android/src/SafeDestruct.java

   1 /* Copyright_License {
   2
   3   XCSoar Glide Computer - http://www.xcsoar.org/
   4   Copyright (C) 2000-2013 The XCSoar Project
   5   A detailed list of copyright holders can be found in the file "AUTHORS".
   6
   7   This program is free software; you can redistribute it and/or
   8   modify it under the terms of the GNU General Public License
   9   as published by the Free Software Foundation; either version 2
  10   of the License, or (at your option) any later version.
  11
  12   This program is distributed in the hope that it will be useful,
  13   but WITHOUT ANY WARRANTY; without even the implied warranty of
  14   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15   GNU General Public License for more details.
  16
  17   You should have received a copy of the GNU General Public License
  18   along with this program; if not, write to the Free Software
  19   Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
  20 }
  21 */
  22
  23 package org.xcsoar;
  24
  25 /**
  26  * This class manages safe destruction of objects that call native
  27  * methods.  It keeps track of how many threads are currently running
  28  * inside native code, and allows waiting for this counter to become
  29  * zero.  As soon as that happens, the native code may free its
  30  * memory.
  31  */
  32 class SafeDestruct {
  33   /**
  34    * The reference counter.
  35    */
  36   private int count = 0;
  37
  38   /**
  39    * A flag that specifies whether shutdown was requested by
  40    * beginShutdown().  When set, the reference counter will never be
  41    * incremented.
  42    */
  43   private boolean shutdown = false;
  44
  45   /**
  46    * Safely increment the reference counter.  Returns false when the
  47    * object is shutting down (no change to the reference counter in
  48    * this case).
  49    */
  50   public synchronized boolean Increment() {
  51     if (shutdown)
  52       return false;
  53
  54     ++count;
  55     return true;
  56   }
  57
  58   /**
  59    * Safely increment the reference counter.  Returns false when the
  60    * object is shutting down (no change to the reference counter in
  61    * this case).
  62    */
  63   public synchronized void Decrement() {
  64     --count;
  65
  66     if (count == 0 && shutdown)
  67       /* notify the thread that is waiting in finishShutdown() */
  68       notifyAll();
  69   }
  70
  71   /**
  72    * Initiate the shutdown of the managed object.  This method is
  73    * asynchronous, it returns immediately.
  74    */
  75   public synchronized void beginShutdown() {
  76     if (shutdown)
  77       throw new IllegalStateException("Already shutting down");
  78
  79     shutdown = true;
  80   }
  81
  82   /**
  83    * Synchronously wait for the shutdown to complete, i.e. waits for
  84    * the reference counter to drop to zero.
  85    */
  86   public synchronized void finishShutdown() {
  87     if (!shutdown)
  88       throw new IllegalStateException("Not shutting down");
  89
  90     while (count > 0) {
  91       try {
  92         wait();
  93       } catch (InterruptedException e) {
  94       }
  95     }
  96   }
  97 }