1 /* 2 * Copyright (c) 2018, 2019, 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 static jdk.vm.ci.hotspot.UnsafeAccess.UNSAFE; 26 27 /** 28 * This class manages a set of {@code jobject} and {@code jmetadata} handles whose lifetimes are 29 * dependent on associated {@link IndirectHotSpotObjectConstantImpl} and 30 * {@link MetaspaceHandleObject} wrapper objects respectively. 31 * 32 * The general theory of operation is that all wrappers are created by calling into the VM which 33 * calls back out to actually create the wrapper instance. During the call the VM keeps the object 34 * or metadata reference alive through the use of handles. Once the call completes the wrapper 35 * object is registered here and will be scanned during metadata scanning. The weakness of the 36 * reference to the wrapper object allows the handles to be reclaimed when they are no longer used. 37 */ 38 final class HandleCleaner extends Cleaner { 39 40 /** 41 * A {@code jmetadata} or {@code jobject} handle. 42 */ 43 private final long handle; 44 45 /** 46 * Specifies if {@link #handle} is a {@code jobject} or {@code jmetadata}. 47 */ 48 private final boolean isJObject; 49 50 private HandleCleaner(Object wrapper, long handle, boolean isJObject) { 51 super(wrapper); 52 this.handle = handle; 53 this.isJObject = isJObject; 54 } 55 56 /** 57 * Releases the resource associated with {@code this.handle}. 58 */ 59 @Override 60 void doCleanup() { 61 if (isJObject) { 62 // The sentinel value used to denote a free handle is 63 // an object on the HotSpot heap so we call into the 64 // VM to set the target of an object handle to this value. 65 CompilerToVM.compilerToVM().deleteGlobalHandle(handle); 66 } else { 67 // Setting the target of a jmetadata handle to 0 enables 68 // the handle to be reused. See MetadataHandles in 69 // metadataHandles.hpp for more info. 70 long value = UNSAFE.getLong(null, handle); 71 UNSAFE.compareAndSetLong(null, handle, value, 0); 72 } 73 } 74 75 /** 76 * Registers a cleaner for {@code handle}. The cleaner will release the handle some time after 77 * {@code wrapper} is detected as unreachable by the garbage collector. 78 */ 79 @SuppressWarnings("unused") 80 static void create(Object wrapper, long handle) { 81 assert wrapper instanceof IndirectHotSpotObjectConstantImpl || wrapper instanceof MetaspaceHandleObject; 82 new HandleCleaner(wrapper, handle, wrapper instanceof IndirectHotSpotObjectConstantImpl); 83 } 84 }