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Java example source code file (WeakIdentityMap.java)

This example Java source code file (WeakIdentityMap.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

entry, help, maximum_capacity, must, null, object, referencequeue, suppresswarnings, weakidentitymap, weakreference

The WeakIdentityMap.java Java example source code

/*
 * Copyright (c) 2013, 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 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();

    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) 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[] 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 {
        private final int hash;
        private volatile T value;
        private volatile Entry<T> next;

        Entry(Object key, int hash, T value, ReferenceQueue<Object> queue, Entry 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());
        }
    }
}




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