Can I extend a Java class in Oracle's JDK7 JSR223 JavaScript engine?

Question

I'm trying to write JavaScript that will define a class that extends an existing Java class, called from a JSR223 ScriptEngine . I know that JavaAdapter works for an Interface, but not a Class .

ScriptEngine js = new ScriptEngineManager().getEngineByExtension("js");
js.eval("new java.lang.Runnable {run: function() { ... } }"); // works
js.eval("new java.util.TimerTask {run: function() { ... } }"); // throws

I know that's what the docs say I should expect. I also know that once I can switch to Nashorn, all this will go away and I'll have lovely access to Java.extend() , etc., but for the time being I'm stuck with JDK7.

Given all that , is there any way to do this? I think my fallback will be switching directly to Mozilla's native Rhino bindings, but I'd prefer to keep this as abstract as possible.

Answer 1

What does it throw?

jjs> var tt = new java.util.TimerTask {run: function() { print("hello"); }}
jjs> tt.run();
hello
null

I usually skip TimerTask altogether

// Daemon flag seems to be ignored.
var timer = new java.util.Timer(false);
timer.schedule(function() print("Hello"), 1000);

// Give timer a chance to run before exit.
java.lang.Thread.sleep(5000);

Answer 2

I think I've figured it out. I was under the impression that I could only implement an interface, but it turns out that you can also extend a concrete class -- only abstract classes throw the error I described.

ScriptEngine js = new ScriptEngineManager().getEngineByExtension("js");
js.eval("new java.lang.Runnable() {run: function() { ... } }"); // works
js.eval("new java.lang.Thread() {run: function() { ... } }"); // works
js.eval("new java.util.TimerTask() {run: function() { ... } }"); // throws

This is good enough for my needs -- the docs could definitely be clearer, though!