001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *     http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing, software
013     * distributed under the License is distributed on an "AS IS" BASIS,
014     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015     * See the License for the specific language governing permissions and
016     * limitations under the License.
017     */
018    
019    package org.apache.hadoop.util;
020    
021    import org.apache.hadoop.classification.InterfaceAudience;
022    import org.apache.hadoop.classification.InterfaceStability;
023    
024    import com.google.common.collect.Interner;
025    import com.google.common.collect.Interners;
026    
027    /**
028     * Provides equivalent behavior to String.intern() to optimize performance, 
029     * whereby does not consume memory in the permanent generation.
030     */
031    @InterfaceAudience.Public
032    @InterfaceStability.Stable
033    public class StringInterner {
034      
035      /**
036       * Retains a strong reference to each string instance it has interned.
037       */
038      private final static Interner<String> strongInterner;
039      
040      /**
041       * Retains a weak reference to each string instance it has interned. 
042       */
043      private final static Interner<String> weakInterner;
044      
045      
046      
047      static {
048        strongInterner = Interners.newStrongInterner();
049        weakInterner = Interners.newWeakInterner();
050      }
051      
052      /**
053       * Interns and returns a reference to the representative instance 
054       * for any of a collection of string instances that are equal to each other.
055       * Retains strong reference to the instance, 
056       * thus preventing it from being garbage-collected. 
057       * 
058       * @param sample string instance to be interned
059       * @return strong reference to interned string instance
060       */
061      public static String strongIntern(String sample) {
062        if (sample == null) {
063          return null;
064        }
065        return strongInterner.intern(sample);
066      }
067      
068      /**
069       * Interns and returns a reference to the representative instance 
070       * for any of a collection of string instances that are equal to each other.
071       * Retains weak reference to the instance, 
072       * and so does not prevent it from being garbage-collected.
073       * 
074       * @param sample string instance to be interned
075       * @return weak reference to interned string instance
076       */
077      public static String weakIntern(String sample) {
078        if (sample == null) {
079          return null;
080        }
081        return weakInterner.intern(sample);
082      }
083    
084    }