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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
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*
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* 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).
*
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* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
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package com.sun.beans.util;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.Objects;
/**
* Hash table based implementation of the cache,
* which allows to use weak or soft references for keys and values.
* An entry in a {@code Cache} will automatically be removed
* when its key or value is no longer in ordinary use.
*
* @author Sergey Malenkov
* @since 1.8
*/
public abstract class Cache<K,V> {
private static final int MAXIMUM_CAPACITY = 1 << 30; // maximum capacity MUST be a power of two <= 1<<30
private final boolean identity; // defines whether the identity comparison is used
private final Kind keyKind; // a reference kind for the cache keys
private final Kind valueKind; // a reference kind for the cache values
private final ReferenceQueue<Object> queue = new ReferenceQueue<>(); // queue for references to remove
private volatile CacheEntry<K,V>[] table = newTable(1 << 3); // table's length MUST be a power of two
private int threshold = 6; // the next size value at which to resize
private int size; // the number of key-value mappings contained in this map
/**
* Creates a corresponding value for the specified key.
*
* @param key a key that can be used to create a value
* @return a corresponding value for the specified key
*/
public abstract V create(K key);
/**
* Constructs an empty {@code Cache}.
* The default initial capacity is 8.
* The default load factor is 0.75.
*
* @param keyKind a reference kind for keys
* @param valueKind a reference kind for values
*
* @throws NullPointerException if {@code keyKind} or {@code valueKind} are {@code null}
*/
public Cache(Kind keyKind, Kind valueKind) {
this(keyKind, valueKind, false);
}
/**
* Constructs an empty {@code Cache}
* with the specified comparison method.
* The default initial capacity is 8.
* The default load factor is 0.75.
*
* @param keyKind a reference kind for keys
* @param valueKind a reference kind for values
* @param identity defines whether reference-equality
* is used in place of object-equality
*
* @throws NullPointerException if {@code keyKind} or {@code valueKind} are {@code null}
*/
public Cache(Kind keyKind, Kind valueKind, boolean identity) {
Objects.requireNonNull(keyKind, "keyKind");
Objects.requireNonNull(valueKind, "valueKind");
this.keyKind = keyKind;
this.valueKind = valueKind;
this.identity = identity;
}
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if there is no mapping for the key.
*
* @param key the key whose cached value is to be returned
* @return a value to which the specified key is mapped,
* or {@code null} if there is no mapping for {@code key}
*
* @throws NullPointerException if {@code key} is {@code null}
* or corresponding value is {@code null}
*/
public final V get(K key) {
Objects.requireNonNull(key, "key");
removeStaleEntries();
int hash = hash(key);
// unsynchronized search improves performance
// the null value does not mean that there are no needed entry
CacheEntry<K,V>[] table = this.table; // unsynchronized access
V current = getEntryValue(key, hash, table[index(hash, table)]);
if (current != null) {
return current;
}
synchronized (this.queue) {
// synchronized search improves stability
// we must create and add new value if there are no needed entry
current = getEntryValue(key, hash, this.table[index(hash, this.table)]);
if (current != null) {
return current;
}
V value = create(key);
Objects.requireNonNull(value, "value");
int index = index(hash, this.table);
this.table[index] = new CacheEntry<>(hash, key, value, this.table[index]);
if (++this.size >= this.threshold) {
if (this.table.length == MAXIMUM_CAPACITY) {
this.threshold = Integer.MAX_VALUE;
} else {
removeStaleEntries();
table = newTable(this.table.length << 1);
transfer(this.table, table);
// If ignoring null elements and processing ref queue caused massive
// shrinkage, then restore old table. This should be rare, but avoids
// unbounded expansion of garbage-filled tables.
if (this.size >= this.threshold / 2) {
this.table = table;
this.threshold <<= 1;
} else {
transfer(table, this.table);
}
removeStaleEntries();
}
}
return value;
}
}
/**
* Removes the cached value that corresponds to the specified key.
*
* @param key the key whose mapping is to be removed from this cache
*/
public final void remove(K key) {
if (key != null) {
synchronized (this.queue) {
removeStaleEntries();
int hash = hash(key);
int index = index(hash, this.table);
CacheEntry<K,V> prev = this.table[index];
CacheEntry<K,V> entry = prev;
while (entry != null) {
CacheEntry<K,V> next = entry.next;
if (entry.matches(hash, key)) {
if (entry == prev) {
this.table[index] = next;
} else {
prev.next = next;
}
entry.unlink();
break;
}
prev = entry;
entry = next;
}
}
}
}
/**
* Removes all of the mappings from this cache.
* It will be empty after this call returns.
*/
public final void clear() {
synchronized (this.queue) {
int index = this.table.length;
while (0 < index--) {
CacheEntry<K,V> entry = this.table[index];
while (entry != null) {
CacheEntry<K,V> next = entry.next;
entry.unlink();
entry = next;
}
this.table[index] = null;
}
while (null != this.queue.poll()) {
// Clear out the reference queue.
}
}
}
/**
* Retrieves object hash code and applies a supplemental hash function
* to the result hash, which defends against poor quality hash functions.
* This is critical because {@code Cache} uses power-of-two length hash tables,
* that otherwise encounter collisions for hashCodes that do not differ
* in lower bits.
*
* @param key the object which hash code is to be calculated
* @return a hash code value for the specified object
*/
private int hash(Object key) {
if (this.identity) {
int hash = System.identityHashCode(key);
return (hash << 1) - (hash << 8);
}
int hash = key.hashCode();
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
hash ^= (hash >>> 20) ^ (hash >>> 12);
return hash ^ (hash >>> 7) ^ (hash >>> 4);
}
/**
* Returns index of the specified hash code in the given table.
* Note that the table size must be a power of two.
*
* @param hash the hash code
* @param table the table
* @return an index of the specified hash code in the given table
*/
private static int index(int hash, Object[] table) {
return hash & (table.length - 1);
}
/**
* Creates a new array for the cache entries.
*
* @param size requested capacity MUST be a power of two
* @return a new array for the cache entries
*/
@SuppressWarnings({"unchecked", "rawtypes"})
private CacheEntry<K,V>[] newTable(int size) {
return (CacheEntry<K,V>[]) new CacheEntry[size];
}
private V getEntryValue(K key, int hash, CacheEntry<K,V> entry) {
while (entry != null) {
if (entry.matches(hash, key)) {
return entry.value.getReferent();
}
entry = entry.next;
}
return null;
}
private void removeStaleEntries() {
Object reference = this.queue.poll();
if (reference != null) {
synchronized (this.queue) {
do {
if (reference instanceof Ref) {
@SuppressWarnings("rawtypes")
Ref ref = (Ref) reference;
@SuppressWarnings("unchecked")
CacheEntry<K,V> owner = (CacheEntry<K,V>) ref.getOwner();
if (owner != null) {
int index = index(owner.hash, this.table);
CacheEntry<K,V> prev = this.table[index];
CacheEntry<K,V> entry = prev;
while (entry != null) {
CacheEntry<K,V> next = entry.next;
if (entry == owner) {
if (entry == prev) {
this.table[index] = next;
} else {
prev.next = next;
}
entry.unlink();
break;
}
prev = entry;
entry = next;
}
}
}
reference = this.queue.poll();
}
while (reference != null);
}
}
}
private void transfer(CacheEntry<K,V>[] oldTable, CacheEntry<K,V>[] newTable) {
int oldIndex = oldTable.length;
while (0 < oldIndex--) {
CacheEntry<K,V> entry = oldTable[oldIndex];
oldTable[oldIndex] = null;
while (entry != null) {
CacheEntry<K,V> next = entry.next;
if (entry.key.isStale() || entry.value.isStale()) {
entry.unlink();
} else {
int newIndex = index(entry.hash, newTable);
entry.next = newTable[newIndex];
newTable[newIndex] = entry;
}
entry = next;
}
}
}
/**
* Represents a cache entry (key-value pair).
*/
private final class CacheEntry<K,V> {
private final int hash;
private final Ref<K> key;
private final Ref<V> value;
private volatile CacheEntry<K,V> next;
/**
* Constructs an entry for the cache.
*
* @param hash the hash code calculated for the entry key
* @param key the entry key
* @param value the initial value of the entry
* @param next the next entry in a chain
*/
private CacheEntry(int hash, K key, V value, CacheEntry<K,V> next) {
this.hash = hash;
this.key = Cache.this.keyKind.create(this, key, Cache.this.queue);
this.value = Cache.this.valueKind.create(this, value, Cache.this.queue);
this.next = next;
}
/**
* Determines whether the entry has the given key with the given hash code.
*
* @param hash an expected hash code
* @param object an object to be compared with the entry key
* @return {@code true} if the entry has the given key with the given hash code;
* {@code false} otherwise
*/
private boolean matches(int hash, Object object) {
if (this.hash != hash) {
return false;
}
Object key = this.key.getReferent();
return (key == object) || !Cache.this.identity && (key != null) && key.equals(object);
}
/**
* Marks the entry as actually removed from the cache.
*/
private void unlink() {
this.next = null;
this.key.removeOwner();
this.value.removeOwner();
Cache.this.size--;
}
}
/**
* Basic interface for references.
* It defines the operations common for the all kind of references.
*
* @param <T> the type of object to refer
*/
private static interface Ref<T> {
/**
* Returns the object that possesses information about the reference.
*
* @return the owner of the reference or {@code null} if the owner is unknown
*/
Object getOwner();
/**
* Returns the object to refer.
*
* @return the referred object or {@code null} if it was collected
*/
T getReferent();
/**
* Determines whether the referred object was taken by the garbage collector or not.
*
* @return {@code true} if the referred object was collected
*/
boolean isStale();
/**
* Marks this reference as removed from the cache.
*/
void removeOwner();
}
/**
* Represents a reference kind.
*/
public static enum Kind {
STRONG {
<T> Ref<T> create(Object owner, T value, ReferenceQueue<? super T> queue) {
return new Strong<>(owner, value);
}
},
SOFT {
<T> Ref<T> create(Object owner, T referent, ReferenceQueue<? super T> queue) {
return (referent == null)
? new Strong<>(owner, referent)
: new Soft<>(owner, referent, queue);
}
},
WEAK {
<T> Ref<T> create(Object owner, T referent, ReferenceQueue<? super T> queue) {
return (referent == null)
? new Strong<>(owner, referent)
: new Weak<>(owner, referent, queue);
}
};
/**
* Creates a reference to the specified object.
*
* @param <T> the type of object to refer
* @param owner the owner of the reference, if needed
* @param referent the object to refer
* @param queue the queue to register the reference with,
* or {@code null} if registration is not required
* @return the reference to the specified object
*/
abstract <T> Ref<T> create(Object owner, T referent, ReferenceQueue<? super T> queue);
/**
* This is an implementation of the {@link Cache.Ref} interface
* that uses the strong references that prevent their referents
* from being made finalizable, finalized, and then reclaimed.
*
* @param <T> the type of object to refer
*/
private static final class Strong<T> implements Ref<T> {
private Object owner;
private final T referent;
/**
* Creates a strong reference to the specified object.
*
* @param owner the owner of the reference, if needed
* @param referent the non-null object to refer
*/
private Strong(Object owner, T referent) {
this.owner = owner;
this.referent = referent;
}
/**
* Returns the object that possesses information about the reference.
*
* @return the owner of the reference or {@code null} if the owner is unknown
*/
public Object getOwner() {
return this.owner;
}
/**
* Returns the object to refer.
*
* @return the referred object
*/
public T getReferent() {
return this.referent;
}
/**
* Determines whether the referred object was taken by the garbage collector or not.
*
* @return {@code true} if the referred object was collected
*/
public boolean isStale() {
return false;
}
/**
* Marks this reference as removed from the cache.
*/
public void removeOwner() {
this.owner = null;
}
}
/**
* This is an implementation of the {@link Cache.Ref} interface
* that uses the soft references that are cleared at the discretion
* of the garbage collector in response to a memory request.
*
* @param <T> the type of object to refer
* @see java.lang.ref.SoftReference
*/
private static final class Soft<T> extends SoftReference<T> implements Ref<T> {
private Object owner;
/**
* Creates a soft reference to the specified object.
*
* @param owner the owner of the reference, if needed
* @param referent the non-null object to refer
* @param queue the queue to register the reference with,
* or {@code null} if registration is not required
*/
private Soft(Object owner, T referent, ReferenceQueue<? super T> queue) {
super(referent, queue);
this.owner = owner;
}
/**
* Returns the object that possesses information about the reference.
*
* @return the owner of the reference or {@code null} if the owner is unknown
*/
public Object getOwner() {
return this.owner;
}
/**
* Returns the object to refer.
*
* @return the referred object or {@code null} if it was collected
*/
public T getReferent() {
return get();
}
/**
* Determines whether the referred object was taken by the garbage collector or not.
*
* @return {@code true} if the referred object was collected
*/
public boolean isStale() {
return null == get();
}
/**
* Marks this reference as removed from the cache.
*/
public void removeOwner() {
this.owner = null;
}
}
/**
* This is an implementation of the {@link Cache.Ref} interface
* that uses the weak references that do not prevent their referents
* from being made finalizable, finalized, and then reclaimed.
*
* @param <T> the type of object to refer
* @see java.lang.ref.WeakReference
*/
private static final class Weak<T> extends WeakReference<T> implements Ref<T> {
private Object owner;
/**
* Creates a weak reference to the specified object.
*
* @param owner the owner of the reference, if needed
* @param referent the non-null object to refer
* @param queue the queue to register the reference with,
* or {@code null} if registration is not required
*/
private Weak(Object owner, T referent, ReferenceQueue<? super T> queue) {
super(referent, queue);
this.owner = owner;
}
/**
* Returns the object that possesses information about the reference.
*
* @return the owner of the reference or {@code null} if the owner is unknown
*/
public Object getOwner() {
return this.owner;
}
/**
* Returns the object to refer.
*
* @return the referred object or {@code null} if it was collected
*/
public T getReferent() {
return get();
}
/**
* Determines whether the referred object was taken by the garbage collector or not.
*
* @return {@code true} if the referred object was collected
*/
public boolean isStale() {
return null == get();
}
/**
* Marks this reference as removed from the cache.
*/
public void removeOwner() {
this.owner = null;
}
}
}
}