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
019package org.apache.hadoop.util;
020
021import org.apache.hadoop.classification.InterfaceAudience;
022import org.apache.hadoop.classification.InterfaceStability;
023
024import com.google.common.collect.Interner;
025import 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
033public 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}