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.JVMCIMetaAccessContext;
34 import jdk.vm.ci.meta.JavaKind;
35 import jdk.vm.ci.meta.ResolvedJavaType;
36
37 /**
38 * This class manages the set of metadata roots that must be scanned during garbage collection.
39 * Because of class redefinition Method* and ConstantPool* can be freed if they don't appear to be
40 * in use so they must be tracked when there are live references to them from Java.
41 *
42 * The general theory of operation is that all {@link MetaspaceWrapperObject}s are created by
43 * calling into the VM which calls back out to actually create the wrapper instance. During the call
44 * the VM keeps the metadata reference alive through the use of metadata handles. Once the call
45 * completes the wrapper object is registered here and will be scanned during metadata scanning. The
46 * weakness of the reference to the wrapper object allows them to be reclaimed when they are no
47 * longer used.
48 *
49 */
50 public class HotSpotJVMCIMetaAccessContext implements JVMCIMetaAccessContext {
51
52 /**
53 * The set of currently live contexts used for tracking of live metadata. Examined from the VM
54 * during garbage collection.
55 */
56 private static WeakReference<?>[] allContexts = new WeakReference<?>[0];
57
58 /**
59 * This is a chunked list of metadata roots. It can be read from VM native code so it's been
60 * marked volatile to ensure the order of updates are respected.
61 */
62 private volatile Object[] metadataRoots;
63
64 private ChunkedList<WeakReference<MetaspaceWrapperObject>> list = new ChunkedList<>();
65
66 /**
67 * The number of weak references freed since the last time the list was shrunk.
68 */
69 private int freed;
70
71 /**
72 * The {@link ReferenceQueue} tracking the weak references created by this context.
73 */
74 private final ReferenceQueue<MetaspaceWrapperObject> queue = new ReferenceQueue<>();
75
76 static synchronized void add(HotSpotJVMCIMetaAccessContext context) {
77 for (int i = 0; i < allContexts.length; i++) {
78 if (allContexts[i] == null || allContexts[i].get() == null) {
79 allContexts[i] = new WeakReference<>(context);
80 return;
81 }
82 }
83 int index = allContexts.length;
84 allContexts = Arrays.copyOf(allContexts, index + 2);
85 allContexts[index] = new WeakReference<>(context);
86 }
87
88 HotSpotJVMCIMetaAccessContext() {
89 add(this);
90 }
91
92 /**
93 * Periodically trim the list of tracked metadata. A new list is created to replace the old to
94 * avoid concurrent scanning issues.
95 */
96 private void clean() {
97 Reference<?> ref = queue.poll();
98 if (ref == null) {
99 return;
100 }
101 while (ref != null) {
102 freed++;
103 ref = queue.poll();
104 }
105 if (freed > list.size() / 2) {
106 ChunkedList<WeakReference<MetaspaceWrapperObject>> newList = new ChunkedList<>();
107 for (WeakReference<MetaspaceWrapperObject> element : list) {
108 /*
109 * The referent could become null anywhere in here but it doesn't matter. It will
110 * get cleaned up next time.
111 */
112 if (element != null && element.get() != null) {
113 newList.add(element);
114 }
115 }
116 list = newList;
117 metadataRoots = list.getHead();
118 freed = 0;
119 }
120 }
121
122 /**
123 * Add a {@link MetaspaceWrapperObject} to tracked by the GC. It's assumed that the caller is
124 * responsible for keeping the reference alive for the duration of the call. Once registration
125 * is complete then the VM will ensure it's kept alive.
126 *
127 * @param metaspaceObject
128 */
129
130 public synchronized void add(MetaspaceWrapperObject metaspaceObject) {
131 clean();
132 list.add(new WeakReference<>(metaspaceObject, queue));
133 if (list.getHead() != metadataRoots) {
134 /*
135 * The list enlarged so update the head.
136 */
137 metadataRoots = list.getHead();
138 }
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 @Override
153 public synchronized ResolvedJavaType fromClass(Class<?> javaClass) {
154 WeakReference<ResolvedJavaType> typeRef = typeMap.get(javaClass);
155 ResolvedJavaType type = typeRef != null ? typeRef.get() : null;
156 if (type == null) {
157 type = createClass(javaClass);
158 typeMap.put(javaClass, new WeakReference<>(type));
159 }
160 return type;
161 }
162
163 /**
164 * A very simple append only chunked list implementation.
165 */
166 static class ChunkedList<T> implements Iterable<T> {
167 private static final int CHUNK_SIZE = 32;
168
169 private static final int NEXT_CHUNK_INDEX = CHUNK_SIZE - 1;
170
171 private Object[] head;
172 private int index;
173 private int size;
174
175 ChunkedList() {
176 head = new Object[CHUNK_SIZE];
177 index = 0;
178 }
179
180 void add(T element) {
181 if (index == NEXT_CHUNK_INDEX) {
182 Object[] newHead = new Object[CHUNK_SIZE];
183 newHead[index] = head;
184 head = newHead;
185 index = 0;
186 }
187 head[index++] = element;
188 size++;
189 }
190
191 Object[] getHead() {
192 return head;
193 }
194
195 public Iterator<T> iterator() {
196 return new ChunkIterator<>();
197 }
198
199 int size() {
200 return size;
201 }
202
203 class ChunkIterator<V> implements Iterator<V> {
204
205 ChunkIterator() {
206 currentChunk = head;
207 currentIndex = -1;
208 findNext();
209 }
210
211 Object[] currentChunk;
212 int currentIndex;
213 V next;
214
215 @SuppressWarnings("unchecked")
216 V findNext() {
217 V result;
218 do {
219 currentIndex++;
220 if (currentIndex == NEXT_CHUNK_INDEX) {
221 currentChunk = (Object[]) currentChunk[currentIndex];
222 currentIndex = 0;
223 if (currentChunk == null) {
224 return null;
225 }
226 }
227 result = (V) currentChunk[currentIndex];
228 } while (result == null);
229 return result;
230 }
231
232 public boolean hasNext() {
233 return next != null;
234 }
235
236 public V next() {
237 V result = next;
238 next = findNext();
239 return result;
240 }
241
242 }
243
244 }
245 }