A Map implementation that dumps its content when memory runs low. : Map « Collections Data Structure « Java






  1. Java
  2. Collections Data Structure
  3. Map

A Map implementation that dumps its content when memory runs low.

     
//     package com.croftsoft.core.util;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.AbstractMap;
import java.util.HashSet;
import java.util.Set;
import java.util.WeakHashMap;

/*********************************************************************
 * A Map implementation that dumps its content when memory runs low.
 * 
 * <P>
 * 
 * Backed by a WeakHashMap. Note that an entry will not be garbage collected if
 * its key remains strongly reachable.
 * 
 * <P>
 * 
 * @see java.util.WeakHashMap
 * @see java.lang.ref.SoftReference
 * 
 * @version 1999-04-20
 * @author <a href="http://www.CroftSoft.com/">David Wallace Croft</a>
 *********************************************************************/

public class SoftHashMap extends AbstractMap
// ////////////////////////////////////////////////////////////////////
// ////////////////////////////////////////////////////////////////////
{

  private WeakHashMap weakHashMap = new WeakHashMap();
  private ReferenceQueue referenceQueue = new ReferenceQueue();
  private Set softSet = new HashSet();

  // ////////////////////////////////////////////////////////////////////
  // ////////////////////////////////////////////////////////////////////

  public static void main(String[] args)
  // ////////////////////////////////////////////////////////////////////
  {
    System.out.println(test());
  }

  public static boolean test()
  // ////////////////////////////////////////////////////////////////////
  {
    try {
      SoftHashMap softHashMap = new SoftHashMap();

      softHashMap.put("key", "value");

      if (!softHashMap.remove("key").equals("value")
          || (softHashMap.size() > 0)) {
        return false;
      }

      Runtime runtime = Runtime.getRuntime();

      for (int i = 0; i < 1000000; i++) {
        if (i % 10000 == 0) {
          System.out.println(i + ":  " + softHashMap.size()
              + " entries, " + runtime.freeMemory() + " / "
              + runtime.totalMemory() + " memory usage");
        }

        Integer value = new Integer(i);
        softHashMap.put(value.toString(), value);
      }
    } catch (Throwable t) {
      t.printStackTrace();

      return false;
    }

    return true;
  }

  // ////////////////////////////////////////////////////////////////////
  // ////////////////////////////////////////////////////////////////////

  public Set entrySet()
  // ////////////////////////////////////////////////////////////////////
  {
    clearQueue();

    return weakHashMap.entrySet();
  }

  public Object put(Object key, Object value)
  // ////////////////////////////////////////////////////////////////////
  {
    clearQueue();

    softSet.add(new SoftReference(key, referenceQueue));

    return weakHashMap.put(key, value);
  }

  // ////////////////////////////////////////////////////////////////////
  // ////////////////////////////////////////////////////////////////////

  public void clearQueue()
  // ////////////////////////////////////////////////////////////////////
  {
    Reference reference = null;
    while ((reference = referenceQueue.poll()) != null) {
      softSet.remove(reference);
    }
  }

  // ////////////////////////////////////////////////////////////////////
  // ////////////////////////////////////////////////////////////////////
}








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