Monday, December 31, 2012

SoftHashMap - hashmap with soft values

The biggest problem with WeakHashMap is, it will only weakly reference the keys but not the values. This may lead to memory issues if we are using weakhashmap as a cache.

We can try this!!

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.AbstractMap;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;
import java.util.Set;

public class SoftHashMap<K, V> extends AbstractMap<K, V> {

/** The internal HashMap that will hold the SoftReference. */
private final Map<K, SoftValue<V>> hash = new HashMap<K, SoftValue<V>>();

/** The number of "hard" references to hold internally. */
private final int HARD_SIZE;

/** 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 queue = new ReferenceQueue();

public SoftHashMap() {

public SoftHashMap(int hardSize) {
HARD_SIZE = hardSize;

public V get(Object key) {
V result = null;
// We get the SoftReference represented by that key
SoftReference<V> soft_ref = hash.get(key);
if (soft_ref != null) {
// From the SoftReference we get the value, which can be
// null if it was not in the map, or it was removed in
// the processQueue() method defined below
result = soft_ref.get();
if (result == null) {
// If the value has been garbage collected, remove the
// entry from the HashMap.
} 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.
if (hardCache.size() > HARD_SIZE) {
// Remove the last entry if list longer than HARD_SIZE
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<V> extends SoftReference<V> {
private final Object key; // always make data member final

private SoftValue(V obj, Object key, ReferenceQueue queue) {
super(obj, queue);
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.
private void processQueue() {
SoftValue sv;
while ((sv = (SoftValue) queue.poll()) != null) {
hash.remove(sv.key); // we can access private data!

* Here we put the key, value pair into the HashMap using a SoftValue
* object.
public V put(K key, V value) {
processQueue(); // throw out garbage collected values first
SoftValue<V> softValue = new SoftValue<V>(value, key, queue);
SoftValue<V> put = hash.put(key, softValue);
if (put != null) {
return put.get();
return null;

public V remove(Object key) {
processQueue(); // throw out garbage collected values first
SoftValue<V> remove = hash.remove(key);
if (remove != null)  {
return remove.get();
return null;

public void clear() {
processQueue(); // throw out garbage collected values

public int size() {
processQueue(); // throw out garbage collected values first
return hash.size();

public Set entrySet() {
throw new UnsupportedOperationException();

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