hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/StringInterner.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.hadoop.util;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

import com.google.common.collect.Interner;
import com.google.common.collect.Interners;

/**
 * Provides string interning utility methods. For weak interning,
 * we use the standard String.intern() call, that performs very well
 * (no problems with PermGen overflowing, etc.) starting from JDK 7.
 */
@InterfaceAudience.Public
@InterfaceStability.Stable
public class StringInterner {
  
  /**
   * Retains a strong reference to each string instance it has interned.
   */
  private final static Interner<String> STRONG_INTERNER =
      Interners.newStrongInterner();

  /**
   * Interns and returns a reference to the representative instance 
   * for any of a collection of string instances that are equal to each other.
   * Retains strong reference to the instance, 
   * thus preventing it from being garbage-collected. 
   * 
   * @param sample string instance to be interned
   * @return strong reference to interned string instance
   */
  public static String strongIntern(String sample) {
    if (sample == null) {
      return null;
    }
    return STRONG_INTERNER.intern(sample);
  }
  
  /**
   * Interns and returns a reference to the representative instance 
   * for any of a collection of string instances that are equal to each other.
   * Retains weak reference to the instance, 
   * and so does not prevent it from being garbage-collected.
   * 
   * @param sample string instance to be interned
   * @return weak reference to interned string instance
   */
  public static String weakIntern(String sample) {
    if (sample == null) {
      return null;
    }
    return sample.intern();
  }

  /**
   * Interns all the strings in the given array in place,
   * returning the same array.
   */
  public static String[] internStringsInArray(String[] strings) {
    for (int i = 0; i < strings.length; i++) {
      strings[i] = weakIntern(strings[i]);
    }
    return strings;
  }

}