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1   /*
2    * ====================================================================
3    * Licensed to the Apache Software Foundation (ASF) under one
4    * or more contributor license agreements.  See the NOTICE file
5    * distributed with this work for additional information
6    * regarding copyright ownership.  The ASF licenses this file
7    * to you under the Apache License, Version 2.0 (the
8    * "License"); you may not use this file except in compliance
9    * with 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,
14   * software distributed under the License is distributed on an
15   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
16   * KIND, either express or implied.  See the License for the
17   * specific language governing permissions and limitations
18   * under the License.
19   * ====================================================================
20   *
21   * This software consists of voluntary contributions made by many
22   * individuals on behalf of the Apache Software Foundation.  For more
23   * information on the Apache Software Foundation, please see
24   * <http://www.apache.org/>.
25   *
26   */
27  package org.apache.http.annotation;
28  
29  import java.lang.annotation.Documented;
30  import java.lang.annotation.ElementType;
31  import java.lang.annotation.Retention;
32  import java.lang.annotation.RetentionPolicy;
33  import java.lang.annotation.Target;
34  
35  /**
36   * The class to which this annotation is applied is immutable.  This means that
37   * its state cannot be seen to change by callers, which implies that
38   * <ul>
39   * <li> all public fields are final, </li>
40   * <li> all public final reference fields refer to other immutable objects, and </li>
41   * <li> constructors and methods do not publish references to any internal state
42   *      which is potentially mutable by the implementation. </li>
43   * </ul>
44   * Immutable objects may still have internal mutable state for purposes of performance
45   * optimization; some state variables may be lazily computed, so long as they are computed
46   * from immutable state and that callers cannot tell the difference.
47   * <p>
48   * Immutable objects are inherently thread-safe; they may be passed between threads or
49   * published without synchronization.
50   * <p>
51   * Based on code developed by Brian Goetz and Tim Peierls and concepts
52   * published in 'Java Concurrency in Practice' by Brian Goetz, Tim Peierls,
53   * Joshua Bloch, Joseph Bowbeer, David Holmes and Doug Lea.
54   */
55  @Documented
56  @Target(ElementType.TYPE)
57  @Retention(RetentionPolicy.CLASS) // The original version used RUNTIME
58  public @interface Immutable {
59  }