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 -- Escape the Java Trap with GNU Classpath! http://www.gnu.org/philosophy/java-trap.html Join the community at http://planet.classpath.org/
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