refcountpeergroupinterface.java

JXTA&#8482 is a set of open, generalized peer-to-peer (P2P) protocols that allow any networked devi

/*
 * Copyright (c) 2002-2007 Sun Microsystems, Inc.  All rights reserved.
 *  
 *  The Sun Project JXTA(TM) Software License
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 *  modification, are permitted provided that the following conditions are met:
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 *     developed by Sun Microsystems, Inc. for JXTA(TM) technology." 
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package net.jxta.impl.peergroup;

import net.jxta.exception.ServiceNotFoundException;
import net.jxta.id.ID;
import net.jxta.peergroup.PeerGroup;
import net.jxta.service.Service;

import java.util.Arrays;
import java.util.Collections;
import java.util.Iterator;
import java.util.Map;

/**
 * RefCountPeerGroupInterface is a PeerGroupInterface object that
 * also serves as a peergroup very-strong reference. When the last
 * such goes away, the peergroup terminates itself despite the existence
 * of aeternal strong references from the various service's threads
 * that would prevent it from ever being finalized.
 * The alternative: to give only weak references to threads seems impractical.
 */
class RefCountPeerGroupInterface extends PeerGroupInterface {

    private Map roleMap;

    /**
     * Constructs an interface object that front-ends a given
     * PeerGroup object.
     *
     * @param theRealThing the peer group
     */
    RefCountPeerGroupInterface(GenericPeerGroup theRealThing) {
        super(theRealThing);
    }

    RefCountPeerGroupInterface(GenericPeerGroup theRealThing, Map roleMap) {
        super(theRealThing);
        this.roleMap = roleMap;
    }

    /**
     * {@inheritDoc}
     * <p/>
     * Normally it is ignored. By definition, the interface object
     * protects the real object's start/stop methods from being called.
     * <p/>
     * However we have to make an exception for groups: even the creator
     * of a group does not have access to the real object. So the interface
     * has to forward startApp to the group, which is responsible for
     * ensuring that it is executed only once (if needed).
     *
     * @param arg A table of strings arguments.
     * @return int status indication.
     */
    @Override
    public int startApp(String[] arg) {
        // Unlike our superclass's method, we do call the real
        // startApp method.
        PeerGroup temp = groupImpl;

        if (null == temp) {
            throw new IllegalStateException("This Peer Group interface object has been unreferenced and can no longer be used.");
        }

        return temp.startApp(arg);
    }

    /**
     * {@inheritDoc}
     * <p/>
     * This is here for temporary class hierarchy reasons.
     * it is normally ignored. By definition, the interface object
     * protects the real object's start/stop methods from being called
     * <p/>
     * In that case we have to make an exception. Most applications
     * currently assume that they do not share the group object and that they do
     * refer to the real object directly. They call stopApp to signify their
     * intention of no-longer using the group. Now that groups are shared,
     * we convert stopApp() to unref() for compatibility.
     * We could also just do nothing and let the interface be GC'd but
     * calling unref() makes the group go away immediately if not shared,
     * which is what applications that call stopApp expect.
     */
    @Override
    public void stopApp() {
        unref();
    }

    /**
     * {@inheritDoc}
     * <p/>
     * Since THIS is already such an object, it could return itself.
     * However, we want the group to know about the number of interfaces
     * objects floating around, so, we'll have the group make a new one.
     * That way, applications which want to use unref() on interfaces can
     * avoid sharing interface objects by using getInterface() as a sort of
     * clone with the additional ref-counting semantics.
     *
     * @return Service An interface object that implements
     *         this service and nothing more.
     */
    @Override
    public Service getInterface() {
        PeerGroup temp = groupImpl;
        if (null == temp) {
            throw new IllegalStateException("This Peer Group interface object has been unreferenced and can no longer be used.");
        }
        return temp.getInterface();
    }

    /**
     * {@inheritDoc}
     * <p/>
     * Returns a weak interface object that refers to this interface
     * object rather than to the group directly. The reason for that
     * is that we want the owner of this interface object to be able
     * to invalidate all weak interface objects made out of this interface
     * object, without them keeping a reference to the group object, and
     * without necessarily having to terminate the group.
     *
     * @return PeerGroup A weak interface object that implements
     *         this group and nothing more.
     */

    @Override
    public PeerGroup getWeakInterface() {
        return new PeerGroupInterface(this);
    }

    /**
     * Can only be called once. After that the reference is no-longer usuable.
     */
    @Override
    public void unref() {
        GenericPeerGroup theGrp;

        synchronized (this) {
            if (groupImpl == null) {
                return;
            }

            theGrp = (GenericPeerGroup) groupImpl;
            groupImpl = null;
        }
        theGrp.decRefCount();
    }

    /**
     * Service-specific role mapping is implemented here.
     **/

    /**
     * {@inheritDoc}
     */
    @Override
    public Service lookupService(ID name) throws ServiceNotFoundException {

        return lookupService(name, 0);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Service lookupService(ID name, int roleIndex) throws ServiceNotFoundException {

        if (roleMap != null) {
            ID[] map = (ID[]) roleMap.get(name);

            // If there is a map, remap; else, identity is the default for
            // role 0 only; the default mapping has only index 0.

            if (map != null) {
                if (roleIndex < 0 || roleIndex >= map.length) {
                    throw new ServiceNotFoundException(name + "[" + roleIndex + "]");
                }

                // We have a translation; look it up directly
                return groupImpl.lookupService(map[roleIndex]);
            }
        }

        // No translation; use the name as-is, provided roleIndex is 0.
        // Do not call groupImpl.lookupService(name, id); group impls
        // should not have to implement it at all.

        if (roleIndex != 0) {
            throw new ServiceNotFoundException(name + "[" + roleIndex + "]");
        }
        return groupImpl.lookupService(name);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Iterator getRoleMap(ID name) {

        if (roleMap != null) {
            ID[] map = (ID[]) roleMap.get(name);

            // If there is a map, remap; else, identity is the default for
            // role 0 only; the default mapping has only index 0.

            if (map != null) {
                // return an iterator on it.
                return Collections.unmodifiableList(Arrays.asList(map)).iterator();
            }
        }
        // No translation; use the given name in a singleton.
        return Collections.singletonList(name).iterator();
    }
}