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package java.beans;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
Hash table based mapping, which uses weak references to store keys
and reference-equality in place of object-equality to compare them.
An entry will automatically be removed when its key is no longer
in ordinary use. Both null values and the null key are supported.
This class does not require additional synchronization.
A thread-safety is provided by a fragile combination
of synchronized blocks and volatile fields.
Be very careful during editing!
See Also: - IdentityHashMap
- WeakHashMap
/**
* Hash table based mapping, which uses weak references to store keys
* and reference-equality in place of object-equality to compare them.
* An entry will automatically be removed when its key is no longer
* in ordinary use. Both null values and the null key are supported.
* This class does not require additional synchronization.
* A thread-safety is provided by a fragile combination
* of synchronized blocks and volatile fields.
* Be very careful during editing!
*
* @see java.util.IdentityHashMap
* @see java.util.WeakHashMap
*/
abstract class WeakIdentityMap<T> {
private static final int MAXIMUM_CAPACITY = 1 << 30; // it MUST be a power of two
private static final Object NULL = new Object(); // special object for null key
private final ReferenceQueue<Object> queue = new ReferenceQueue<Object>();
private volatile Entry<T>[] 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 = 0; // the number of key-value mappings
public T get(Object key) {
removeStaleEntries();
if (key == null) {
key = NULL;
}
int hash = key.hashCode();
Entry<T>[] table = this.table;
// unsynchronized search improves performance
// the null value does not mean that there are no needed entry
int index = getIndex(table, hash);
for (Entry<T> entry = table[index]; entry != null; entry = entry.next) {
if (entry.isMatched(key, hash)) {
return entry.value;
}
}
synchronized (NULL) {
// synchronized search improves stability
// we must create and add new value if there are no needed entry
index = getIndex(this.table, hash);
for (Entry<T> entry = this.table[index]; entry != null; entry = entry.next) {
if (entry.isMatched(key, hash)) {
return entry.value;
}
}
T value = create(key);
this.table[index] = new Entry<T>(key, hash, value, this.queue, 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 * 2);
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 *= 2;
}
else {
transfer(table, this.table);
}
}
}
return value;
}
}
protected abstract T create(Object key);
private void removeStaleEntries() {
Object ref = this.queue.poll();
if (ref != null) {
synchronized (NULL) {
do {
@SuppressWarnings("unchecked")
Entry<T> entry = (Entry<T>) ref;
int index = getIndex(this.table, entry.hash);
Entry<T> prev = this.table[index];
Entry<T> current = prev;
while (current != null) {
Entry<T> next = current.next;
if (current == entry) {
if (prev == entry) {
this.table[index] = next;
}
else {
prev.next = next;
}
entry.value = null; // Help GC
entry.next = null; // Help GC
this.size--;
break;
}
prev = current;
current = next;
}
ref = this.queue.poll();
}
while (ref != null);
}
}
}
private void transfer(Entry<T>[] oldTable, Entry<T>[] newTable) {
for (int i = 0; i < oldTable.length; i++) {
Entry<T> entry = oldTable[i];
oldTable[i] = null;
while (entry != null) {
Entry<T> next = entry.next;
Object key = entry.get();
if (key == null) {
entry.value = null; // Help GC
entry.next = null; // Help GC
this.size--;
}
else {
int index = getIndex(newTable, entry.hash);
entry.next = newTable[index];
newTable[index] = entry;
}
entry = next;
}
}
}
@SuppressWarnings("unchecked")
private Entry<T>[] newTable(int length) {
return (Entry<T>[]) new Entry<?>[length];
}
private static int getIndex(Entry<?>[] table, int hash) {
return hash & (table.length - 1);
}
private static class Entry<T> extends WeakReference<Object> {
private final int hash;
private volatile T value;
private volatile Entry<T> next;
Entry(Object key, int hash, T value, ReferenceQueue<Object> queue, Entry<T> next) {
super(key, queue);
this.hash = hash;
this.value = value;
this.next = next;
}
boolean isMatched(Object key, int hash) {
return (this.hash == hash) && (key == get());
}
}
}