* This class implements all the features necessary for a subclass reference
* hash-based map. Key-value entries are stored in instances of the
* ReferenceEntry class which can be overridden and replaced.
* The iterators can similarly be replaced, without the need to replace the KeySet,
* EntrySet and Values view classes.
*
* Overridable methods are provided to change the default hashing behaviour, and * to change how entries are added to and removed from the map. Hopefully, all you * need for unusual subclasses is here. *
* When you construct an AbstractReferenceMap, you can specify what
* kind of references are used to store the map's keys and values.
* If non-hard references are used, then the garbage collector can remove
* mappings if a key or value becomes unreachable, or if the JVM's memory is
* running low. For information on how the different reference types behave,
* see {@link Reference}.
*
* Different types of references can be specified for keys and values.
* The keys can be configured to be weak but the values hard,
* in which case this class will behave like a
*
* WeakHashMap. However, you can also specify hard keys and
* weak values, or any other combination. The default constructor uses
* hard keys and soft values, providing a memory-sensitive cache.
*
* This {@link Map} implementation does not allow null elements.
* Attempting to add a null key or value to the map will raise a
* NullPointerException.
*
* All the available iterators can be reset back to the start by casting to
* ResettableIterator and calling reset().
*
* This implementation is not synchronized.
* You can use {@link java.util.Collections#synchronizedMap} to
* provide synchronized access to a ReferenceMap.
*
* @see java.lang.ref.Reference
* @since Commons Collections 3.1 (extracted from ReferenceMap in 3.0)
* @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
*
* @author Paul Jack
* @author Stephen Colebourne
*/
public abstract class AbstractReferenceMap extends AbstractHashedMap {
/** Constant indicating that hard references should be used */
public static final int HARD = 0;
/** Constant indicating that soft references should be used */
public static final int SOFT = 1;
/** Constant indicating that weak references should be used */
public static final int WEAK = 2;
/**
* The reference type for keys. Must be HARD, SOFT, WEAK.
* @serial
*/
protected int keyType;
/**
* The reference type for values. Must be HARD, SOFT, WEAK.
* @serial
*/
protected int valueType;
/**
* Should the value be automatically purged when the associated key has been collected?
*/
protected boolean purgeValues;
/**
* ReferenceQueue used to eliminate stale mappings.
* See purge.
*/
private transient ReferenceQueue queue;
//-----------------------------------------------------------------------
/**
* Constructor used during deserialization.
*/
protected AbstractReferenceMap() {
super();
}
/**
* Constructs a new empty map with the specified reference types,
* load factor and initial capacity.
*
* @param keyType the type of reference to use for keys;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
* @param valueType the type of reference to use for values;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
* @param capacity the initial capacity for the map
* @param loadFactor the load factor for the map
* @param purgeValues should the value be automatically purged when the
* key is garbage collected
*/
protected AbstractReferenceMap(
int keyType, int valueType, int capacity,
float loadFactor, boolean purgeValues) {
super(capacity, loadFactor);
verify("keyType", keyType);
verify("valueType", valueType);
this.keyType = keyType;
this.valueType = valueType;
this.purgeValues = purgeValues;
}
/**
* Initialise this subclass during construction, cloning or deserialization.
*/
protected void init() {
queue = new ReferenceQueue();
}
//-----------------------------------------------------------------------
/**
* Checks the type int is a valid value.
*
* @param name the name for error messages
* @param type the type value to check
* @throws IllegalArgumentException if the value if invalid
*/
private static void verify(String name, int type) {
if ((type < HARD) || (type > WEAK)) {
throw new IllegalArgumentException(name + " must be HARD, SOFT, WEAK.");
}
}
//-----------------------------------------------------------------------
/**
* Gets the size of the map.
*
* @return the size
*/
public int size() {
purgeBeforeRead();
return super.size();
}
/**
* Checks whether the map is currently empty.
*
* @return true if the map is currently size zero
*/
public boolean isEmpty() {
purgeBeforeRead();
return super.isEmpty();
}
/**
* Checks whether the map contains the specified key.
*
* @param key the key to search for
* @return true if the map contains the key
*/
public boolean containsKey(Object key) {
purgeBeforeRead();
Entry entry = getEntry(key);
if (entry == null) {
return false;
}
return (entry.getValue() != null);
}
/**
* Checks whether the map contains the specified value.
*
* @param value the value to search for
* @return true if the map contains the value
*/
public boolean containsValue(Object value) {
purgeBeforeRead();
if (value == null) {
return false;
}
return super.containsValue(value);
}
/**
* Gets the value mapped to the key specified.
*
* @param key the key
* @return the mapped value, null if no match
*/
public Object get(Object key) {
purgeBeforeRead();
Entry entry = getEntry(key);
if (entry == null) {
return null;
}
return entry.getValue();
}
/**
* Puts a key-value mapping into this map.
* Neither the key nor the value may be null.
*
* @param key the key to add, must not be null
* @param value the value to add, must not be null
* @return the value previously mapped to this key, null if none
* @throws NullPointerException if either the key or value is null
*/
public Object put(Object key, Object value) {
if (key == null) {
throw new NullPointerException("null keys not allowed");
}
if (value == null) {
throw new NullPointerException("null values not allowed");
}
purgeBeforeWrite();
return super.put(key, value);
}
/**
* Removes the specified mapping from this map.
*
* @param key the mapping to remove
* @return the value mapped to the removed key, null if key not in map
*/
public Object remove(Object key) {
if (key == null) {
return null;
}
purgeBeforeWrite();
return super.remove(key);
}
/**
* Clears this map.
*/
public void clear() {
super.clear();
while (queue.poll() != null) {} // drain the queue
}
//-----------------------------------------------------------------------
/**
* Gets a MapIterator over the reference map.
* The iterator only returns valid key/value pairs.
*
* @return a map iterator
*/
public MapIterator mapIterator() {
return new ReferenceMapIterator(this);
}
/**
* Returns a set view of this map's entries.
* An iterator returned entry is valid until next() is called again.
* The setValue() method on the toArray entries has no effect.
*
* @return a set view of this map's entries
*/
public Set entrySet() {
if (entrySet == null) {
entrySet = new ReferenceEntrySet(this);
}
return entrySet;
}
/**
* Returns a set view of this map's keys.
*
* @return a set view of this map's keys
*/
public Set keySet() {
if (keySet == null) {
keySet = new ReferenceKeySet(this);
}
return keySet;
}
/**
* Returns a collection view of this map's values.
*
* @return a set view of this map's values
*/
public Collection values() {
if (values == null) {
values = new ReferenceValues(this);
}
return values;
}
//-----------------------------------------------------------------------
/**
* Purges stale mappings from this map before read operations.
*
* This implementation calls {@link #purge()} to maintain a consistent state. */ protected void purgeBeforeRead() { purge(); } /** * Purges stale mappings from this map before write operations. *
* This implementation calls {@link #purge()} to maintain a consistent state. */ protected void purgeBeforeWrite() { purge(); } /** * Purges stale mappings from this map. *
* Note that this method is not synchronized! Special * care must be taken if, for instance, you want stale * mappings to be removed on a periodic basis by some * background thread. */ protected void purge() { Reference ref = queue.poll(); while (ref != null) { purge(ref); ref = queue.poll(); } } /** * Purges the specified reference. * * @param ref the reference to purge */ protected void purge(Reference ref) { // The hashCode of the reference is the hashCode of the // mapping key, even if the reference refers to the // mapping value... int hash = ref.hashCode(); int index = hashIndex(hash, data.length); HashEntry previous = null; HashEntry entry = data[index]; while (entry != null) { if (((ReferenceEntry) entry).purge(ref)) { if (previous == null) { data[index] = entry.next; } else { previous.next = entry.next; } this.size--; return; } previous = entry; entry = entry.next; } } //----------------------------------------------------------------------- /** * Gets the entry mapped to the key specified. * * @param key the key * @return the entry, null if no match */ protected HashEntry getEntry(Object key) { if (key == null) { return null; } else { return super.getEntry(key); } } /** * Gets the hash code for a MapEntry. * Subclasses can override this, for example to use the identityHashCode. * * @param key the key to get a hash code for, may be null * @param value the value to get a hash code for, may be null * @return the hash code, as per the MapEntry specification */ protected int hashEntry(Object key, Object value) { return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode()); } /** * Compares two keys, in internal converted form, to see if they are equal. *
* This implementation converts the key from the entry to a real reference
* before comparison.
*
* @param key1 the first key to compare passed in from outside
* @param key2 the second key extracted from the entry via entry.key
* @return true if equal
*/
protected boolean isEqualKey(Object key1, Object key2) {
key2 = (keyType > HARD ? ((Reference) key2).get() : key2);
return (key1 == key2 || key1.equals(key2));
}
/**
* Creates a ReferenceEntry instead of a HashEntry.
*
* @param next the next entry in sequence
* @param hashCode the hash code to use
* @param key the key to store
* @param value the value to store
* @return the newly created entry
*/
protected HashEntry createEntry(HashEntry next, int hashCode, Object key, Object value) {
return new ReferenceEntry(this, next, hashCode, key, value);
}
/**
* Creates an entry set iterator.
*
* @return the entrySet iterator
*/
protected Iterator createEntrySetIterator() {
return new ReferenceEntrySetIterator(this);
}
/**
* Creates an key set iterator.
*
* @return the keySet iterator
*/
protected Iterator createKeySetIterator() {
return new ReferenceKeySetIterator(this);
}
/**
* Creates an values iterator.
*
* @return the values iterator
*/
protected Iterator createValuesIterator() {
return new ReferenceValuesIterator(this);
}
//-----------------------------------------------------------------------
/**
* EntrySet implementation.
*/
static class ReferenceEntrySet extends EntrySet {
protected ReferenceEntrySet(AbstractHashedMap parent) {
super(parent);
}
public Object[] toArray() {
return toArray(new Object[0]);
}
public Object[] toArray(Object[] arr) {
// special implementation to handle disappearing entries
ArrayList list = new ArrayList();
Iterator iterator = iterator();
while (iterator.hasNext()) {
Entry e = (Entry) iterator.next();
list.add(new DefaultMapEntry(e.getKey(), e.getValue()));
}
return list.toArray(arr);
}
}
//-----------------------------------------------------------------------
/**
* KeySet implementation.
*/
static class ReferenceKeySet extends KeySet {
protected ReferenceKeySet(AbstractHashedMap parent) {
super(parent);
}
public Object[] toArray() {
return toArray(new Object[0]);
}
public Object[] toArray(Object[] arr) {
// special implementation to handle disappearing keys
List list = new ArrayList(parent.size());
for (Iterator it = iterator(); it.hasNext(); ) {
list.add(it.next());
}
return list.toArray(arr);
}
}
//-----------------------------------------------------------------------
/**
* Values implementation.
*/
static class ReferenceValues extends Values {
protected ReferenceValues(AbstractHashedMap parent) {
super(parent);
}
public Object[] toArray() {
return toArray(new Object[0]);
}
public Object[] toArray(Object[] arr) {
// special implementation to handle disappearing values
List list = new ArrayList(parent.size());
for (Iterator it = iterator(); it.hasNext(); ) {
list.add(it.next());
}
return list.toArray(arr);
}
}
//-----------------------------------------------------------------------
/**
* A MapEntry implementation for the map.
*
* If getKey() or getValue() returns null, it means * the mapping is stale and should be removed. * * @since Commons Collections 3.1 */ protected static class ReferenceEntry extends HashEntry { /** The parent map */ protected final AbstractReferenceMap parent; /** * Creates a new entry object for the ReferenceMap. * * @param parent the parent map * @param next the next entry in the hash bucket * @param hashCode the hash code of the key * @param key the key * @param value the value */ public ReferenceEntry(AbstractReferenceMap parent, HashEntry next, int hashCode, Object key, Object value) { super(next, hashCode, null, null); this.parent = parent; this.key = toReference(parent.keyType, key, hashCode); this.value = toReference(parent.valueType, value, hashCode); // the key hashCode is passed in deliberately } /** * Gets the key from the entry. * This method dereferences weak and soft keys and thus may return null. * * @return the key, which may be null if it was garbage collected */ public Object getKey() { return (parent.keyType > HARD) ? ((Reference) key).get() : key; } /** * Gets the value from the entry. * This method dereferences weak and soft value and thus may return null. * * @return the value, which may be null if it was garbage collected */ public Object getValue() { return (parent.valueType > HARD) ? ((Reference) value).get() : value; } /** * Sets the value of the entry. * * @param obj the object to store * @return the previous value */ public Object setValue(Object obj) { Object old = getValue(); if (parent.valueType > HARD) { ((Reference)value).clear(); } value = toReference(parent.valueType, obj, hashCode); return old; } /** * Compares this map entry to another. *
* This implementation uses isEqualKey and
* isEqualValue on the main map for comparison.
*
* @param obj the other map entry to compare to
* @return true if equal, false if not
*/
public boolean equals(Object obj) {
if (obj == this) {
return true;
}
if (obj instanceof Map.Entry == false) {
return false;
}
Map.Entry entry = (Map.Entry)obj;
Object entryKey = entry.getKey(); // convert to hard reference
Object entryValue = entry.getValue(); // convert to hard reference
if ((entryKey == null) || (entryValue == null)) {
return false;
}
// compare using map methods, aiding identity subclass
// note that key is direct access and value is via method
return parent.isEqualKey(entryKey, key) &&
parent.isEqualValue(entryValue, getValue());
}
/**
* Gets the hashcode of the entry using temporary hard references.
*
* This implementation uses hashEntry on the main map.
*
* @return the hashcode of the entry
*/
public int hashCode() {
return parent.hashEntry(getKey(), getValue());
}
/**
* Constructs a reference of the given type to the given referent.
* The reference is registered with the queue for later purging.
*
* @param type HARD, SOFT or WEAK
* @param referent the object to refer to
* @param hash the hash code of the key of the mapping;
* this number might be different from referent.hashCode() if
* the referent represents a value and not a key
*/
protected Object toReference(int type, Object referent, int hash) {
switch (type) {
case HARD: return referent;
case SOFT: return new SoftRef(hash, referent, parent.queue);
case WEAK: return new WeakRef(hash, referent, parent.queue);
default: throw new Error();
}
}
/**
* Purges the specified reference
* @param ref the reference to purge
* @return true or false
*/
boolean purge(Reference ref) {
boolean r = (parent.keyType > HARD) && (key == ref);
r = r || ((parent.valueType > HARD) && (value == ref));
if (r) {
if (parent.keyType > HARD) {
((Reference)key).clear();
}
if (parent.valueType > HARD) {
((Reference)value).clear();
} else if (parent.purgeValues) {
value = null;
}
}
return r;
}
/**
* Gets the next entry in the bucket.
*
* @return the next entry in the bucket
*/
protected ReferenceEntry next() {
return (ReferenceEntry) next;
}
}
//-----------------------------------------------------------------------
/**
* The EntrySet iterator.
*/
static class ReferenceEntrySetIterator implements Iterator {
/** The parent map */
final AbstractReferenceMap parent;
// These fields keep track of where we are in the table.
int index;
ReferenceEntry entry;
ReferenceEntry previous;
// These Object fields provide hard references to the
// current and next entry; this assures that if hasNext()
// returns true, next() will actually return a valid element.
Object nextKey, nextValue;
Object currentKey, currentValue;
int expectedModCount;
public ReferenceEntrySetIterator(AbstractReferenceMap parent) {
super();
this.parent = parent;
index = (parent.size() != 0 ? parent.data.length : 0);
// have to do this here! size() invocation above
// may have altered the modCount.
expectedModCount = parent.modCount;
}
public boolean hasNext() {
checkMod();
while (nextNull()) {
ReferenceEntry e = entry;
int i = index;
while ((e == null) && (i > 0)) {
i--;
e = (ReferenceEntry) parent.data[i];
}
entry = e;
index = i;
if (e == null) {
currentKey = null;
currentValue = null;
return false;
}
nextKey = e.getKey();
nextValue = e.getValue();
if (nextNull()) {
entry = entry.next();
}
}
return true;
}
private void checkMod() {
if (parent.modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
private boolean nextNull() {
return (nextKey == null) || (nextValue == null);
}
protected ReferenceEntry nextEntry() {
checkMod();
if (nextNull() && !hasNext()) {
throw new NoSuchElementException();
}
previous = entry;
entry = entry.next();
currentKey = nextKey;
currentValue = nextValue;
nextKey = null;
nextValue = null;
return previous;
}
protected ReferenceEntry currentEntry() {
checkMod();
return previous;
}
public Object next() {
return nextEntry();
}
public void remove() {
checkMod();
if (previous == null) {
throw new IllegalStateException();
}
parent.remove(currentKey);
previous = null;
currentKey = null;
currentValue = null;
expectedModCount = parent.modCount;
}
}
/**
* The keySet iterator.
*/
static class ReferenceKeySetIterator extends ReferenceEntrySetIterator {
ReferenceKeySetIterator(AbstractReferenceMap parent) {
super(parent);
}
public Object next() {
return nextEntry().getKey();
}
}
/**
* The values iterator.
*/
static class ReferenceValuesIterator extends ReferenceEntrySetIterator {
ReferenceValuesIterator(AbstractReferenceMap parent) {
super(parent);
}
public Object next() {
return nextEntry().getValue();
}
}
/**
* The MapIterator implementation.
*/
static class ReferenceMapIterator extends ReferenceEntrySetIterator implements MapIterator {
protected ReferenceMapIterator(AbstractReferenceMap parent) {
super(parent);
}
public Object next() {
return nextEntry().getKey();
}
public Object getKey() {
HashEntry current = currentEntry();
if (current == null) {
throw new IllegalStateException(AbstractHashedMap.GETKEY_INVALID);
}
return current.getKey();
}
public Object getValue() {
HashEntry current = currentEntry();
if (current == null) {
throw new IllegalStateException(AbstractHashedMap.GETVALUE_INVALID);
}
return current.getValue();
}
public Object setValue(Object value) {
HashEntry current = currentEntry();
if (current == null) {
throw new IllegalStateException(AbstractHashedMap.SETVALUE_INVALID);
}
return current.setValue(value);
}
}
//-----------------------------------------------------------------------
// These two classes store the hashCode of the key of
// of the mapping, so that after they're dequeued a quick
// lookup of the bucket in the table can occur.
/**
* A soft reference holder.
*/
static class SoftRef extends SoftReference {
/** the hashCode of the key (even if the reference points to a value) */
private int hash;
public SoftRef(int hash, Object r, ReferenceQueue q) {
super(r, q);
this.hash = hash;
}
public int hashCode() {
return hash;
}
}
/**
* A weak reference holder.
*/
static class WeakRef extends WeakReference {
/** the hashCode of the key (even if the reference points to a value) */
private int hash;
public WeakRef(int hash, Object r, ReferenceQueue q) {
super(r, q);
this.hash = hash;
}
public int hashCode() {
return hash;
}
}
//-----------------------------------------------------------------------
/**
* Replaces the superclass method to store the state of this class.
*
* Serialization is not one of the JDK's nicest topics. Normal serialization will
* initialise the superclass before the subclass. Sometimes however, this isn't
* what you want, as in this case the put() method on read can be
* affected by subclass state.
*
* The solution adopted here is to serialize the state data of this class in
* this protected method. This method must be called by the
* writeObject() of the first serializable subclass.
*
* Subclasses may override if they have a specific field that must be present * on read before this implementation will work. Generally, the read determines * what must be serialized here, if anything. * * @param out the output stream */ protected void doWriteObject(ObjectOutputStream out) throws IOException { out.writeInt(keyType); out.writeInt(valueType); out.writeBoolean(purgeValues); out.writeFloat(loadFactor); out.writeInt(data.length); for (MapIterator it = mapIterator(); it.hasNext();) { out.writeObject(it.next()); out.writeObject(it.getValue()); } out.writeObject(null); // null terminate map // do not call super.doWriteObject() as code there doesn't work for reference map } /** * Replaces the superclassm method to read the state of this class. *
* Serialization is not one of the JDK's nicest topics. Normal serialization will
* initialise the superclass before the subclass. Sometimes however, this isn't
* what you want, as in this case the put() method on read can be
* affected by subclass state.
*
* The solution adopted here is to deserialize the state data of this class in
* this protected method. This method must be called by the
* readObject() of the first serializable subclass.
*
* Subclasses may override if the subclass has a specific field that must be present
* before put() or calculateThreshold() will work correctly.
*
* @param in the input stream
*/
protected void doReadObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
this.keyType = in.readInt();
this.valueType = in.readInt();
this.purgeValues = in.readBoolean();
this.loadFactor = in.readFloat();
int capacity = in.readInt();
init();
data = new HashEntry[capacity];
while (true) {
Object key = in.readObject();
if (key == null) {
break;
}
Object value = in.readObject();
put(key, value);
}
threshold = calculateThreshold(data.length, loadFactor);
// do not call super.doReadObject() as code there doesn't work for reference map
}
}