1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one
3 * or more contributor license agreements. See the NOTICE file
4 * distributed with this work for additional information
5 * regarding copyright ownership. The ASF licenses this file
6 * to you under the Apache License, Version 2.0 (the
7 * "License"); you may not use this file except in compliance
8 * with the License. You may obtain a copy of the License at
9 *
10 * http://www.apache.org/licenses/LICENSE-2.0
11 *
12 * Unless required by applicable law or agreed to in writing, software
13 * distributed under the License is distributed on an "AS IS" BASIS,
14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15 * See the License for the specific language governing permissions and
16 * limitations under the License.
17 */
18
19 package org.apache.hadoop.hbase.zookeeper;
20
21 import java.util.concurrent.CountDownLatch;
22
23 /**
24 * Placeholder of an instance which will be accessed by other threads
25 * but is not yet created. Thread safe.
26 */
27 class InstancePending<T> {
28 // Based on a subtle part of the Java Language Specification,
29 // in order to avoid a slight overhead of synchronization for each access.
30
31 private final CountDownLatch pendingLatch = new CountDownLatch(1);
32
33 /** Piggybacking on {@code pendingLatch}. */
34 private InstanceHolder<T> instanceHolder;
35
36 private static class InstanceHolder<T> {
37 // The JLS ensures the visibility of a final field and its contents
38 // unless they are exposed to another thread while the construction.
39 final T instance;
40
41 InstanceHolder(T instance) {
42 this.instance = instance;
43 }
44 }
45
46 /**
47 * Returns the instance given by the method {@link #prepare}.
48 * This is an interruptible blocking method
49 * and the interruption flag will be set just before returning if any.
50 */
51 T get() {
52 InstanceHolder<T> instanceHolder;
53 boolean interrupted = false;
54
55 while ((instanceHolder = this.instanceHolder) == null) {
56 try {
57 pendingLatch.await();
58 } catch (InterruptedException e) {
59 interrupted = true;
60 }
61 }
62
63 if (interrupted) {
64 Thread.currentThread().interrupt();
65 }
66 return instanceHolder.instance;
67 }
68
69 /**
70 * Associates the given instance for the method {@link #get}.
71 * This method should be called once, and {@code instance} should be non-null.
72 * This method is expected to call as soon as possible
73 * because the method {@code get} is uninterruptibly blocked until this method is called.
74 */
75 void prepare(T instance) {
76 assert instance != null;
77 instanceHolder = new InstanceHolder<T>(instance);
78 pendingLatch.countDown();
79 }
80 }