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 }