1: /* java.lang.ref.Reference 2: Copyright (C) 1999, 2002, 2003, 2004, 2006 Free Software Foundation, Inc. 3: 4: This file is part of GNU Classpath. 5: 6: GNU Classpath is free software; you can redistribute it and/or modify 7: it under the terms of the GNU General Public License as published by 8: the Free Software Foundation; either version 2, or (at your option) 9: any later version. 10: 11: GNU Classpath is distributed in the hope that it will be useful, but 12: WITHOUT ANY WARRANTY; without even the implied warranty of 13: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 14: General Public License for more details. 15: 16: You should have received a copy of the GNU General Public License 17: along with GNU Classpath; see the file COPYING. If not, write to the 18: Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 19: 02110-1301 USA. 20: 21: Linking this library statically or dynamically with other modules is 22: making a combined work based on this library. Thus, the terms and 23: conditions of the GNU General Public License cover the whole 24: combination. 25: 26: As a special exception, the copyright holders of this library give you 27: permission to link this library with independent modules to produce an 28: executable, regardless of the license terms of these independent 29: modules, and to copy and distribute the resulting executable under 30: terms of your choice, provided that you also meet, for each linked 31: independent module, the terms and conditions of the license of that 32: module. An independent module is a module which is not derived from 33: or based on this library. If you modify this library, you may extend 34: this exception to your version of the library, but you are not 35: obligated to do so. If you do not wish to do so, delete this 36: exception statement from your version. */ 37: 38: 39: package java.lang.ref; 40: 41: /** 42: * This is the base class of all references. A reference allows 43: * refering to an object without preventing the garbage collector to 44: * collect it. The only way to get the referred object is via the 45: * <code>get()</code>-method. This method will return 46: * <code>null</code> if the object was collected. <br> 47: * 48: * A reference may be registered with a queue. When a referred 49: * element gets collected the reference will be put on the queue, so 50: * that you will be notified. <br> 51: * 52: * There are currently three types of references: soft reference, 53: * weak reference and phantom reference. <br> 54: * 55: * Soft references will be cleared if the garbage collector is told 56: * to free some memory and there are no unreferenced or weakly referenced 57: * objects. It is useful for caches. <br> 58: * 59: * Weak references will be cleared as soon as the garbage collector 60: * determines that the refered object is only weakly reachable. They 61: * are useful as keys in hashtables (see <code>WeakHashtable</code>) as 62: * you get notified when nobody has the key anymore. 63: * 64: * Phantom references don't prevent finalization. If an object is only 65: * phantom reachable, it will be finalized, and the reference will be 66: * enqueued, but not cleared. Since you mustn't access an finalized 67: * object, the <code>get</code> method of a phantom reference will never 68: * work. It is useful to keep track, when an object is finalized. 69: * 70: * @author Jochen Hoenicke 71: * @see java.util.WeakHashMap 72: */ 73: public abstract class Reference<T> 74: { 75: /** 76: * The underlying object. This field is handled in a special way by 77: * the garbage collector. 78: */ 79: T referent; 80: 81: /** 82: * The queue this reference is registered on. This is null, if this 83: * wasn't registered to any queue or reference was already enqueued. 84: */ 85: volatile ReferenceQueue<? super T> queue; 86: 87: /** 88: * Link to the next entry on the queue. If this is null, this 89: * reference is not enqueued. Otherwise it points to the next 90: * reference. The last reference on a queue will point to itself 91: * (not to null, that value is used to mark a not enqueued 92: * reference). 93: */ 94: volatile Reference nextOnQueue; 95: 96: /** 97: * This lock should be taken by the garbage collector, before 98: * determining reachability. It will prevent the get()-method to 99: * return the reference so that reachability doesn't change. 100: */ 101: static Object lock = new Object(); 102: 103: /** 104: * Creates a new reference that is not registered to any queue. 105: * Since it is package private, it is not possible to overload this 106: * class in a different package. 107: * @param ref the object we refer to. 108: */ 109: Reference(T ref) 110: { 111: referent = ref; 112: } 113: 114: /** 115: * Creates a reference that is registered to a queue. Since this is 116: * package private, it is not possible to overload this class in a 117: * different package. 118: * @param ref the object we refer to. 119: * @param q the reference queue to register on. 120: * @exception NullPointerException if q is null. 121: */ 122: Reference(T ref, ReferenceQueue<? super T> q) 123: { 124: if (q == null) 125: throw new NullPointerException(); 126: referent = ref; 127: queue = q; 128: } 129: 130: /** 131: * Returns the object, this reference refers to. 132: * @return the object, this reference refers to, or null if the 133: * reference was cleared. 134: */ 135: public T get() 136: { 137: synchronized (lock) 138: { 139: return referent; 140: } 141: } 142: 143: /** 144: * Clears the reference, so that it doesn't refer to its object 145: * anymore. For soft and weak references this is called by the 146: * garbage collector. For phantom references you should call 147: * this when enqueuing the reference. 148: */ 149: public void clear() 150: { 151: referent = null; 152: } 153: 154: /** 155: * Tells if the object is enqueued on a reference queue. 156: * @return true if it is enqueued, false otherwise. 157: */ 158: public boolean isEnqueued() 159: { 160: return nextOnQueue != null; 161: } 162: 163: /** 164: * Enqueue an object on a reference queue. This is normally executed 165: * by the garbage collector. 166: */ 167: public boolean enqueue() 168: { 169: ReferenceQueue q = queue; 170: if (q != null) 171: { 172: return q.enqueue(this); 173: } 174: return false; 175: } 176: }