Addition operation in Nashorn javascript engine -java8 - nashorn

Question

I have 2 variables to add using engine.eval()- nashorn javascript engine in java8.?
I've used code like this:
ScriptEngineManager sem=new ScriptEngineManager();
ScriptEngine engine = sem.getEngineByName("nashorn");
Integer sum = 20;
Integer var2=30;
try {
sum=(Integer)engine.eval("sum+var2");
} catch(Exception e) {
System.out.println("Error executing script: "+ e.getMessage());
}
The eval function takes only string parameter. So it shows error.how to perform this operation?

Answer 1

Nashorn ScriptEngine is a separate runtime. It doesn't have knowledge of data outside it's scope unless explicitly specified.
Here sum and var2 are outside the scope of Nashorn. You can pass this data to Nashorn via multiple methods. One of them is using bindings.
Sample code :
ScriptEngineManager sem = new ScriptEngineManager();
ScriptEngine engine = sem.getEngineByName("nashorn");
Integer sum = 20;
Integer var2 = 30;
try {
Bindings bindings = engine.createBindings();
bindings.put("sum", 20);
bindings.put("var2", 30);
double result = (double) engine.eval("sum+var2", bindings);
System.out.println(result);
} catch (Exception e) {
System.out.println("Error executing script: " + e.getMessage());
}
The type conversion from integer to double is a nuance explained here.
But this kind of script evaluation is rather limiting. You can use Invocable api to invoke javascript functions. This tutorial is a good starting point.