feature. For instance, reflection API, dynamic proxies, and essential classes such as Class and Method have a lot of interactions with javascript using JSNI.
When you run the application in debug mode (“debug=true” at querystring), you can execute each single JSNI call, for example to take a look at DOM changes step by step. Take a look at:

To accomplish these interactions JSNI provides a set of methods located at com.dragome.commons.javascript.ScriptHelper class, these are:

  1. put
  2. eval
  3. evalInt / evalLong / evalFloat / evalDouble / evalChar / evalBoolean
  4. evalNoResult

Note: all these JSNI methods receive as last argument a reference to the caller, this is necessary to detect the method (context) where the execution is performed. If you are calling it from a static method you must set it to null, otherwise always set it to “this”.

JSNI methods description

'put' method


void put(String s, Object value, Object callerInstance)

Creates a variable in current javascript context.

'eval' method


Object eval(String expression, Object callerInstance)

Returns the resulting value of evaluating the expression in javascript context.

'evalInt|Long|Float|Double|Char|Boolean' methods


Object eval[Type](String expression, Object callerInstance)

Returns the resulting value of corresponding type evaluating the expression in javascript context.

'evalNoResult' method


void evalNoResult(String expression, Object callerInstance)

Evaluates the expression in javascript context returning no value.


If we want to execute a JQuery selector to make desired elements draggable and resizable:

ScriptHelper.evalNoResult("$('.draggable').draggable().resizable()", this);

Creating a variable with value 10, and getting this value plus 4:

ScriptHelper.put("x", 10, this);
int newValue= ScriptHelper.evalInt("x + 4", this);

LocalStorage implementation

public class LocalStorage
    public <T> T load(String aKey)
        ScriptHelper.put("key", aKey, this);
        String serializedValue= (String) ScriptHelper.eval("localStorage.getItem(key)", this);
        if (serializedValue != null && !serializedValue.isEmpty())
            return (T) ServiceLocator.getInstance().getSerializationService().deserialize(serializedValue);
            return null;

    public <T> void save(String aKey, T aValue)
        String serializedValue= ServiceLocator.getInstance().getSerializationService().serialize(aValue);
        ScriptHelper.put("key", aKey, this);
        ScriptHelper.put("serialized", serializedValue, this);
        ScriptHelper.eval("localStorage.setItem(key, serialized)", this);