jurt/com/sun/star/uno/WeakReference.java

   1 /* -*- Mode: Java; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
   2 /*
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   9  * This file incorporates work covered by the following license notice:
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  15  *   License, Version 2.0 (the "License"); you may not use this file
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  17  *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
  18  */
  19
  20 package com.sun.star.uno;
  21
  22 /**
  23  * This class holds weak reference to an object.
  24  *
  25  * <p>It actually holds a reference to a <code>com.sun.star.XAdapter</code>
  26  * implementation and obtains a hard reference if necessary.
  27  */
  28 public class WeakReference
  29 {
  30     private OWeakRefListener m_listener;
  31     // There is no default constructor. Every instance must register itself with the
  32     // XAdapter interface, which is done in the constructors. Assume we have this code
  33     // WeakReference ref= new WeakReference();
  34     // ref = someOtherWeakReference;
  35     //
  36     // ref would not be notified (XReference.dispose()) because it did not register
  37     // itself. Therefore the XAdapter would be kept alive although this is not
  38     // necessary.
  39
  40     /**
  41      * Creates an instance of this class.
  42      *
  43      * @param obj another instance that is to be copied.
  44      */
  45     public WeakReference(WeakReference obj)
  46     {
  47         if (obj == null) {
  48             return;
  49         }
  50         Object weakImpl = obj.get();
  51         if (weakImpl == null) {
  52             return;
  53         }
  54         XWeak weak = UnoRuntime.queryInterface(XWeak.class, weakImpl);
  55         if (weak != null) {
  56             XAdapter adapter = weak.queryAdapter();
  57             if (adapter != null)
  58                 m_listener = new OWeakRefListener(adapter);
  59         }
  60     }
  61
  62     /**
  63      * Creates an instance of this class.
  64      *
  65      * @param obj XWeak implementation.
  66      */
  67     public WeakReference(Object obj)
  68     {
  69         XWeak weak= UnoRuntime.queryInterface(XWeak.class, obj);
  70         if (weak != null)
  71         {
  72             XAdapter adapter= weak.queryAdapter();
  73             if (adapter != null)
  74                 m_listener= new OWeakRefListener(adapter);
  75         }
  76     }
  77
  78     /**
  79      * Returns a hard reference to the object that is kept weak by this class.
  80      *
  81      * @return a hard reference to the XWeak implementation.
  82      */
  83     public Object get()
  84     {
  85         if (m_listener != null)
  86             return m_listener.get();
  87         return null;
  88     }
  89 }
  90
  91 /**
  92  * Implementation of com.sun.star.uno.XReference for use with WeakReference.
  93  *
  94  * <p>It keeps the XAdapter implementation and registers always with it.
  95  * Deregistering occurs on notification by the adapter and the adapter is
  96  * released.</p>
  97  */
  98 class OWeakRefListener implements XReference
  99 {
 100     private XAdapter m_adapter;
 101
 102     /**
 103      * The constructor registered this object with adapter.
 104      *
 105      * @param adapter the XAdapter implementation.
 106      */
 107     OWeakRefListener( XAdapter adapter)
 108     {
 109         m_adapter= adapter;
 110         m_adapter.addReference(this);
 111     }
 112
 113     /**
 114      * Method of <code>com.sun.star.uno.XReference</code>.
 115      *
 116      * <p>When called, it deregisters this object with the adapter and releases
 117      * the reference to it.</p>
 118      */
 119     synchronized public void dispose()
 120     {
 121         if (m_adapter != null)
 122         {
 123             m_adapter.removeReference(this);
 124             m_adapter= null;
 125         }
 126     }
 127
 128     /**
 129      * Obtains a hard reference to the object which is kept weak by the adapter
 130      * and returns it.
 131      *
 132      * @return hard reference to the otherwise weakly kept object.
 133      */
 134     synchronized Object get()
 135     {
 136         Object retVal= null;
 137         if (m_adapter != null)
 138         {
 139             retVal= m_adapter.queryAdapted();
 140             if (retVal == null)
 141             {
 142                 // If this object registered as listener with XAdapter while it was notifying
 143                 // the listeners then this object might not have been notified. If queryAdapted
 144                 // returned null then the weak kept object is dead and the listeners have already
 145                 // been notified. And we missed it.
 146                 m_adapter.removeReference(this);
 147                 m_adapter= null;
 148             }
 149         }
 150         return retVal;
 151     }
 152 }
 153
 154 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */