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.util;
20 
21 import org.apache.hadoop.classification.InterfaceAudience;
22 import org.apache.hadoop.classification.InterfaceStability;
23 
24 import com.google.common.collect.Interner;
25 import com.google.common.collect.Interners;
26 
27 /**
28  * Provides equivalent behavior to String.intern() to optimize performance,
29  * whereby does not consume memory in the permanent generation.
30  */
31 @InterfaceAudience.Public
32 @InterfaceStability.Stable
33 public class StringInterner {
34 
35   /**
36    * Retains a strong reference to each string instance it has interned.
37    */
38   private final static Interner<String> strongInterner;
39 
40   /**
41    * Retains a weak reference to each string instance it has interned.
42    */
43   private final static Interner<String> weakInterner;
44 
45 
46 
47   static {
48     strongInterner = Interners.newStrongInterner();
49     weakInterner = Interners.newWeakInterner();
50   }
51 
52   /**
53    * Interns and returns a reference to the representative instance
54    * for any of a collection of string instances that are equal to each other.
55    * Retains strong reference to the instance,
56    * thus preventing it from being garbage-collected.
57    *
58    * @param sample string instance to be interned
59    * @return strong reference to interned string instance
60    */
strongIntern(String sample)61   public static String strongIntern(String sample) {
62     if (sample == null) {
63       return null;
64     }
65     return strongInterner.intern(sample);
66   }
67 
68   /**
69    * Interns and returns a reference to the representative instance
70    * for any of a collection of string instances that are equal to each other.
71    * Retains weak reference to the instance,
72    * and so does not prevent it from being garbage-collected.
73    *
74    * @param sample string instance to be interned
75    * @return weak reference to interned string instance
76    */
weakIntern(String sample)77   public static String weakIntern(String sample) {
78     if (sample == null) {
79       return null;
80     }
81     return weakInterner.intern(sample);
82   }
83 
84 }
85