How to provide a seperate context on each eval of the java ScriptEngine?

Question

Working on a java based game, using javax.script for AI;

public class AI implements Runnable {
private boolean alive = true;

String scriptStr;
ScriptEngine scriptEngine;
ScriptContext scriptContext;
Bindings engineScope;

public AI() {
    ScriptEngineManager scriptEngineManager = new ScriptEngineManager();
    scriptEngine = scriptEngineManager.getEngineByName("JavaScript");
    scriptContext = new SimpleScriptContext();
    engineScope = scriptContext.getBindings(ScriptContext.ENGINE_SCOPE);
    engineScope.put("scene", Scene.instance);

    try {
        scriptStr = new Scanner(new File(getClass().getResource("ai.js").getPath())).useDelimiter("\\Z").next();
    } catch (FileNotFoundException e) {
        e.printStackTrace(); 
    }
}

@Override
public void run() {
    while (alive) {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        Iterable<IUnit> units = Scene.instance.getUnits();
        for (IUnit unit : units) {
            if (unit.isAlive()) {
                engineScope.put("unit", unit);
                try {
                    scriptEngine.eval(scriptStr, scriptContext);
                } catch (ScriptException e) {
                    e.printStackTrace();  
                }
            }
        }
    }
}
}

in the code above I have a scene which provides units to iterate. I want to call the very same js code ("ai.js") for all of the units in which units will determine what to do.

The problem is that I want every unit to have their own context data. so each time I evaluate ai.js for a unit they will not be stateless and memoryless.

What is the correct way?

Should I explicitly use something like SpringContext for each of the units?

Or is it possible to run ai.js as if it is a function in the Unit class context?

Answer 1

I accomplished this using a single ScriptEngine with a SimpleScriptContext per execution - although it probably could have been per thread.

Once the context is created, you must execute it within that context before it is available for use by your scriptlets.

To optimize the repeated inclusion of the library, I compiled it to a CompiledScript once, at the time of the creation of the ScriptEngine.

I compile each of my scriptlets objects, then use eval(context) method to execute them.

This is sufficient in my tests to create thread safety.