/*
* Copyright 2014 Attila Szegedi, Daniel Dekany, Jonathan Revusky
*
* 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
*
* http://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 freemarker.cache;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.HashMap;
import java.util.Map;
/**
* A cache storage that implements a two-level Most Recently Used cache. In the
* first level, items are strongly referenced up to the specified maximum. When
* the maximum is exceeded, the least recently used item is moved into the
* second level cache, where they are softly referenced, up to another
* specified maximum. When the second level maximum is also exceeded, the least
* recently used item is discarded altogether. This cache storage is a
* generalization of both {@link StrongCacheStorage} and
* {@link SoftCacheStorage} - the effect of both of them can be achieved by
* setting one maximum to zero and the other to the largest positive integer.
* On the other hand, if you wish to use this storage in a strong-only mode, or
* in a soft-only mode, you might consider using {@link StrongCacheStorage} or
* {@link SoftCacheStorage} instead, as they can be used by
* {@link TemplateCache} concurrently without any synchronization on a 5.0 or
* later JRE.
*
* <p>This class is <em>NOT</em> thread-safe. If it's accessed from multiple
* threads concurrently, proper synchronization must be provided by the callers.
* Note that {@link TemplateCache}, the natural user of this class provides the
* necessary synchronizations when it uses the class.
* Also you might consider whether you need this sort of a mixed storage at all
* in your solution, as in most cases SoftCacheStorage can also be sufficient.
* SoftCacheStorage will use Java soft references, and they already use access
* timestamps internally to bias the garbage collector against clearing
* recently used references, so you can get reasonably good (and
* memory-sensitive) most-recently-used caching through
* {@link SoftCacheStorage} as well.
*
* @see freemarker.template.Configuration#setCacheStorage(CacheStorage)
*/
public class MruCacheStorage implements CacheStorageWithGetSize {
private final MruEntry strongHead = new MruEntry();
private final MruEntry softHead = new MruEntry();
{
softHead.linkAfter(strongHead);
}
private final Map map = new HashMap();
private final ReferenceQueue refQueue = new ReferenceQueue();
private final int strongSizeLimit;
private final int softSizeLimit;
private int strongSize = 0;
private int softSize = 0;
/**
* Creates a new MRU cache storage with specified maximum cache sizes. Each
* cache size can vary between 0 and {@link Integer#MAX_VALUE}.
* @param strongSizeLimit the maximum number of strongly referenced templates; when exceeded, the entry used
* the least recently will be moved into the soft cache.
* @param softSizeLimit the maximum number of softly referenced templates; when exceeded, the entry used
* the least recently will be discarded.
*/
public MruCacheStorage(int strongSizeLimit, int softSizeLimit) {
if (strongSizeLimit < 0) throw new IllegalArgumentException("strongSizeLimit < 0");
if (softSizeLimit < 0) throw new IllegalArgumentException("softSizeLimit < 0");
this.strongSizeLimit = strongSizeLimit;
this.softSizeLimit = softSizeLimit;
}
public Object get(Object key) {
removeClearedReferences();
MruEntry entry = (MruEntry) map.get(key);
if (entry == null) {
return null;
}
relinkEntryAfterStrongHead(entry, null);
Object value = entry.getValue();
if (value instanceof MruReference) {
// This can only happen with strongSizeLimit == 0
return ((MruReference) value).get();
}
return value;
}
public void put(Object key, Object value) {
removeClearedReferences();
MruEntry entry = (MruEntry) map.get(key);
if (entry == null) {
entry = new MruEntry(key, value);
map.put(key, entry);
linkAfterStrongHead(entry);
} else {
relinkEntryAfterStrongHead(entry, value);
}
}
public void remove(Object key) {
removeClearedReferences();
removeInternal(key);
}
private void removeInternal(Object key) {
MruEntry entry = (MruEntry) map.remove(key);
if (entry != null) {
unlinkEntryAndInspectIfSoft(entry);
}
}
public void clear() {
strongHead.makeHead();
softHead.linkAfter(strongHead);
map.clear();
strongSize = softSize = 0;
// Quick refQueue processing
while (refQueue.poll() != null);
}
private void relinkEntryAfterStrongHead(MruEntry entry, Object newValue) {
if (unlinkEntryAndInspectIfSoft(entry) && newValue == null) {
// Turn soft reference into strong reference, unless is was cleared
MruReference mref = (MruReference) entry.getValue();
Object strongValue = mref.get();
if (strongValue != null) {
entry.setValue(strongValue);
linkAfterStrongHead(entry);
} else {
map.remove(mref.getKey());
}
} else {
if (newValue != null) {
entry.setValue(newValue);
}
linkAfterStrongHead(entry);
}
}
private void linkAfterStrongHead(MruEntry entry) {
entry.linkAfter(strongHead);
if (strongSize == strongSizeLimit) {
// softHead.previous is LRU strong entry
MruEntry lruStrong = softHead.getPrevious();
// Attila: This is equaivalent to strongSizeLimit != 0
// DD: But entry.linkAfter(strongHead) was just executed above, so
// lruStrong != strongHead is true even if strongSizeLimit == 0.
if (lruStrong != strongHead) {
lruStrong.unlink();
if (softSizeLimit > 0) {
lruStrong.linkAfter(softHead);
lruStrong.setValue(new MruReference(lruStrong, refQueue));
if (softSize == softSizeLimit) {
// List is circular, so strongHead.previous is LRU soft entry
MruEntry lruSoft = strongHead.getPrevious();
lruSoft.unlink();
map.remove(lruSoft.getKey());
} else {
++softSize;
}
} else {
map.remove(lruStrong.getKey());
}
}
} else {
++strongSize;
}
}
private boolean unlinkEntryAndInspectIfSoft(MruEntry entry) {
entry.unlink();
if (entry.getValue() instanceof MruReference) {
--softSize;
return true;
} else {
--strongSize;
return false;
}
}
private void removeClearedReferences() {
for (; ; ) {
MruReference ref = (MruReference) refQueue.poll();
if (ref == null) {
break;
}
removeInternal(ref.getKey());
}
}
/**
* Returns the configured upper limit of the number of strong cache entries.
*
* @since 2.3.21
*/
public int getStrongSizeLimit() {
return strongSizeLimit;
}
/**
* Returns the configured upper limit of the number of soft cache entries.
*
* @since 2.3.21
*/
public int getSoftSizeLimit() {
return softSizeLimit;
}
/**
* Returns the <em>current</em> number of strong cache entries.
*
* @see #getStrongSizeLimit()
* @since 2.3.21
*/
public int getStrongSize() {
return strongSize;
}
/**
* Returns a close approximation of the <em>current</em> number of soft cache entries.
*
* @see #getSoftSizeLimit()
* @since 2.3.21
*/
public int getSoftSize() {
removeClearedReferences();
return softSize;
}
/**
* Returns a close approximation of the current number of cache entries.
*
* @see #getStrongSize()
* @see #getSoftSize()
* @since 2.3.21
*/
public int getSize() {
return getSoftSize() + getStrongSize();
}
private static final class MruEntry {
private MruEntry prev;
private MruEntry next;
private final Object key;
private Object value;
/**
* Used solely to construct the head element
*/
MruEntry() {
makeHead();
key = value = null;
}
MruEntry(Object key, Object value) {
this.key = key;
this.value = value;
}
Object getKey() {
return key;
}
Object getValue() {
return value;
}
void setValue(Object value) {
this.value = value;
}
MruEntry getPrevious() {
return prev;
}
void linkAfter(MruEntry entry) {
next = entry.next;
entry.next = this;
prev = entry;
next.prev = this;
}
void unlink() {
next.prev = prev;
prev.next = next;
prev = null;
next = null;
}
void makeHead() {
prev = next = this;
}
}
private static class MruReference extends SoftReference {
private final Object key;
MruReference(MruEntry entry, ReferenceQueue queue) {
super(entry.getValue(), queue);
this.key = entry.getKey();
}
Object getKey() {
return key;
}
}
}