Source Code Cross Referenced for WeakCache.java in  » 6.0-JDK-Modules » j2me » sun » misc » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » 6.0 JDK Modules » j2me » sun.misc 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        /*
002:         * @(#)WeakCache.java	1.14 06/10/10
003:         *
004:         * Copyright  1990-2006 Sun Microsystems, Inc. All Rights Reserved.
005:         * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER
006:         * 
007:         * This program is free software; you can redistribute it and/or
008:         * modify it under the terms of the GNU General Public License version
009:         * 2 only, as published by the Free Software Foundation. 
010:         * 
011:         * This program is distributed in the hope that it will be useful, but
012:         * WITHOUT ANY WARRANTY; without even the implied warranty of
013:         * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
014:         * General Public License version 2 for more details (a copy is
015:         * included at /legal/license.txt). 
016:         * 
017:         * You should have received a copy of the GNU General Public License
018:         * version 2 along with this work; if not, write to the Free Software
019:         * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
020:         * 02110-1301 USA 
021:         * 
022:         * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa
023:         * Clara, CA 95054 or visit www.sun.com if you need additional
024:         * information or have any questions. 
025:         *
026:         */
027:
028:        package sun.misc;
029:
030:        /**
031:         * This is a hash table that uses a sun.misc.Ref to store its values.
032:         * With each operation, this hash table will put a little
033:         * bit of work into checking for ref's that have been nulled out by the
034:         * GC.  When it detects that this has happened, it removes the corresponding
035:         * key from the hashtable.  This prevents the hashtable from leaking keys.
036:         */
037:
038:        public class WeakCache extends java.util.Hashtable {
039:            private java.util.Vector keyVector;
040:            private int cleanupIndex = 0;
041:
042:            //
043:            // We use a weak ref to store the value.  sun.misc.Ref is abstract,
044:            // and we need to override reconstitute.  We don't want it to actually
045:            // do anything, though -- if a value disappears from the cache, it just
046:            // goes away.
047:            //
048:            private static class Value extends Ref {
049:                public Value(Object t) {
050:                    super ();
051:                    setThing(t);
052:                }
053:
054:                public Object reconstitute() {
055:                    return null;
056:                }
057:            }
058:
059:            public WeakCache() {
060:                super ();
061:                keyVector = new java.util.Vector();
062:            }
063:
064:            public WeakCache(int initialCapacity) {
065:                super (initialCapacity);
066:                keyVector = new java.util.Vector(initialCapacity);
067:            }
068:
069:            public synchronized Object get(Object key) {
070:                Object returnVal = super .get(key);
071:                doSomeCleanup(1);
072:                if (returnVal != null) {
073:                    return ((Value) returnVal).get();
074:                } else {
075:                    return null;
076:                }
077:            }
078:
079:            public synchronized Object put(Object key, Object value) {
080:                if (!(value instanceof  Value)) {
081:                    value = new Value(value);
082:                }
083:                Object returnVal = super .put(key, value);
084:                if (returnVal == null) {
085:                    keyVector.addElement(key);
086:                }
087:                doSomeCleanup(2);
088:                return returnVal;
089:            }
090:
091:            public synchronized Object remove(Object key) {
092:                Object returnVal = super .remove(key);
093:                keyVector.removeElement(key);
094:                doSomeCleanup(2);
095:                return returnVal;
096:            }
097:
098:            protected synchronized void doSomeCleanup(int amountOfCleanup) {
099:                if (keyVector.size() == 0)
100:                    return;
101:                for (; amountOfCleanup != 0; amountOfCleanup--) {
102:                    // In the case where the cleanupIndex is at the end of the
103:                    // Vector and then an element is removed, the cleanupIndex
104:                    // will be out of bounds.  So, before the cleanupIndex is
105:                    // used, make sure it is in bounds.
106:                    if (cleanupIndex >= keyVector.size()) {
107:                        cleanupIndex = 0;
108:                        break;
109:                    }
110:                    Object key = keyVector.elementAt(cleanupIndex);
111:                    // If the value of this key in the hashtable has been
112:                    // garbage collected, remove the key.
113:                    Object v = super .get(key);
114:                    if (v instanceof  sun.misc.Ref
115:                            && ((sun.misc.Ref) v).check() == null) {
116:                        super .remove(key);
117:                        keyVector.removeElement(key);
118:                        if (keyVector.size() == 0) {
119:                            cleanupIndex = 0;
120:                            return;
121:                        }
122:                    } else {
123:                        // Otherwise, move the pointer to the next
124:                        // element in the vector
125:                        cleanupIndex++;
126:                    }
127:                    // Finally, modulate the index to loop back around to
128:                    // the start of the vector once you hit the end.
129:                    cleanupIndex = cleanupIndex % keyVector.size();
130:                }
131:            }
132:        }
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