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Copy pathSoftHashMap.java
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268 lines (147 loc) · 4.8 KB
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/*
* PDB web development code
*
* This code may be freely distributed and modified under the
* terms of the GNU Lesser General Public Licence. This should
* be distributed with the code. If you do not have a copy,
* see:
*
* http://www.gnu.org/copyleft/lesser.html
*
* Copyright for this code is held jointly by the individual
* authors. These should be listed in @author doc comments.
*
*
* Created on Mar 26, 2009
* Created by ap3
*
*/
package org.biojava.nbio.core.util;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.*;
/** A in memory cache using soft references. (can be garbage collected)
*
* This code is based on: http://java-interview-faqs.blogspot.com/2008/09/building-faster-and-efficient-cache.html
* */
public class SoftHashMap<K, V> extends AbstractMap<K, V> {
private final static Logger logger = LoggerFactory.getLogger(SoftHashMap.class);
public static final int DEFAULT_LIMIT = 1;
/** The internal HashMap that stores SoftReference to actual data. */
private final Map<K, SoftReference<V>> map = new HashMap<K, SoftReference<V>>();
/** Maximum Number of references you dont want GC to collect. */
private final int max_limit;
/** The FIFO list of hard references, order of last access. */
private final LinkedList<V> hardCache = new LinkedList<V>();
/** Reference queue for cleared SoftReference objects. */
private final ReferenceQueue<V> queue = new ReferenceQueue<V>();
public SoftHashMap() {
this(1000);
}
public SoftHashMap(int hardSize) {
max_limit = hardSize;
}
@Override
public V get(Object key) {
V result = null;
// We get the SoftReference represented by that key
SoftReference<V> soft_ref = map.get(key);
if (soft_ref != null) {
try {
// From the SoftReference we get the value, which can be
// null if it was not in the map, or it was removed in
// the clearGCCollected() method defined below
result = soft_ref.get();
if (result == null) {
// If the value has been garbage collected, remove the
// entry from the HashMap.
map.remove(key);
} else {
// We now add this object to the beginning of the hard
// reference queue. One reference can occur more than
// once, because lookups of the FIFO queue are slow, so
// we don't want to search through it each time to remove
// duplicates.
synchronized (hardCache){
hardCache.addFirst(result);
if (hardCache.size() > max_limit) {
// Remove the last entry if list greater than MAX_LIMIT
hardCache.removeLast();
}
}
}
} catch (Exception e){
logger.error("Exception: ", e);
}
}
return result;
}
/**
* We define our own subclass of SoftReference which contains not only the
* value but also the key to make it easier to find the entry in the HashMap
* after it's been garbage collected.
*/
private static class SoftValue<K, V> extends SoftReference<V> {
private final Object key; // always make data member final
/**
* Did you know that an outer class can access private data members and
* methods of an inner class? I didn't know that! I thought it was only
* the inner class who could access the outer class's private
* information. An outer class can also access private members of an
* inner class inside its inner class.
*/
private SoftValue(V k, K key, ReferenceQueue<? super V> q) {
super(k, q);
this.key = key;
}
}
/**
* Here we go through the ReferenceQueue and remove garbage collected
* SoftValue objects from the HashMap by looking them up using the
* SoftValue.key data member.
*/
@SuppressWarnings("unchecked") // every Reference in queue is stored as a SoftValue
private void clearGCCollected() {
SoftValue<K, V> sv;
while ((sv = (SoftValue<K, V>) queue.poll()) != null) {
map.remove(sv.key); // we can access private data!
}
}
/**
* Here we put the key, value pair into the HashMap using a SoftValue
* object.
*/
@Override
public synchronized V put(K key, V value) {
clearGCCollected();
logger.debug("Putting {} on cache. size: {}", key, size());
map.put(key, new SoftValue<K, V>(value, key, queue));
return value;
}
@Override
public V remove(Object key) {
clearGCCollected();
logger.debug("Removing {} from cache. size: {}", key, size());
return map.remove(key).get();
}
@Override
public void clear() {
synchronized (hardCache){
hardCache.clear();
}
clearGCCollected();
logger.debug("clearing cache");
map.clear();
}
@Override
public int size() {
clearGCCollected();
return map.size();
}
@Override
public Set<Map.Entry<K, V>> entrySet() {
throw new UnsupportedOperationException();
}
}