AbstractCachingMapDecorator.java
/*
* Copyright 2004-2020 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.binding.collection;
import java.io.Serializable;
import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.Map;
import java.util.Set;
import java.util.WeakHashMap;
import org.springframework.util.Assert;
/**
* A simple decorator for a Map, encapsulating the workflow for caching
* expensive values in a target Map. Supports caching weak or strong keys.
* <p>
* This class is an abstract template. Caching Map implementations should
* subclass and override the <code>create(key)</code> method which encapsulates
* expensive creation of a new object.
*
* @author Keith Donald
* @author Juergen Hoeller
* @since 2.4
*/
@SuppressWarnings("serial")
public abstract class AbstractCachingMapDecorator<K, V> implements Map<K, V>, Serializable {
private static Object NULL_VALUE = new Object();
private final Map<K, Object> targetMap;
private final boolean synchronize;
private final boolean weak;
/**
* Create a CachingMapDecorator with strong keys,
* using an underlying synchronized Map.
*/
public AbstractCachingMapDecorator() {
this(false);
}
/**
* Create a CachingMapDecorator,
* using an underlying synchronized Map.
* @param weak whether to use weak references for keys and values
*/
public AbstractCachingMapDecorator(boolean weak) {
Map<K, Object> internalMap = (weak ? new WeakHashMap<>() : new HashMap<>());
this.targetMap = Collections.synchronizedMap(internalMap);
this.synchronize = true;
this.weak = weak;
}
/**
* Create a CachingMapDecorator with initial size,
* using an underlying synchronized Map.
* @param weak whether to use weak references for keys and values
* @param size the initial cache size
*/
public AbstractCachingMapDecorator(boolean weak, int size) {
Map<K, Object> internalMap = weak ? new WeakHashMap<>(size) : new HashMap<>(size);
this.targetMap = Collections.synchronizedMap(internalMap);
this.synchronize = true;
this.weak = weak;
}
/**
* Create a CachingMapDecorator for the given Map.
* <p>The passed-in Map won't get synchronized explicitly,
* so make sure to pass in a properly synchronized Map, if desired.
* @param targetMap the Map to decorate
*/
public AbstractCachingMapDecorator(Map<K, V> targetMap) {
this(targetMap, false, false);
}
/**
* Create a CachingMapDecorator for the given Map.
* <p>The passed-in Map won't get synchronized explicitly unless
* you specify "synchronize" as "true".
* @param targetMap the Map to decorate
* @param synchronize whether to synchronize on the given Map
* @param weak whether to use weak references for values
*/
@SuppressWarnings("unchecked")
public AbstractCachingMapDecorator(Map<K, V> targetMap, boolean synchronize, boolean weak) {
Assert.notNull(targetMap, "'targetMap' must not be null");
this.targetMap = (Map<K, Object>) (synchronize ? Collections.synchronizedMap(targetMap) : targetMap);
this.synchronize = synchronize;
this.weak = weak;
}
public int size() {
return this.targetMap.size();
}
public boolean isEmpty() {
return this.targetMap.isEmpty();
}
public boolean containsKey(Object key) {
return this.targetMap.containsKey(key);
}
public boolean containsValue(Object value) {
Object valueToCheck = (value != null ? value : NULL_VALUE);
if (this.synchronize) {
synchronized (this.targetMap) {
return containsValueOrReference(valueToCheck);
}
}
else {
return containsValueOrReference(valueToCheck);
}
}
private boolean containsValueOrReference(Object value) {
if (this.targetMap.containsValue(value)) {
return true;
}
for (Object mapVal : this.targetMap.values()) {
if (mapVal instanceof Reference && value.equals(((Reference) mapVal).get())) {
return true;
}
}
return false;
}
public V remove(Object key) {
return unwrapReturnValue(this.targetMap.remove(key));
}
@SuppressWarnings("unchecked")
private V unwrapReturnValue(Object value) {
Object returnValue = value;
if (returnValue instanceof Reference) {
returnValue = ((Reference) returnValue).get();
}
return (returnValue == NULL_VALUE ? null : (V) returnValue);
}
public void putAll(Map<? extends K, ? extends V> map) {
this.targetMap.putAll(map);
}
public void clear() {
this.targetMap.clear();
}
public Set<K> keySet() {
if (this.synchronize) {
synchronized (this.targetMap) {
return new LinkedHashSet<>(this.targetMap.keySet());
}
}
else {
return new LinkedHashSet<>(this.targetMap.keySet());
}
}
public Collection<V> values() {
if (this.synchronize) {
synchronized (this.targetMap) {
return valuesCopy();
}
}
else {
return valuesCopy();
}
}
@SuppressWarnings("unchecked")
private Collection<V> valuesCopy() {
LinkedList<V> values = new LinkedList<>();
for (Iterator<Object> it = this.targetMap.values().iterator(); it.hasNext();) {
Object value = it.next();
if (value instanceof Reference) {
value = ((Reference) value).get();
if (value == null) {
it.remove();
continue;
}
}
values.add(value == NULL_VALUE ? null : (V) value);
}
return values;
}
public Set<Map.Entry<K, V>> entrySet() {
if (this.synchronize) {
synchronized (this.targetMap) {
return entryCopy();
}
}
else {
return entryCopy();
}
}
@SuppressWarnings("unchecked")
private Set<Map.Entry<K, V>> entryCopy() {
Map<K,V> entries = new LinkedHashMap<>();
for (Iterator<Entry<K, Object>> it = this.targetMap.entrySet().iterator(); it.hasNext();) {
Entry<K, Object> entry = it.next();
Object value = entry.getValue();
if (value instanceof Reference) {
value = ((Reference) value).get();
if (value == null) {
it.remove();
continue;
}
}
entries.put(entry.getKey(), value == NULL_VALUE ? null : (V) value);
}
return entries.entrySet();
}
/**
* Put an object into the cache, possibly wrapping it with a weak
* reference.
* @see #useWeakValue(Object, Object)
*/
public V put(K key, V value) {
Object newValue = value;
if (value == null) {
newValue = NULL_VALUE;
}
else if (useWeakValue(key, value)) {
newValue = new WeakReference<>(newValue);
}
return unwrapReturnValue(this.targetMap.put(key, newValue));
}
/**
* Decide whether to use a weak reference for the value of
* the given key-value pair.
* @param key the candidate key
* @param value the candidate value
* @return <code>true</code> in order to use a weak reference;
* <code>false</code> otherwise.
*/
protected boolean useWeakValue(K key, V value) {
return this.weak;
}
/**
* Get value for key.
* Creates and caches value if it doesn't already exist in the cache.
* <p>This implementation is <i>not</i> synchronized: This is highly
* concurrent but does not guarantee unique instances in the cache,
* as multiple values for the same key could get created in parallel.
* Consider overriding this method to synchronize it, if desired.
* @see #create(Object)
*/
@SuppressWarnings("unchecked")
public V get(Object key) {
Object value = this.targetMap.get(key);
if (value instanceof Reference) {
value = ((Reference) value).get();
}
if (value == null) {
V newValue = create((K) key);
put((K) key, newValue);
return newValue;
}
return (value == NULL_VALUE ? null : (V) value);
}
/**
* Create a value to cache for the given key.
* Called by <code>get</code> if there is no value cached already.
* @param key the cache key
* @see #get(Object)
*/
protected abstract V create(K key);
@Override
public String toString() {
return "CachingMapDecorator [" + getClass().getName() + "]:" + this.targetMap;
}
}