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The Cache.java Java example source code
/*
* Copyright (c) 2002, 2011, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package sun.security.util;
import java.util.*;
import java.lang.ref.*;
/**
* Abstract base class and factory for caches. A cache is a key-value mapping.
* It has properties that make it more suitable for caching than a Map.
*
* The factory methods can be used to obtain two different implementations.
* They have the following properties:
*
* . keys and values reside in memory
*
* . keys and values must be non-null
*
* . maximum size. Replacements are made in LRU order.
*
* . optional lifetime, specified in seconds.
*
* . safe for concurrent use by multiple threads
*
* . values are held by either standard references or via SoftReferences.
* SoftReferences have the advantage that they are automatically cleared
* by the garbage collector in response to memory demand. This makes it
* possible to simple set the maximum size to a very large value and let
* the GC automatically size the cache dynamically depending on the
* amount of available memory.
*
* However, note that because of the way SoftReferences are implemented in
* HotSpot at the moment, this may not work perfectly as it clears them fairly
* eagerly. Performance may be improved if the Java heap size is set to larger
* value using e.g. java -ms64M -mx128M foo.Test
*
* Cache sizing: the memory cache is implemented on top of a LinkedHashMap.
* In its current implementation, the number of buckets (NOT entries) in
* (Linked)HashMaps is always a power of two. It is recommended to set the
* maximum cache size to value that uses those buckets fully. For example,
* if a cache with somewhere between 500 and 1000 entries is desired, a
* maximum size of 750 would be a good choice: try 1024 buckets, with a
* load factor of 0.75f, the number of entries can be calculated as
* buckets / 4 * 3. As mentioned above, with a SoftReference cache, it is
* generally reasonable to set the size to a fairly large value.
*
* @author Andreas Sterbenz
*/
public abstract class Cache<K,V> {
protected Cache() {
// empty
}
/**
* Return the number of currently valid entries in the cache.
*/
public abstract int size();
/**
* Remove all entries from the cache.
*/
public abstract void clear();
/**
* Add an entry to the cache.
*/
public abstract void put(K key, V value);
/**
* Get a value from the cache.
*/
public abstract V get(Object key);
/**
* Remove an entry from the cache.
*/
public abstract void remove(Object key);
/**
* Set the maximum size.
*/
public abstract void setCapacity(int size);
/**
* Set the timeout(in seconds).
*/
public abstract void setTimeout(int timeout);
/**
* accept a visitor
*/
public abstract void accept(CacheVisitor<K,V> visitor);
/**
* Return a new memory cache with the specified maximum size, unlimited
* lifetime for entries, with the values held by SoftReferences.
*/
public static <K,V> Cache newSoftMemoryCache(int size) {
return new MemoryCache<>(true, size);
}
/**
* Return a new memory cache with the specified maximum size, the
* specified maximum lifetime (in seconds), with the values held
* by SoftReferences.
*/
public static <K,V> Cache newSoftMemoryCache(int size, int timeout) {
return new MemoryCache<>(true, size, timeout);
}
/**
* Return a new memory cache with the specified maximum size, unlimited
* lifetime for entries, with the values held by standard references.
*/
public static <K,V> Cache newHardMemoryCache(int size) {
return new MemoryCache<>(false, size);
}
/**
* Return a dummy cache that does nothing.
*/
@SuppressWarnings("unchecked")
public static <K,V> Cache newNullCache() {
return (Cache<K,V>) NullCache.INSTANCE;
}
/**
* Return a new memory cache with the specified maximum size, the
* specified maximum lifetime (in seconds), with the values held
* by standard references.
*/
public static <K,V> Cache newHardMemoryCache(int size, int timeout) {
return new MemoryCache<>(false, size, timeout);
}
/**
* Utility class that wraps a byte array and implements the equals()
* and hashCode() contract in a way suitable for Maps and caches.
*/
public static class EqualByteArray {
private final byte[] b;
private volatile int hash;
public EqualByteArray(byte[] b) {
this.b = b;
}
public int hashCode() {
int h = hash;
if (h == 0) {
h = b.length + 1;
for (int i = 0; i < b.length; i++) {
h += (b[i] & 0xff) * 37;
}
hash = h;
}
return h;
}
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj instanceof EqualByteArray == false) {
return false;
}
EqualByteArray other = (EqualByteArray)obj;
return Arrays.equals(this.b, other.b);
}
}
public interface CacheVisitor<K,V> {
public void visit(Map<K,V> map);
}
}
class NullCache<K,V> extends Cache {
final static Cache<Object,Object> INSTANCE = new NullCache<>();
private NullCache() {
// empty
}
public int size() {
return 0;
}
public void clear() {
// empty
}
public void put(K key, V value) {
// empty
}
public V get(Object key) {
return null;
}
public void remove(Object key) {
// empty
}
public void setCapacity(int size) {
// empty
}
public void setTimeout(int timeout) {
// empty
}
public void accept(CacheVisitor<K,V> visitor) {
// empty
}
}
class MemoryCache<K,V> extends Cache {
private final static float LOAD_FACTOR = 0.75f;
// XXXX
private final static boolean DEBUG = false;
private final Map<K, CacheEntry cacheMap;
private int maxSize;
private long lifetime;
// ReferenceQueue is of type V instead of Cache<K,V>
// to allow SoftCacheEntry to extend SoftReference<V>
private final ReferenceQueue<V> queue;
public MemoryCache(boolean soft, int maxSize) {
this(soft, maxSize, 0);
}
public MemoryCache(boolean soft, int maxSize, int lifetime) {
this.maxSize = maxSize;
this.lifetime = lifetime * 1000;
if (soft)
this.queue = new ReferenceQueue<>();
else
this.queue = null;
int buckets = (int)(maxSize / LOAD_FACTOR) + 1;
cacheMap = new LinkedHashMap<>(buckets, LOAD_FACTOR, true);
}
/**
* Empty the reference queue and remove all corresponding entries
* from the cache.
*
* This method should be called at the beginning of each public
* method.
*/
private void emptyQueue() {
if (queue == null) {
return;
}
int startSize = cacheMap.size();
while (true) {
@SuppressWarnings("unchecked")
CacheEntry<K,V> entry = (CacheEntry)queue.poll();
if (entry == null) {
break;
}
K key = entry.getKey();
if (key == null) {
// key is null, entry has already been removed
continue;
}
CacheEntry<K,V> currentEntry = cacheMap.remove(key);
// check if the entry in the map corresponds to the expired
// entry. If not, readd the entry
if ((currentEntry != null) && (entry != currentEntry)) {
cacheMap.put(key, currentEntry);
}
}
if (DEBUG) {
int endSize = cacheMap.size();
if (startSize != endSize) {
System.out.println("*** Expunged " + (startSize - endSize)
+ " entries, " + endSize + " entries left");
}
}
}
/**
* Scan all entries and remove all expired ones.
*/
private void expungeExpiredEntries() {
emptyQueue();
if (lifetime == 0) {
return;
}
int cnt = 0;
long time = System.currentTimeMillis();
for (Iterator<CacheEntry t = cacheMap.values().iterator();
t.hasNext(); ) {
CacheEntry<K,V> entry = t.next();
if (entry.isValid(time) == false) {
t.remove();
cnt++;
}
}
if (DEBUG) {
if (cnt != 0) {
System.out.println("Removed " + cnt
+ " expired entries, remaining " + cacheMap.size());
}
}
}
public synchronized int size() {
expungeExpiredEntries();
return cacheMap.size();
}
public synchronized void clear() {
if (queue != null) {
// if this is a SoftReference cache, first invalidate() all
// entries so that GC does not have to enqueue them
for (CacheEntry<K,V> entry : cacheMap.values()) {
entry.invalidate();
}
while (queue.poll() != null) {
// empty
}
}
cacheMap.clear();
}
public synchronized void put(K key, V value) {
emptyQueue();
long expirationTime = (lifetime == 0) ? 0 :
System.currentTimeMillis() + lifetime;
CacheEntry<K,V> newEntry = newEntry(key, value, expirationTime, queue);
CacheEntry<K,V> oldEntry = cacheMap.put(key, newEntry);
if (oldEntry != null) {
oldEntry.invalidate();
return;
}
if (maxSize > 0 && cacheMap.size() > maxSize) {
expungeExpiredEntries();
if (cacheMap.size() > maxSize) { // still too large?
Iterator<CacheEntry t = cacheMap.values().iterator();
CacheEntry<K,V> lruEntry = t.next();
if (DEBUG) {
System.out.println("** Overflow removal "
+ lruEntry.getKey() + " | " + lruEntry.getValue());
}
t.remove();
lruEntry.invalidate();
}
}
}
public synchronized V get(Object key) {
emptyQueue();
CacheEntry<K,V> entry = cacheMap.get(key);
if (entry == null) {
return null;
}
long time = (lifetime == 0) ? 0 : System.currentTimeMillis();
if (entry.isValid(time) == false) {
if (DEBUG) {
System.out.println("Ignoring expired entry");
}
cacheMap.remove(key);
return null;
}
return entry.getValue();
}
public synchronized void remove(Object key) {
emptyQueue();
CacheEntry<K,V> entry = cacheMap.remove(key);
if (entry != null) {
entry.invalidate();
}
}
public synchronized void setCapacity(int size) {
expungeExpiredEntries();
if (size > 0 && cacheMap.size() > size) {
Iterator<CacheEntry t = cacheMap.values().iterator();
for (int i = cacheMap.size() - size; i > 0; i--) {
CacheEntry<K,V> lruEntry = t.next();
if (DEBUG) {
System.out.println("** capacity reset removal "
+ lruEntry.getKey() + " | " + lruEntry.getValue());
}
t.remove();
lruEntry.invalidate();
}
}
maxSize = size > 0 ? size : 0;
if (DEBUG) {
System.out.println("** capacity reset to " + size);
}
}
public synchronized void setTimeout(int timeout) {
emptyQueue();
lifetime = timeout > 0 ? timeout * 1000L : 0L;
if (DEBUG) {
System.out.println("** lifetime reset to " + timeout);
}
}
// it is a heavyweight method.
public synchronized void accept(CacheVisitor<K,V> visitor) {
expungeExpiredEntries();
Map<K,V> cached = getCachedEntries();
visitor.visit(cached);
}
private Map<K,V> getCachedEntries() {
Map<K,V> kvmap = new HashMap<>(cacheMap.size());
for (CacheEntry<K,V> entry : cacheMap.values()) {
kvmap.put(entry.getKey(), entry.getValue());
}
return kvmap;
}
protected CacheEntry<K,V> newEntry(K key, V value,
long expirationTime, ReferenceQueue<V> queue) {
if (queue != null) {
return new SoftCacheEntry<>(key, value, expirationTime, queue);
} else {
return new HardCacheEntry<>(key, value, expirationTime);
}
}
private static interface CacheEntry<K,V> {
boolean isValid(long currentTime);
void invalidate();
K getKey();
V getValue();
}
private static class HardCacheEntry<K,V> implements CacheEntry {
private K key;
private V value;
private long expirationTime;
HardCacheEntry(K key, V value, long expirationTime) {
this.key = key;
this.value = value;
this.expirationTime = expirationTime;
}
public K getKey() {
return key;
}
public V getValue() {
return value;
}
public boolean isValid(long currentTime) {
boolean valid = (currentTime <= expirationTime);
if (valid == false) {
invalidate();
}
return valid;
}
public void invalidate() {
key = null;
value = null;
expirationTime = -1;
}
}
private static class SoftCacheEntry<K,V>
extends SoftReference<V>
implements CacheEntry<K,V> {
private K key;
private long expirationTime;
SoftCacheEntry(K key, V value, long expirationTime,
ReferenceQueue<V> queue) {
super(value, queue);
this.key = key;
this.expirationTime = expirationTime;
}
public K getKey() {
return key;
}
public V getValue() {
return get();
}
public boolean isValid(long currentTime) {
boolean valid = (currentTime <= expirationTime) && (get() != null);
if (valid == false) {
invalidate();
}
return valid;
}
public void invalidate() {
clear();
key = null;
expirationTime = -1;
}
}
}
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