Java Scripting API -- Property from JavaScript object is always null

Question

Below is the code I am running in my Java compiler:

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

engine.put("person", "{name: 'Bob', favoriteColor: 'red'}");
System.out.println(engine.get("person.name"));

I would expect this to evaluate to "Bob", but instead it gives me null. If I try printing just the user object, it properly gives me this output:

{name: 'Bob', favoriteColor: 'red'}

Why is person.nameevaluating to null? Any help would be appreciated!

You've `put` a variable called `person` and then you are trying to fetch values from `user`. Try using `person.name` instead. — neuronaut, Jun 22 '15 at 19:45
Sorry! Just an error in copying, I have changed it. It should be 'person.name' — kshah, Jun 22 '15 at 19:47
So many == 1? But yeah, it looks like it might be a typo/syntax error. — markspace, Jun 22 '15 at 19:47
The documentation for `ScriptEngine.get()` describes the string parameter as a "key", not an "expression". Is it possible that it's not evaluating the expression you gave it? — markspace, Jun 22 '15 at 19:54
Yes, that may be the case. If I use the JavaScript print function to print the boolean instead of embedding `engine.get` into a Java print statement, it works fine — kshah, Jun 22 '15 at 20:08

score 2 · Accepted Answer · answered Jun 22 '15 at 20:01

Apparently the put method doesn't evaluate the value passed, but assumes it is meant as a literal. For example script.put("test", 5) will put the literal integer value 5 into a variable called test.

The second thing that's wrong is that get also doesn't evaluate its parameter, but assumes it, too, is the literal name of the variable.

However, it is possible to do what you are trying to accomplish. Try this:

engine.eval ("var person = {name: 'Bob', favoriteColor: 'red'}");
System.out.println(((Bindings) engine.get("person")).get ("name"));

Java Scripting API -- Property from JavaScript object is always null

1 Answers1