1 /* 2 * Copyright (c) 2015, 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. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 */ 23 package jdk.vm.ci.hotspot; 24 25 import java.lang.ref.Reference; 26 import java.lang.ref.ReferenceQueue; 27 import java.lang.ref.WeakReference; 28 import java.util.Arrays; 29 import java.util.Iterator; 30 import java.util.Map; 31 import java.util.WeakHashMap; 32 33 import jdk.vm.ci.meta.JavaKind; 34 import jdk.vm.ci.meta.ResolvedJavaType; 35 36 /** 37 * This class manages the set of metadata roots that must be scanned during garbage collection. 38 * Because of class redefinition Method* and ConstantPool* can be freed if they don't appear to be 39 * in use so they must be tracked when there are live references to them from Java. 40 * 41 * The general theory of operation is that all {@link MetaspaceWrapperObject}s are created by 42 * calling into the VM which calls back out to actually create the wrapper instance. During the call 43 * the VM keeps the metadata reference alive through the use of metadata handles. Once the call 44 * completes the wrapper object is registered here and will be scanned during metadata scanning. The 45 * weakness of the reference to the wrapper object allows them to be reclaimed when they are no 46 * longer used. 47 * 48 */ 49 public class HotSpotJVMCIMetaAccessContext { 50 51 /** 52 * The set of currently live contexts used for tracking of live metadata. Examined from the VM 53 * during garbage collection. 54 */ 55 private static WeakReference<?>[] allContexts = new WeakReference<?>[0]; 56 57 /** 58 * This is a chunked list of metadata roots. It can be read from VM native code so it's been 59 * marked volatile to ensure the order of updates are respected. 60 */ 61 private volatile Object[] metadataRoots; 62 63 private ChunkedList<WeakReference<MetaspaceWrapperObject>> list = new ChunkedList<>(); 64 65 /** 66 * The number of weak references freed since the last time the list was shrunk. 67 */ 68 private int freed; 69 70 /** 71 * The {@link ReferenceQueue} tracking the weak references created by this context. 72 */ 73 private final ReferenceQueue<MetaspaceWrapperObject> queue = new ReferenceQueue<>(); 74 75 static synchronized void add(HotSpotJVMCIMetaAccessContext context) { 76 for (int i = 0; i < allContexts.length; i++) { 77 if (allContexts[i] == null || allContexts[i].get() == null) { 78 allContexts[i] = new WeakReference<>(context); 79 return; 80 } 81 } 82 int index = allContexts.length; 83 allContexts = Arrays.copyOf(allContexts, index + 2); 84 allContexts[index] = new WeakReference<>(context); 85 } 86 87 HotSpotJVMCIMetaAccessContext() { 88 add(this); 89 } 90 91 /** 92 * Periodically trim the list of tracked metadata. A new list is created to replace the old to 93 * avoid concurrent scanning issues. 94 */ 95 private void clean() { 96 Reference<?> ref = queue.poll(); 97 if (ref == null) { 98 return; 99 } 100 while (ref != null) { 101 freed++; 102 ref = queue.poll(); 103 } 104 if (freed > list.size() / 2) { 105 ChunkedList<WeakReference<MetaspaceWrapperObject>> newList = new ChunkedList<>(); 106 for (WeakReference<MetaspaceWrapperObject> element : list) { 107 /* 108 * The referent could become null anywhere in here but it doesn't matter. It will 109 * get cleaned up next time. 110 */ 111 if (element != null && element.get() != null) { 112 newList.add(element); 113 } 114 } 115 list = newList; 116 metadataRoots = list.getHead(); 117 freed = 0; 118 } 119 } 120 121 /** 122 * Add a {@link MetaspaceWrapperObject} to tracked by the GC. It's assumed that the caller is 123 * responsible for keeping the reference alive for the duration of the call. Once registration 124 * is complete then the VM will ensure it's kept alive. 125 * 126 * @param metaspaceObject 127 */ 128 129 public synchronized void add(MetaspaceWrapperObject metaspaceObject) { 130 clean(); 131 list.add(new WeakReference<>(metaspaceObject, queue)); 132 if (list.getHead() != metadataRoots) { 133 /* 134 * The list enlarged so update the head. 135 */ 136 metadataRoots = list.getHead(); 137 } 138 assert isRegistered(metaspaceObject); 139 } 140 141 protected ResolvedJavaType createClass(Class<?> javaClass) { 142 if (javaClass.isPrimitive()) { 143 JavaKind kind = JavaKind.fromJavaClass(javaClass); 144 return new HotSpotResolvedPrimitiveType(kind); 145 } else { 146 return new HotSpotResolvedObjectTypeImpl(javaClass, this); 147 } 148 } 149 150 private final Map<Class<?>, WeakReference<ResolvedJavaType>> typeMap = new WeakHashMap<>(); 151 152 /** 153 * Gets the JVMCI mirror for a {@link Class} object. 154 * 155 * @return the {@link ResolvedJavaType} corresponding to {@code javaClass} 156 */ 157 public synchronized ResolvedJavaType fromClass(Class<?> javaClass) { 158 WeakReference<ResolvedJavaType> typeRef = typeMap.get(javaClass); 159 ResolvedJavaType type = typeRef != null ? typeRef.get() : null; 160 if (type == null) { 161 type = createClass(javaClass); 162 typeMap.put(javaClass, new WeakReference<>(type)); 163 } 164 return type; 165 } 166 167 /** 168 * A very simple append only chunked list implementation. 169 */ 170 static class ChunkedList<T> implements Iterable<T> { 171 private static final int CHUNK_SIZE = 32; 172 173 private static final int NEXT_CHUNK_INDEX = CHUNK_SIZE - 1; 174 175 private Object[] head; 176 private int index; 177 private int size; 178 179 ChunkedList() { 180 head = new Object[CHUNK_SIZE]; 181 index = 0; 182 } 183 184 void add(T element) { 185 if (index == NEXT_CHUNK_INDEX) { 186 Object[] newHead = new Object[CHUNK_SIZE]; 187 newHead[index] = head; 188 head = newHead; 189 index = 0; 190 } 191 head[index++] = element; 192 size++; 193 } 194 195 Object[] getHead() { 196 return head; 197 } 198 199 public Iterator<T> iterator() { 200 return new ChunkIterator<>(); 201 } 202 203 int size() { 204 return size; 205 } 206 207 class ChunkIterator<V> implements Iterator<V> { 208 209 ChunkIterator() { 210 currentChunk = head; 211 currentIndex = -1; 212 next = findNext(); 213 } 214 215 Object[] currentChunk; 216 int currentIndex; 217 V next; 218 219 @SuppressWarnings("unchecked") 220 V findNext() { 221 V result; 222 do { 223 currentIndex++; 224 if (currentIndex == NEXT_CHUNK_INDEX) { 225 currentChunk = (Object[]) currentChunk[currentIndex]; 226 currentIndex = 0; 227 if (currentChunk == null) { 228 return null; 229 } 230 } 231 result = (V) currentChunk[currentIndex]; 232 } while (result == null); 233 return result; 234 } 235 236 public boolean hasNext() { 237 return next != null; 238 } 239 240 public V next() { 241 V result = next; 242 next = findNext(); 243 return result; 244 } 245 246 } 247 248 } 249 250 synchronized boolean isRegistered(MetaspaceWrapperObject wrapper) { 251 for (WeakReference<MetaspaceWrapperObject> m : list) { 252 if (m != null && m.get() == wrapper) { 253 return true; 254 } 255 } 256 return false; 257 } 258 }