1 /*
2 * Licensed to The Apereo Foundation under one or more contributor license
3 * agreements. See the NOTICE file distributed with this work for additional
4 * information regarding copyright ownership.
5 *
6 *
7 * The Apereo Foundation licenses this file to you under the Educational
8 * Community License, Version 2.0 (the "License"); you may not use this file
9 * except in compliance with the License. You may obtain a copy of the License
10 * at:
11 *
12 * http://opensource.org/licenses/ecl2.txt
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, WITHOUT
16 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
17 * License for the specific language governing permissions and limitations under
18 * the License.
19 *
20 */
21
22 package org.opencastproject.util;
23
24 import static org.opencastproject.util.data.Option.none;
25 import static org.opencastproject.util.data.Option.some;
26
27 import org.opencastproject.util.data.Function0;
28 import org.opencastproject.util.data.Option;
29
30 import java.util.concurrent.atomic.AtomicBoolean;
31
32 /** Only one function application can be threaded through the needle eye at a time. */
33 public final class NeedleEye {
34 private final AtomicBoolean running = new AtomicBoolean(false);
35
36 /**
37 * Apply function <code>f</code> only if no other thread currently applies a function using this needle eye. Please
38 * note that <code>f</code> must <em>not</em> return null, so please do not use
39 * {@link org.opencastproject.util.data.Effect0}.
40 *
41 * @return the result of <code>f</code> or none if another function is currently being applied.
42 */
43 public <A> Option<A> apply(Function0<A> f) {
44 if (running.compareAndSet(false, true)) {
45 try {
46 return some(f.apply());
47 } finally {
48 running.set(false);
49 }
50 } else {
51 return none();
52 }
53 }
54 }