Re: rmi vs cajo

[Mark Wielaard]

Hi Andrew,

I CCed John Catherino the main Cajo developer. He might have more
insights (or correct me if I am just plain wrong about cajo).

On Tue, 2005-09-13 at 15:03 +0100, Andrew Haley wrote:
> Mark Wielaard writes:
>  > 
>  > While playing a bit with Cajo
>  > (http://wiki.java.net/bin/view/Communications/ProxyUsage) I got the
>  > following error:
>  > 
>  > java.lang.NullPointerException
>  >    at gnu.cajo.invoke.Remote.hashCode (Remote.java:510)
>  >    at java.util.Hashtable.hash (Hashtable.java:822)
>  >    at java.util.Hashtable.put (Hashtable.java:432)
>  >    at gnu.java.rmi.server.UnicastServer.exportObject 
> (UnicastServer.java:66=
>  > )
>  >    at gnu.java.rmi.server.UnicastServerRef.exportObject 
> (UnicastServerRef.j=
>  > ava:110)
>  >    at java.rmi.server.UnicastRemoteObject.exportObject 
> (UnicastRemoteObject=
>  > .java:83)
>  >    at java.rmi.server.UnicastRemoteObject.<init> 
> (UnicastRemoteObject.java:=
>  > 69)
>  >    at gnu.cajo.invoke.Remote.<init> (Remote.java:486)
>  >    at gnu.cajo.utils.ItemServer.bind (ItemServer.java:206)
>  >    at ProxyTest.main (ProxyTest.java:38)
>  > 
>  > It seems we are to eager to export the Remote object immediately (from
>  > the constructor). We want to put it in a Hashtable and call hasCode(),
>  > but for the cajo Remote object the item field used to calculate the
>  > hashCode() hasn't been set yet so that gives a NullPointerException.
> 
> Isn't this a simple failure by gnu.cajo.invoke.Remote to satisfy the
> contract for hashCode()?  As I understand it, hashCode() should not
> throw any exception.

Normally hasCode() should not. But I think we are a little evil here in
the UnicastRemoteObject constructor. I don't have the precise source
code here, but it looks something like:

class Remote extends UnicastRemoteObject
{
  private final string name;

  Remote(String name)
  {
    super(); // Either explicitly or implicitly.
    this.name = name;
  }

  public int hashCode()
  {
    return name.hashCode();
  }
}

And in UnicastRemoteObject initializer we are trying to call the
hashCode() of the not-really-fully-initialized-yet Remote object.
I am not sure how else cajo could do this, except special casing
hashCode() to expect being called from the (super-class) constructor
which seems terribly messy.

Cheers,

Mark

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