CachedMap.java
package org.codehaus.plexus.util;
/*
* J.A.D.E. Java(TM) Addition to Default Environment.
* Latest release available at http://jade.dautelle.com/
* This class is public domain (not copyrighted).
*/
import java.util.Collection;
import java.util.Collections;
import java.util.Map;
import java.util.Set;
/**
* <p>
* This class provides cache access to <code>Map</code> collections.
* </p>
* <p>
* Instance of this class can be used as "proxy" for any collection implementing the <code>java.util.Map</code>
* interface.
* </p>
* <p>
* Typically, {@link CachedMap} are used to accelerate access to large collections when the access to the collection is
* not evenly distributed (associative cache). The performance gain is about 50% for the fastest hash map collections
* (e.g. {@link FastMap}). For slower collections such as <code>java.util.TreeMap</code>, non-resizable {@link FastMap}
* (real-time) or database access, performance can be of several orders of magnitude.
* </p>
* <p>
* <b>Note:</b> The keys used to access elements of a {@link CachedMap} do not need to be immutable as they are not
* stored in the cache (only keys specified by the {@link #put} method are). In other words, access can be performed
* using mutable keys as long as these keys can be compared for equality with the real map's keys (e.g. same
* <code>hashCode</code> values).
* </p>
* <p>
* This implementation is not synchronized. Multiple threads accessing or modifying the collection must be synchronized
* externally.
* </p>
* <p>
* <i> This class is <b>public domain</b> (not copyrighted).</i>
* </p>
*
* @author <a href="mailto:jean-marie@dautelle.com">Jean-Marie Dautelle</a>
* @version 5.3, October 30, 2003
*/
public final class CachedMap implements Map {
/**
* Holds the FastMap backing this collection (<code>null</code> if generic backing map).
*/
private final FastMap _backingFastMap;
/**
* Holds the generic map backing this collection.
*/
private final Map _backingMap;
/**
* Holds the keys of the backing map (key-to-key mapping). (<code>null</code> if FastMap backing map).
*/
private final FastMap _keysMap;
/**
* Holds the cache's mask (capacity - 1).
*/
private final int _mask;
/**
* Holds the keys being cached.
*/
private final Object[] _keys;
/**
* Holds the values being cached.
*/
private final Object[] _values;
/**
* Creates a cached map backed by a {@link FastMap}. The default cache size and map capacity is set to
* <code>256</code> entries.
*/
public CachedMap() {
this(256, new FastMap());
}
/**
* Creates a cached map backed by a {@link FastMap} and having the specified cache size.
*
* @param cacheSize the cache size, the actual cache size is the first power of 2 greater or equal to
* <code>cacheSize</code>. This is also the initial capacity of the backing map.
*/
public CachedMap(int cacheSize) {
this(cacheSize, new FastMap(cacheSize));
}
/**
* Creates a cached map backed by the specified map and having the specified cache size. In order to maintain cache
* veracity, it is critical that <b>all</b> update to the backing map is accomplished through the {@link CachedMap}
* instance; otherwise {@link #flush} has to be called.
*
* @param cacheSize the cache size, the actual cache size is the first power of 2 greater or equal to
* <code>cacheSize</code>.
* @param backingMap the backing map to be "wrapped" in a cached map.
*/
public CachedMap(int cacheSize, Map backingMap) {
// Find a power of 2 >= minimalCache
int actualCacheSize = 1;
while (actualCacheSize < cacheSize) {
actualCacheSize <<= 1;
}
// Sets up cache.
_keys = new Object[actualCacheSize];
_values = new Object[actualCacheSize];
_mask = actualCacheSize - 1;
// Sets backing map references.
if (backingMap instanceof FastMap) {
_backingFastMap = (FastMap) backingMap;
_backingMap = _backingFastMap;
_keysMap = null;
} else {
_backingFastMap = null;
_backingMap = backingMap;
_keysMap = new FastMap(backingMap.size());
for (Object key : backingMap.keySet()) {
_keysMap.put(key, key);
}
}
}
/**
* Returns the actual cache size.
*
* @return the cache size (power of 2).
*/
public int getCacheSize() {
return _keys.length;
}
/**
* Returns the backing map. If the backing map is modified directly, this {@link CachedMap} has to be flushed.
*
* @return the backing map.
* @see #flush
*/
public Map getBackingMap() {
return (_backingFastMap != null) ? _backingFastMap : _backingMap;
}
/**
* Flushes the key/value pairs being cached. This method should be called if the backing map is externally modified.
*/
public void flush() {
for (int i = 0; i < _keys.length; i++) {
_keys[i] = null;
_values[i] = null;
}
if (_keysMap != null) {
// Re-populates keys from backing map.
for (Object key : _backingMap.keySet()) {
_keysMap.put(key, key);
}
}
}
/**
* Returns the value to which this map maps the specified key. First, the cache is being checked, then if the cache
* does not contains the specified key, the backing map is accessed and the key/value pair is stored in the cache.
*
* @param key the key whose associated value is to be returned.
* @return the value to which this map maps the specified key, or <code>null</code> if the map contains no mapping
* for this key.
* @throws ClassCastException if the key is of an inappropriate type for the backing map (optional).
* @throws NullPointerException if the key is <code>null</code>.
*/
@Override
public Object get(Object key) {
int index = key.hashCode() & _mask;
return key.equals(_keys[index]) ? _values[index] : getCacheMissed(key, index);
}
private Object getCacheMissed(Object key, int index) {
if (_backingFastMap != null) {
Map.Entry entry = _backingFastMap.getEntry(key);
if (entry != null) {
_keys[index] = entry.getKey();
Object value = entry.getValue();
_values[index] = value;
return value;
} else {
return null;
}
} else { // Generic backing map.
Object mapKey = _keysMap.get(key);
if (mapKey != null) {
_keys[index] = mapKey;
Object value = _backingMap.get(key);
_values[index] = value;
return value;
} else {
return null;
}
}
}
/**
* Associates the specified value with the specified key in this map.
*
* @param key the key with which the specified value is to be associated.
* @param value the value to be associated with the specified key.
* @return the previous value associated with specified key, or <code>null</code> if there was no mapping for the
* key.
* @throws UnsupportedOperationException if the <code>put</code> operation is not supported by the backing map.
* @throws ClassCastException if the class of the specified key or value prevents it from being stored in this map.
* @throws IllegalArgumentException if some aspect of this key or value prevents it from being stored in this map.
* @throws NullPointerException if the key is <code>null</code>.
*/
@Override
public Object put(Object key, Object value) {
// Updates the cache.
int index = key.hashCode() & _mask;
if (key.equals(_keys[index])) {
_values[index] = value;
} else if (_keysMap != null) { // Possibly a new key.
_keysMap.put(key, key);
}
// Updates the backing map.
return _backingMap.put(key, value);
}
/**
* Removes the mapping for this key from this map if it is present.
*
* @param key key whose mapping is to be removed from the map.
* @return previous value associated with specified key, or <code>null</code> if there was no mapping for key.
* @throws ClassCastException if the key is of an inappropriate type for the backing map (optional).
* @throws NullPointerException if the key is <code>null</code>.
* @throws UnsupportedOperationException if the <code>remove</code> method is not supported by the backing map.
*/
@Override
public Object remove(Object key) {
// Removes from cache.
int index = key.hashCode() & _mask;
if (key.equals(_keys[index])) {
_keys[index] = null;
}
// Removes from key map.
if (_keysMap != null) {
_keysMap.remove(key);
}
// Removes from backing map.
return _backingMap.remove(key);
}
/**
* Indicates if this map contains a mapping for the specified key.
*
* @param key the key whose presence in this map is to be tested.
* @return <code>true</code> if this map contains a mapping for the specified key; <code>false</code> otherwise.
*/
@Override
public boolean containsKey(Object key) {
// Checks the cache.
int index = key.hashCode() & _mask;
if (key.equals(_keys[index])) {
return true;
} else { // Checks the backing map.
return _backingMap.containsKey(key);
}
}
/**
* Returns the number of key-value mappings in this map. If the map contains more than
* <code>Integer.MAX_VALUE</code> elements, returns <code>Integer.MAX_VALUE</code>.
*
* @return the number of key-value mappings in this map.
*/
@Override
public int size() {
return _backingMap.size();
}
/**
* Returns <code>true</code> if this map contains no key-value mappings.
*
* @return <code>true</code> if this map contains no key-value mappings.
*/
@Override
public boolean isEmpty() {
return _backingMap.isEmpty();
}
/**
* Returns <code>true</code> if this map maps one or more keys to the specified value.
*
* @param value value whose presence in this map is to be tested.
* @return <code>true</code> if this map maps one or more keys to the specified value.
* @throws ClassCastException if the value is of an inappropriate type for the backing map (optional).
* @throws NullPointerException if the value is <code>null</code> and the backing map does not not permit
* <code>null</code> values.
*/
@Override
public boolean containsValue(Object value) {
return _backingMap.containsValue(value);
}
/**
* Copies all of the mappings from the specified map to this map (optional operation). This method automatically
* flushes the cache.
*
* @param map the mappings to be stored in this map.
* @throws UnsupportedOperationException if the <code>putAll</code> method is not supported by the backing map.
* @throws ClassCastException if the class of a key or value in the specified map prevents it from being stored in
* this map.
* @throws IllegalArgumentException some aspect of a key or value in the specified map prevents it from being stored
* in this map.
* @throws NullPointerException the specified map is <code>null</code>, or if the backing map does not permit
* <code>null</code> keys or values, and the specified map contains <code>null</code> keys or values.
*/
@Override
public void putAll(Map map) {
_backingMap.putAll(map);
flush();
}
/**
* Removes all mappings from this map (optional operation). This method automatically flushes the cache.
*
* @throws UnsupportedOperationException if clear is not supported by the backing map.
*/
@Override
public void clear() {
_backingMap.clear();
flush();
}
/**
* Returns an <b>unmodifiable</b> view of the keys contained in this map.
*
* @return an unmodifiable view of the keys contained in this map.
*/
@Override
public Set keySet() {
return Collections.unmodifiableSet(_backingMap.keySet());
}
/**
* Returns an <b>unmodifiable</b> view of the values contained in this map.
*
* @return an unmodifiable view of the values contained in this map.
*/
@Override
public Collection values() {
return Collections.unmodifiableCollection(_backingMap.values());
}
/**
* Returns an <b>unmodifiable</b> view of the mappings contained in this map. Each element in the returned set is a
* <code>Map.Entry</code>.
*
* @return an unmodifiable view of the mappings contained in this map.
*/
@Override
public Set entrySet() {
return Collections.unmodifiableSet(_backingMap.entrySet());
}
/**
* Compares the specified object with this map for equality. Returns <code>true</code> if the given object is also a map
* and the two Maps represent the same mappings.
*
* @param o object to be compared for equality with this map.
* @return <code>true</code> if the specified object is equal to this map.
*/
@Override
public boolean equals(Object o) {
return _backingMap.equals(o);
}
/**
* Returns the hash code value for this map.
*
* @return the hash code value for this map.
*/
@Override
public int hashCode() {
return _backingMap.hashCode();
}
}