1 /* 2 * Copyright (c) 1995, 2004, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 26 package sun.misc; 27 import java.lang.ref.SoftReference; 28 29 30 /** 31 * A "Ref" is an indirect reference to an object that the garbage collector 32 * knows about. An application should override the reconstitute() method with one 33 * that will construct the object based on information in the Ref, often by 34 * reading from a file. The get() method retains a cache of the result of the last call to 35 * reconstitute() in the Ref. When space gets tight, the garbage collector 36 * will clear old Ref cache entries when there are no other pointers to the 37 * object. In normal usage, Ref will always be subclassed. The subclass will add the 38 * instance variables necessary for the reconstitute() method to work. It will also add a 39 * constructor to set them up, and write a version of reconstitute(). 40 * 41 * @deprecated This class has been replaced by 42 * <code>java.util.SoftReference</code>. 43 * 44 * @see java.util.SoftReference 45 * 46 */ 47 @Deprecated 48 49 public abstract class Ref { 50 51 private SoftReference soft = null; 52 53 /** 54 * Returns a pointer to the object referenced by this Ref. If the object 55 * has been thrown away by the garbage collector, it will be 56 * reconstituted. This method does everything necessary to ensure that the garbage 57 * collector throws things away in Least Recently Used(LRU) order. Applications should 58 * never override this method. The get() method effectively caches calls to 59 * reconstitute(). 60 */ 61 public synchronized Object get() { 62 Object t = check(); 63 if (t == null) { 64 t = reconstitute(); 65 setThing(t); 66 } 67 return t; 68 } 69 70 /** 71 * Returns a pointer to the object referenced by this Ref by 72 * reconstituting it from some external source (such as a file). This method should not 73 * bother with caching since the method get() will deal with that. 74 * <p> 75 * In normal usage, Ref will always be subclassed. The subclass will add 76 * the instance variables necessary for reconstitute() to work. It will 77 * also add a constructor to set them up, and write a version of 78 * reconstitute(). 79 */ 80 public abstract Object reconstitute(); 81 82 /** 83 * Flushes the cached object. Forces the next invocation of get() to 84 * invoke reconstitute(). 85 */ 86 public synchronized void flush() { 87 SoftReference s = soft; 88 if (s != null) s.clear(); 89 soft = null; 90 } 91 92 /** 93 * Sets the thing to the specified object. 94 * @param thing the specified object 95 */ 96 public synchronized void setThing(Object thing) { 97 flush(); 98 soft = new SoftReference(thing); 99 } 100 101 /** 102 * Checks to see what object is being pointed at by this Ref and returns it. 103 */ 104 public synchronized Object check() { 105 SoftReference s = soft; 106 if (s == null) return null; 107 return s.get(); 108 } 109 110 /** 111 * Constructs a new Ref. 112 */ 113 public Ref() { } 114 115 /** 116 * Constructs a new Ref that initially points to thing. 117 */ 118 public Ref(Object thing) { 119 setThing(thing); 120 } 121 122 }