javascript variable lifecycle in the java method

Question

I'm using javax.script to execute javascript from a java method.

In my java method I invoke different functions defined in javascript. On the javascript side I want to keep a global variable so the output of a call depends on the previous ones.

java method

public void myMethod(){
  ScriptEngineManager factory = new ScriptEngineManager();
  ScriptEngine engine = factory.getEngineByName("JavaScript");

  engine.eval(new java.io.FileReader("myTest.js"));

  Invocable inv = (Invocable) engine;

  Object obj = engine.get("obj");
  inv.invokeMethod(obj, "method1");

  inv.invokeMethod(obj, "method2");
}

myTest.js

var obj=new Object();
var myStatus=1;

obj.method1 = function(){
  myStatus++;
};

obj.method2 = function(){
  for (var i=0; i<myStatus)
    println('Hello world');
}

What is the scope of the variable declared in the script? If I add a global variable to the script using

engine.put("globalVariable", myVariable)

what is the scope of this variable?

Thanks

ltebean · Accepted Answer · 2013-05-10 06:54:30Z

engine.put("globalVariable", myVariable)

meas this variable is belonged to the engine, every script the engines runs shares this variable, here's an example:

ScriptEngineManager factory = new ScriptEngineManager();
ScriptEngine engine = factory.getEngineByName("JavaScript");   
engine.put("status",0);
engine.eval("status++; println(status);"); //print 1
engine.eval("status++; println(status);"); //print 2

If you want to pass some script scope parameters to you script, you should use bindings

ScriptEngineManager factory = new ScriptEngineManager();
ScriptEngine engine = factory.getEngineByName("JavaScript");

Bindings bindings=engine.createBindings();
bindings.put("status",0);

Bindings bindings2=engine.createBindings();
bindings2.put("status",0);

engine.eval("status++; println(status);",bindings); //print 1
engine.eval("status++; println(status);",bindings2); //print 1

Next, the variable defined in the script, if you don't use bindings, they are all engine scope:

ScriptEngineManager factory = new ScriptEngineManager();
ScriptEngine engine = factory.getEngineByName("JavaScript");
engine.eval("var status=0; status++; println(status);"); //print 1
engine.eval("status++; println(status);"); //print 2

If you use bindings, the variable defined in the script is binding scope, it will not pollute the engines scope.

ScriptEngineManager factory = new ScriptEngineManager();
ScriptEngine engine = factory.getEngineByName("JavaScript");

Bindings bindings=engine.createBindings();
//bindings.put("status",0);

Bindings bindings2=engine.createBindings();
//bindings2.put("status",0);

engine.eval("var status=0; status++; println(status);",bindings); //print 1
engine.eval("status++; println(status);",bindings2); // exception, status not defined

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