Re: [Waterken-server] how to resolve a promise in java code?

[Tyler C. <tyl...@gm...>]

On Fri, Jun 29, 2012 at 9:14 PM, Tyler Close <tyl...@gm...> wrote:
> On Fri, Jun 29, 2012 at 8:33 AM, Hunter Kelly <re...@gm...> wrote:
>> We've been trying to use promise.call() to get the results from a
>> promise, but we get the following stack trace:
>>
>> org.ref_send.promise.Unresolved
>>        at org.ref_send.promise.Eventual$Tail.call(Eventual.java:789)
>>        at com.viscis.poc.Administrator$X.lookupMessage(Administrator.java:74)
>>        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
>>        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
>>        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
>>        at java.lang.reflect.Method.invoke(Method.java:597)
>>        at org.joe_e.reflect.Reflection.invoke(Reflection.java:343)
>>        at org.waterken.remote.http.Callee$1.apply(Callee.java:176)
>>        at org.waterken.remote.http.ServerSideSession.once(ServerSideSession.java:87)
>>        at org.waterken.remote.http.Callee.apply(Callee.java:130)
>>        at org.waterken.remote.http.AMP$3.apply(AMP.java:142)
>>        at org.waterken.remote.http.AMP$3.apply(AMP.java:137)
>>        at org.waterken.jos.JODB.enter(JODB.java:196)
>>        at org.waterken.remote.http.AMP$1$1.call(AMP.java:102)
>>        at org.waterken.remote.http.AMP$1$1.call(AMP.java:92)
>>        at org.waterken.thread.Loop$1.run(Loop.java:73)
>>        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
>>        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
>>        at java.util.concurrent.FutureTask.run(FutureTask.java:138)
>>        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
>>        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
>>        at java.lang.Thread.run(Thread.java:680)
>>
>>
>> Any ideas on what we are doing wrong?
>>
>> the lookupMessage method looks like this:
>>
>>        @Override
>>        public Promise<String> lookupMessage(String name) {
>>            Vat<User> v = users.get(name);
>>            if (v == null)
>>                throw new IllegalArgumentException("Invalid user/pass");
>>            String message = null;
>>            try {
>>                message = v.top.getMessage().call();
>
> The "top" field of a Vat is a remote reference to the object created
> in a child vat. So that getMessage() invocation in the code above is
> an asynchronous invocation that returns a promise that will only be
> resolved after the message has been sent to the child vat, processed
> and returned. Since you're invoking the call() method right away, none
> of that has happened yet, so the promise is throwing the Unresolved
> exception.
>
> So you want to rewrite this method as:
>
>   public Promise<String> lookupMessage(String name) {
>    Vat<User> v = users.get(name);
>     if (v == null)
>       throw new IllegalArgumentException("Invalid user/pass");
>    return v.top.getMessage();
>  }
>
> If the caller of this method needs the eventual String value of the
> promise, they'll need to use an Eventual.when() invocation.

That when invocation looks like:

  class Hello extends Do<String,Promise<String>> implements Serializable {
    public Promise<String>
    fulfill(final String message) {
      return Eventual.ref(name + " sez: " + message);
    }
  }
  return _.when(v.top.getMessage(), new Hello());

--Tyler

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