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1   /*
2    * ====================================================================
3    *
4    *  Licensed to the Apache Software Foundation (ASF) under one or more
5    *  contributor license agreements.  See the NOTICE file distributed with
6    *  this work for additional information regarding copyright ownership.
7    *  The ASF licenses this file to You under the Apache License, Version 2.0
8    *  (the "License"); you may not use this file except in compliance with
9    *  the License.  You may obtain a copy of the License at
10   *
11   *      http://www.apache.org/licenses/LICENSE-2.0
12   *
13   *  Unless required by applicable law or agreed to in writing, software
14   *  distributed under the License is distributed on an "AS IS" BASIS,
15   *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
16   *  See the License for the specific language governing permissions and
17   *  limitations under the License.
18   * ====================================================================
19   *
20   * This software consists of voluntary contributions made by many
21   * individuals on behalf of the Apache Software Foundation.  For more
22   * information on the Apache Software Foundation, please see
23   * <http://www.apache.org/>.
24   *
25   */
26  package org.apache.http.annotation;
27  
28  import java.lang.annotation.Documented;
29  import java.lang.annotation.ElementType;
30  import java.lang.annotation.Retention;
31  import java.lang.annotation.RetentionPolicy;
32  import java.lang.annotation.Target;
33  
34  /**
35   * The class to which this annotation is applied is immutable.  This means that
36   * its state cannot be seen to change by callers, which implies that
37   * <ul>
38   * <li> all public fields are final, </li>
39   * <li> all public final reference fields refer to other immutable objects, and </li>
40   * <li> constructors and methods do not publish references to any internal state
41   *      which is potentially mutable by the implementation. </li>
42   * </ul>
43   * Immutable objects may still have internal mutable state for purposes of performance
44   * optimization; some state variables may be lazily computed, so long as they are computed
45   * from immutable state and that callers cannot tell the difference.
46   * <p>
47   * Immutable objects are inherently thread-safe; they may be passed between threads or
48   * published without synchronization.
49   * <p>
50   * Based on code developed by Brian Goetz and Tim Peierls and concepts
51   * published in 'Java Concurrency in Practice' by Brian Goetz, Tim Peierls,
52   * Joshua Bloch, Joseph Bowbeer, David Holmes and Doug Lea.
53   */
54  @Documented
55  @Target(ElementType.TYPE)
56  @Retention(RetentionPolicy.CLASS) // The original version used RUNTIME
57  public @interface Immutable {
58  }