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/*
* Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package javac.internal.jimage;
import java.lang.ref.WeakReference;
import java.nio.ByteBuffer;
import java.util.AbstractMap;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Map;
/**
* @implNote This class needs to maintain JDK 8 source compatibility.
*
* It is used internally in the JDK to implement jimage/jrtfs access,
* but also compiled and delivered as part of the jrt-fs.jar to support access
* to the jimage file provided by the shipped JDK by tools running on JDK 8.
*/
class ImageBufferCache {
private static final int MAX_CACHED_BUFFERS = 3;
private static final int LARGE_BUFFER = 0x10000;
/*
* We used to have a class BufferReference extending from WeakReference.
* BufferReference class had an instance field called "capacity". This field was
* used to make DECREASING_CAPACITY_NULLS_LAST comparator stable in the presence
* of GC clearing the WeakReference concurrently.
*
* But this scheme results in metaspace leak. The thread local is alive till the
* the thread is alive. And so ImageBufferCache$BufferReference class was kept alive.
* Because this class and ImageBufferCache$BufferReference are all loaded by a URL
* class loader from jrt-fs.jar, the class loader and so all the classes loaded by it
* were alive!
*
* Solution is to avoid using a URL loader loaded class type with thread local. All we
* need is a pair of WeakReference, Integer (saved capacity for stability
* of comparator). We use Map.Entry as pair implementation. With this, all types used
* with thread local are bootstrap types and so no metaspace leak.
*/
@SuppressWarnings("unchecked")
private static final ThreadLocal, Integer>[]> CACHE =
new ThreadLocal, Integer>[]>() {
@Override
protected Map.Entry, Integer>[] initialValue() {
// 1 extra slot to simplify logic of releaseBuffer()
return (Map.Entry, Integer>[])new Map.Entry[MAX_CACHED_BUFFERS + 1];
}
};
private static ByteBuffer allocateBuffer(long size) {
return ByteBuffer.allocateDirect((int)((size + 0xFFF) & ~0xFFF));
}
static ByteBuffer getBuffer(long size) {
if (size < 0 || Integer.MAX_VALUE < size) {
throw new IndexOutOfBoundsException("size");
}
ByteBuffer result = null;
if (size > LARGE_BUFFER) {
result = allocateBuffer(size);
} else {
Map.Entry, Integer>[] cache = CACHE.get();
// buffers are ordered by decreasing capacity
// cache[MAX_CACHED_BUFFERS] is always null
for (int i = MAX_CACHED_BUFFERS - 1; i >= 0; i--) {
Map.Entry, Integer> reference = cache[i];
if (reference != null) {
ByteBuffer buffer = getByteBuffer(reference);
if (buffer != null && size <= buffer.capacity()) {
cache[i] = null;
result = buffer;
result.rewind();
break;
}
}
}
if (result == null) {
result = allocateBuffer(size);
}
}
result.limit((int)size);
return result;
}
static void releaseBuffer(ByteBuffer buffer) {
if (buffer.capacity() > LARGE_BUFFER) {
return;
}
Map.Entry, Integer>[] cache = CACHE.get();
// expunge cleared BufferRef(s)
for (int i = 0; i < MAX_CACHED_BUFFERS; i++) {
Map.Entry, Integer> reference = cache[i];
if (reference != null && getByteBuffer(reference) == null) {
cache[i] = null;
}
}
// insert buffer back with new BufferRef wrapping it
cache[MAX_CACHED_BUFFERS] = newCacheEntry(buffer);
Arrays.sort(cache, DECREASING_CAPACITY_NULLS_LAST);
// squeeze the smallest one out
cache[MAX_CACHED_BUFFERS] = null;
}
private static Map.Entry, Integer> newCacheEntry(ByteBuffer bb) {
return new AbstractMap.SimpleEntry, Integer>(
new WeakReference(bb), bb.capacity());
}
private static int getCapacity(Map.Entry, Integer> e) {
return e == null? 0 : e.getValue();
}
private static ByteBuffer getByteBuffer(Map.Entry, Integer> e) {
return e == null? null : e.getKey().get();
}
private static Comparator, Integer>> DECREASING_CAPACITY_NULLS_LAST =
new Comparator, Integer>>() {
@Override
public int compare(Map.Entry, Integer> br1,
Map.Entry, Integer> br2) {
return Integer.compare(getCapacity(br1), getCapacity(br2));
}
};
}