1 /* 2 * Copyright (c) 2021, 2022, 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. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 26 package jdk.internal.misc; 27 28 import java.util.concurrent.locks.ReentrantLock; 29 30 /** 31 * A reentrant mutual exclusion lock for internal use. The lock does not 32 * implement {@link java.util.concurrent.locks.Lock} or extend {@link 33 * java.util.concurrent.locks.ReentrantLock} so that it can be distinguished 34 * from lock objects accessible to subclasses of {@link java.io.Reader} and 35 * {@link java.io.Writer} (it is possible to create a Reader that uses a 36 * lock object of type ReentrantLock for example). 37 */ 38 public class InternalLock { 39 private static final boolean CAN_USE_INTERNAL_LOCK; 40 static { 41 String s = System.getProperty("jdk.io.useMonitors"); 42 if (s != null && s.equals("false")) { 43 CAN_USE_INTERNAL_LOCK = true; 44 } else { 45 CAN_USE_INTERNAL_LOCK = false; 46 } 47 } 48 49 private final ReentrantLock lock; 50 51 private InternalLock() { 52 this.lock = new ReentrantLock(); 53 } 54 55 /** 56 * Returns a new InternalLock or null. 57 */ 58 public static InternalLock newLockOrNull() { 59 return (CAN_USE_INTERNAL_LOCK) ? new InternalLock() : null; 60 } 61 62 /** 63 * Returns a new InternalLock or the given object. 64 */ 65 public static Object newLockOr(Object obj) { 66 return (CAN_USE_INTERNAL_LOCK) ? new InternalLock() : obj; 67 } 68 69 public boolean tryLock() { 70 return lock.tryLock(); 71 } 72 73 public void lock() { 74 lock.lock(); 75 } 76 77 public void unlock() { 78 lock.unlock(); 79 } 80 81 public boolean isHeldByCurrentThread() { 82 return lock.isHeldByCurrentThread(); 83 } 84 }