Re: Need help about code mobility

Andrea Angeloni wrote:
>> I'm not familiar with the example but I would imagine the producer
>> process has a direct dependency on PrimeJob in that it uses it's
>> constructor and fills in the fields.
>
>
> yes, this is the case under the producer:
>
> PrimeJob job = new PrimeJob(nextNumberToCalculate);

As Dan said, this code, has to executing within the confines of a ClassLoader
hierarchy that can resolve this class.  This is a Java thing!  Anytime that a
class is resolved, the classes that it refers to need to be resolvable as well.
  Thus, the class that the above code is within, must be resolved in a context
where these classes are available.

Thus, if you want your Producer to use downloaded code, you need to create an
instance of your Producer using a ClassLoader that can provide all of the needed
classes either directly, or indirectly, through its parent classloaders.

As a short example, one could do something like the following to create a
service provided by a Java application which allows use of a Javaspace, without
explicit access to the Javaspace.

// Define an interface that has to be everywhere
// to specify the service contract.
public interface WorkDescription {
        public URL[] getClassLoaderURLs();
        public String getProducerClass();
}

// Define a semi generic interface for things that
// produce Entry objects.
public interface EntryProducer {
        public Callable<Entry> getProducer();
}

// Define the main service entrypoint.  You can
// provide some mechanism for an external entity
// to call this method, perhaps via a servlet with
// a URL such as:
//
// http://omino:8085/submitwork.cgi?loaders=...&class=...
//
// The servlet then might put together a WorkDescription
// instance, and call this method.
//
public void processItem( WorkDescription work ) {

   // Create a classloader that can load from the network.
   // We need create class loader permission to do this.
   URLClassLoader cl = new URLClassLoader(
        work.getClassLoaderURLs(),
        Thread.getContextClassLoader() == null ?
                getClass().getClassLoader() :
                Thread.getContextClassLoader() );

   // Get the Producer using a well know class name.
   // This class name could be a parameter that you
   // receive from a remote machine that is submitting
   // work items.
   EntryProducer prc = (EntryProducer )Class.forName(
        work.getProducerClass(), true, cl );

   // Get the current context class loader
   ClassLoader ctxcl = Thread.currentThread().getContextClassLoader();
   try {
        // Set the current context class loader for this thread
        // so that any and all attempts at class loading will
        // include our class loader.
        Thread.currentThread().setContextClassLoader( cl );
        processItem( EntryProducer prc );
   } finally {
        Thread.currentThread().setContextClassLoader( ctxcl );
   }
}

// This is the main work point for any EntryProducer that
// your system might provide services for.  Note that an
// inbound JERI/RMI call would have a context class loader
// set automatically.  So, this method might be the JERI/RMI
// interface to submitting work into your javaspace.
//
public void processItem( EntryProducer prc ) {
   // Get the Entry producer.
   Callable<Entry> cl = prc.getProducer();

   // Loop until no more entries are produced.
   boolean done = false;
   while( !done ) {
        // We'll call directly here, but we
        // could use an Executor implementation to
        // manage a Thread pool for all this work.
        Entry e = cl.call();
        if( e != null ) {
                ... now put the entry into the space ...
        }

        done = e == null;
   }
}
Hopefully this will give you some classes to go look at the javadocs for, and
you should be able to see some of the ways to utilize downloaded code using a
different mechanism than that provided by JERI/RMI.

Gregg Wonderly

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