xref: /aoo41x/main/jurt/com/sun/star/uno/WeakReference.java (revision 2be43276)

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 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
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 * Unless required by applicable law or agreed to in writing,
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package com.sun.star.uno;

import com.sun.star.uno.XWeak;
import com.sun.star.uno.UnoRuntime;
import com.sun.star.uno.XAdapter;
import com.sun.star.uno.XReference;

/** This class holds weak reference to an object. It actually holds a reference to a
    com.sun.star.XAdapter implementation and obtains a hard reference if necessary.
 */
public class WeakReference
{
    private final boolean DEBUG= false;
    private OWeakRefListener m_listener;
    // There is no default constructor. Every instance must register itself with the
    // XAdapter interface, which is done in the constructors. Assume we have this code
    // WeakReference ref= new WeakReference();
    // ref = someOtherWeakReference;
    //
    // ref would not be notified (XReference.dispose()) because it did not register
    // itself. Therefore the XAdapter would be kept aliver although this is not
    // necessary.

    /** Creates an instance of this class.
     *@param obj - another instance that is to be copied
     */
    public WeakReference(WeakReference obj)
    {
        if (obj != null)
        {
            Object weakImpl= obj.get();
            if (weakImpl != null)
            {
                XWeak weak= UnoRuntime.queryInterface(XWeak.class, weakImpl);
                if (weak != null)
                {
                    XAdapter adapter= (XAdapter) weak.queryAdapter();
                    if (adapter != null)
                        m_listener= new OWeakRefListener(adapter);
                }
            }
        }
    }

    /** Creates an instance of this class.
     *@param obj XWeak implementation
     */
    public WeakReference(Object obj)
    {
        XWeak weak= UnoRuntime.queryInterface(XWeak.class, obj);
        if (weak != null)
        {
            XAdapter adapter= (XAdapter) weak.queryAdapter();
            if (adapter != null)
                m_listener= new OWeakRefListener(adapter);
        }
    }
    /** Returns a hard reference to the object that is kept weak by this class.
     *@return a hard reference to the XWeak implementation.
     */
    public Object get()
    {
        if (m_listener != null)
            return m_listener.get();
        return null;
    }
}

/** Implementation of com.sun.star.uno.XReference for use with WeakReference.
 *  It keeps the XAdapter implementation and registers always with it. Deregistering
 *  occurs on notification by the adapter and the adapter is released.
 */
class OWeakRefListener implements XReference
{
    private final boolean DEBUG= false;
    private XAdapter m_adapter;

    /** The constructor registered this object with adapter.
     *@param adapter the XAdapter implementation.
     */
    OWeakRefListener( XAdapter adapter)
    {
        m_adapter= adapter;
        m_adapter.addReference(this);
    }
    /** Method of com.sun.star.uno.XReference. When called, it deregisteres this
     *  object with the adapter and releases the reference to it.
     */
    synchronized public void dispose()
    {
        if (m_adapter != null)
        {
            m_adapter.removeReference(this);
            m_adapter= null;
        }
    }

    /** Obtains a hard reference to the object which is kept weak by the adapter
     *  and returns it.
     *  @return hard reference to the otherwise weakly kept object.
     */
    synchronized Object get()
    {
        Object retVal= null;
        if (m_adapter != null)
        {
            retVal= m_adapter.queryAdapted();
            if (retVal == null)
            {
                // If this object registered as listener with XAdapter while it was notifying
                // the listeners then this object might not have been notified. If queryAdapted
                // returned null then the weak kept object is dead and the listeners have already
                // been notified. And we missed it.
                m_adapter.removeReference(this);
                m_adapter= null;
            }
        }
        return retVal;
    }
}