1 /*
2 * Copyright 2011 The Apache Software Foundation
3 *
4 * Licensed to the Apache Software Foundation (ASF) under one
5 * or more contributor license agreements. See the NOTICE file
6 * distributed with this work for additional information
7 * regarding copyright ownership. The ASF licenses this file
8 * to you under the Apache License, Version 2.0 (the
9 * "License"); you may not use this file except in compliance
10 * with the License. You may obtain a copy of the License at
11 *
12 * http://www.apache.org/licenses/LICENSE-2.0
13 *
14 * Unless required by applicable law or agreed to in writing, software
15 * distributed under the License is distributed on an "AS IS" BASIS,
16 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
17 * See the License for the specific language governing permissions and
18 * limitations under the License.
19 */
20 package org.apache.hadoop.hbase.util;
21
22 import java.io.IOException;
23 import java.io.InterruptedIOException;
24 import java.util.concurrent.ConcurrentHashMap;
25 import java.util.concurrent.ConcurrentMap;
26
27 /**
28 * Allows multiple concurrent clients to lock on a numeric id with a minimal
29 * memory overhead. The intended usage is as follows:
30 *
31 * <pre>
32 * IdLock.Entry lockEntry = idLock.getLockEntry(id);
33 * try {
34 * // User code.
35 * } finally {
36 * idLock.releaseLockEntry(lockEntry);
37 * }</pre>
38 */
39 public class IdLock {
40
41 /** An entry returned to the client as a lock object */
42 public static class Entry {
43 private final long id;
44 private int numWaiters;
45 private boolean isLocked = true;
46
47 private Entry(long id) {
48 this.id = id;
49 }
50
51 public String toString() {
52 return "id=" + id + ", numWaiter=" + numWaiters + ", isLocked="
53 + isLocked;
54 }
55 }
56
57 private ConcurrentMap<Long, Entry> map =
58 new ConcurrentHashMap<Long, Entry>();
59
60 /**
61 * Blocks until the lock corresponding to the given id is acquired.
62 *
63 * @param id an arbitrary number to lock on
64 * @return an "entry" to pass to {@link #releaseLockEntry(Entry)} to release
65 * the lock
66 * @throws IOException if interrupted
67 */
68 public Entry getLockEntry(long id) throws IOException {
69 Entry entry = new Entry(id);
70 Entry existing;
71 while ((existing = map.putIfAbsent(entry.id, entry)) != null) {
72 synchronized (existing) {
73 if (existing.isLocked) {
74 ++existing.numWaiters; // Add ourselves to waiters.
75 while (existing.isLocked) {
76 try {
77 existing.wait();
78 } catch (InterruptedException e) {
79 --existing.numWaiters; // Remove ourselves from waiters.
80 throw new InterruptedIOException(
81 "Interrupted waiting to acquire sparse lock");
82 }
83 }
84
85 --existing.numWaiters; // Remove ourselves from waiters.
86 existing.isLocked = true;
87 return existing;
88 }
89 // If the entry is not locked, it might already be deleted from the
90 // map, so we cannot return it. We need to get our entry into the map
91 // or get someone else's locked entry.
92 }
93 }
94 return entry;
95 }
96
97 /**
98 * Must be called in a finally block to decrease the internal counter and
99 * remove the monitor object for the given id if the caller is the last
100 * client.
101 *
102 * @param entry the return value of {@link #getLockEntry(long)}
103 */
104 public void releaseLockEntry(Entry entry) {
105 synchronized (entry) {
106 entry.isLocked = false;
107 if (entry.numWaiters > 0) {
108 entry.notify();
109 } else {
110 map.remove(entry.id);
111 }
112 }
113 }
114
115 /** For testing */
116 void assertMapEmpty() {
117 assert map.size() == 0;
118 }
119
120 }