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WeakIdentityMap.java
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190 lines (173 loc) · 6.16 KB
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/*
* Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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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 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());
}
}
}