1 /* 2 * Copyright (C) 2014 The Android Open Source Project 3 * Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved. 4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 5 * 6 * This code is free software; you can redistribute it and/or modify it 7 * under the terms of the GNU General Public License version 2 only, as 8 * published by the Free Software Foundation. Oracle designates this 9 * particular file as subject to the "Classpath" exception as provided 10 * by Oracle in the LICENSE file that accompanied this code. 11 * 12 * This code is distributed in the hope that it will be useful, but WITHOUT 13 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 14 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 15 * version 2 for more details (a copy is included in the LICENSE file that 16 * accompanied this code). 17 * 18 * You should have received a copy of the GNU General Public License version 19 * 2 along with this work; if not, write to the Free Software Foundation, 20 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 21 * 22 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 23 * or visit www.oracle.com if you need additional information or have any 24 * questions. 25 */ 26 27 package java.lang.ref; 28 29 import dalvik.annotation.optimization.FastNative; 30 31 32 /** 33 * Abstract base class for reference objects. This class defines the 34 * operations common to all reference objects. Because reference objects are 35 * implemented in close cooperation with the garbage collector, this class may 36 * not be subclassed directly. 37 * 38 * @author Mark Reinhold 39 * @since 1.2 40 */ 41 42 public abstract class Reference<T> { 43 // BEGIN Android-changed: Reimplemented to accommodate a different GC and compiler. 44 // ClassLinker knows about the fields of this class. 45 46 /** 47 * Forces JNI path. 48 * If GC is not in progress (ie: not going through slow path), the referent 49 * can be quickly returned through intrinsic without passing through JNI. 50 * This flag forces the JNI path so that it can be tested and benchmarked. 51 */ 52 private static boolean disableIntrinsic = false; 53 54 /** 55 * Slow path flag for the reference processor. 56 * Used by the reference processor to determine whether or not the referent 57 * can be immediately returned. Because the referent might get swept during 58 * GC, the slow path, which passes through JNI, must be taken. 59 */ 60 private static boolean slowPathEnabled = false; 61 62 // Treated specially by GC. ART's ClassLinker::LinkFields() knows this is the 63 // alphabetically last non-static field. 64 volatile T referent; 65 66 final ReferenceQueue<? super T> queue; 67 68 /* 69 * This field forms a singly-linked list of reference objects that have 70 * been enqueued. The queueNext field is non-null if and only if this 71 * reference has been enqueued. After this reference has been enqueued and 72 * before it has been removed from its queue, the queueNext field points 73 * to the next reference on the queue. The last reference on a queue 74 * points to itself. Once this reference has been removed from the 75 * reference queue, the queueNext field points to the 76 * ReferenceQueue.sQueueNextUnenqueued sentinel reference object for the 77 * rest of this reference's lifetime. 78 * <p> 79 * Access to the queueNext field is guarded by synchronization on a lock 80 * internal to 'queue'. 81 */ 82 Reference queueNext; 83 84 /** 85 * The pendingNext field is initially set by the GC. After the GC forms a 86 * complete circularly linked list, the list is handed off to the 87 * ReferenceQueueDaemon using the ReferenceQueue.class lock. The 88 * ReferenceQueueDaemon can then read the pendingNext fields without 89 * additional synchronization. 90 */ 91 Reference<?> pendingNext; 92 93 /* -- Referent accessor and setters -- */ 94 95 /** 96 * Returns this reference object's referent. If this reference object has 97 * been cleared, either by the program or by the garbage collector, then 98 * this method returns <code>null</code>. 99 * 100 * @return The object to which this reference refers, or 101 * <code>null</code> if this reference object has been cleared 102 */ get()103 public T get() { 104 return getReferent(); 105 } 106 107 @FastNative getReferent()108 private final native T getReferent(); 109 110 /** 111 * Clears this reference object. Invoking this method will not cause this 112 * object to be enqueued. 113 * 114 * <p> This method is invoked only by Java code; when the garbage collector 115 * clears references it does so directly, without invoking this method. 116 */ clear()117 public void clear() { 118 clearReferent(); 119 } 120 121 // Direct access to the referent is prohibited, clearReferent blocks and set 122 // the referent to null when it is safe to do so. 123 @FastNative clearReferent()124 native void clearReferent(); 125 126 /* -- Queue operations -- */ 127 128 /** 129 * Tells whether or not this reference object has been enqueued, either by 130 * the program or by the garbage collector. If this reference object was 131 * not registered with a queue when it was created, then this method will 132 * always return <code>false</code>. 133 * 134 * @return <code>true</code> if and only if this reference object has 135 * been enqueued 136 */ isEnqueued()137 public boolean isEnqueued() { 138 // Contrary to what the documentation says, this method returns false 139 // after this reference object has been removed from its queue 140 // (b/26647823). ReferenceQueue.isEnqueued preserves this historically 141 // incorrect behavior. 142 return queue != null && queue.isEnqueued(this); 143 } 144 145 /** 146 * Adds this reference object to the queue with which it is registered, 147 * if any. 148 * 149 * <p> This method is invoked only by Java code; when the garbage collector 150 * enqueues references it does so directly, without invoking this method. 151 * 152 * @return <code>true</code> if this reference object was successfully 153 * enqueued; <code>false</code> if it was already enqueued or if 154 * it was not registered with a queue when it was created 155 */ enqueue()156 public boolean enqueue() { 157 return queue != null && queue.enqueue(this); 158 } 159 160 /* -- Constructors -- */ 161 Reference(T referent)162 Reference(T referent) { 163 this(referent, null); 164 } 165 Reference(T referent, ReferenceQueue<? super T> queue)166 Reference(T referent, ReferenceQueue<? super T> queue) { 167 this.referent = referent; 168 this.queue = queue; 169 } 170 // END Android-changed: Reimplemented to accommodate a different GC and compiler. 171 172 // BEGIN Android-added: reachabilityFence() from upstream OpenJDK9+181. 173 // The actual implementation differs from OpenJDK9. 174 /** 175 * Ensures that the object referenced by the given reference remains 176 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>, 177 * regardless of any prior actions of the program that might otherwise cause 178 * the object to become unreachable; thus, the referenced object is not 179 * reclaimable by garbage collection at least until after the invocation of 180 * this method. Invocation of this method does not itself initiate garbage 181 * collection or finalization. 182 * 183 * <p> This method establishes an ordering for 184 * <a href="package-summary.html#reachability"><em>strong reachability</em></a> 185 * with respect to garbage collection. It controls relations that are 186 * otherwise only implicit in a program -- the reachability conditions 187 * triggering garbage collection. This method is designed for use in 188 * uncommon situations of premature finalization where using 189 * {@code synchronized} blocks or methods, or using other synchronization 190 * facilities are not possible or do not provide the desired control. This 191 * method is applicable only when reclamation may have visible effects, 192 * which is possible for objects with finalizers (See 193 * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6"> 194 * Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>) 195 * that are implemented in ways that rely on ordering control for correctness. 196 * 197 * @apiNote 198 * Finalization may occur whenever the virtual machine detects that no 199 * reference to an object will ever be stored in the heap: The garbage 200 * collector may reclaim an object even if the fields of that object are 201 * still in use, so long as the object has otherwise become unreachable. 202 * This may have surprising and undesirable effects in cases such as the 203 * following example in which the bookkeeping associated with a class is 204 * managed through array indices. Here, method {@code action} uses a 205 * {@code reachabilityFence} to ensure that the {@code Resource} object is 206 * not reclaimed before bookkeeping on an associated 207 * {@code ExternalResource} has been performed; in particular here, to 208 * ensure that the array slot holding the {@code ExternalResource} is not 209 * nulled out in method {@link Object#finalize}, which may otherwise run 210 * concurrently. 211 * 212 * <pre> {@code 213 * class Resource { 214 * private static ExternalResource[] externalResourceArray = ... 215 * 216 * int myIndex; 217 * Resource(...) { 218 * myIndex = ... 219 * externalResourceArray[myIndex] = ...; 220 * ... 221 * } 222 * protected void finalize() { 223 * externalResourceArray[myIndex] = null; 224 * ... 225 * } 226 * public void action() { 227 * try { 228 * // ... 229 * int i = myIndex; 230 * Resource.update(externalResourceArray[i]); 231 * } finally { 232 * Reference.reachabilityFence(this); 233 * } 234 * } 235 * private static void update(ExternalResource ext) { 236 * ext.status = ...; 237 * } 238 * }}</pre> 239 * 240 * Here, the invocation of {@code reachabilityFence} is nonintuitively 241 * placed <em>after</em> the call to {@code update}, to ensure that the 242 * array slot is not nulled out by {@link Object#finalize} before the 243 * update, even if the call to {@code action} was the last use of this 244 * object. This might be the case if, for example a usage in a user program 245 * had the form {@code new Resource().action();} which retains no other 246 * reference to this {@code Resource}. While probably overkill here, 247 * {@code reachabilityFence} is placed in a {@code finally} block to ensure 248 * that it is invoked across all paths in the method. In a method with more 249 * complex control paths, you might need further precautions to ensure that 250 * {@code reachabilityFence} is encountered along all of them. 251 * 252 * <p> It is sometimes possible to better encapsulate use of 253 * {@code reachabilityFence}. Continuing the above example, if it were 254 * acceptable for the call to method {@code update} to proceed even if the 255 * finalizer had already executed (nulling out slot), then you could 256 * localize use of {@code reachabilityFence}: 257 * 258 * <pre> {@code 259 * public void action2() { 260 * // ... 261 * Resource.update(getExternalResource()); 262 * } 263 * private ExternalResource getExternalResource() { 264 * ExternalResource ext = externalResourceArray[myIndex]; 265 * Reference.reachabilityFence(this); 266 * return ext; 267 * }}</pre> 268 * 269 * <p> Method {@code reachabilityFence} is not required in constructions 270 * that themselves ensure reachability. For example, because objects that 271 * are locked cannot, in general, be reclaimed, it would suffice if all 272 * accesses of the object, in all methods of class {@code Resource} 273 * (including {@code finalize}) were enclosed in {@code synchronized (this)} 274 * blocks. (Further, such blocks must not include infinite loops, or 275 * themselves be unreachable, which fall into the corner case exceptions to 276 * the "in general" disclaimer.) However, method {@code reachabilityFence} 277 * remains a better option in cases where this approach is not as efficient, 278 * desirable, or possible; for example because it would encounter deadlock. 279 * 280 * @param ref the reference. If {@code null}, this method has no effect. 281 * @since 9 282 */ 283 // @DontInline reachabilityFence(Object ref)284 public static void reachabilityFence(Object ref) { 285 // This code is usually replaced by much faster intrinsic implementations. 286 // It will be executed for tests run with the access checks interpreter in 287 // ART, e.g. with --verify-soft-fail. Since this is a volatile store, it 288 // cannot easily be moved up past prior accesses, even if this method is 289 // inlined. 290 SinkHolder.sink = ref; 291 // Leaving SinkHolder set to ref is unpleasant, since it keeps ref live 292 // until the next reachabilityFence call. This causes e.g. 036-finalizer 293 // to fail. Clear it again in a way that's unlikely to be optimizable. 294 // The fact that finalize_count is volatile makes it hard to move the test up. 295 if (SinkHolder.finalize_count == 0) { 296 SinkHolder.sink = null; 297 } 298 } 299 300 private static class SinkHolder { 301 static volatile Object sink; 302 303 // Ensure that sink looks live to even a reasonably clever compiler. 304 private static volatile int finalize_count = 0; 305 306 private static Object sinkUser = new Object() { 307 protected void finalize() { 308 if (sink == null && finalize_count > 0) { 309 throw new AssertionError("Can't get here"); 310 } 311 finalize_count++; 312 } 313 }; 314 } 315 // END Android-added: reachabilityFence() from upstream OpenJDK9+181. 316 } 317