/*
* Copyright 2010 Google Inc.
*
* Licensed 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 com.google.gwt.dev.util;
import java.lang.ref.WeakReference;
import java.util.WeakHashMap;
/**
* A utility class for reducing String memory waste.
*
* <p> We don't use the String.intern() method because it would prevent GC and fill the PermGen
* space. We also don't use Guava (for now) due to a class loader GC issue with the old
* version of Guava we're using. </p>
*
* <p> Thread-safe. The implementation uses shards to reduce thread contention. </p>
*/
public class StringInterner {
// chosen via performance testing with AWFE. (See AWFE load times speadsheet.)
private static int SHARD_BITS = 10;
static int SHARD_COUNT = 1 << SHARD_BITS;
private static int SHARD_MASK = SHARD_COUNT - 1;
private static final StringInterner instance = new StringInterner();
public static StringInterner get() {
return instance;
}
private final Shard[] shards = new Shard[SHARD_COUNT];
protected StringInterner() {
for (int i = 0; i < SHARD_COUNT; i++) {
shards[i] = new Shard();
}
}
/**
* Returns a string equal to the input string, but not always the same.
*/
public String intern(String original) {
int shardId = getShardId(original);
return shards[shardId].intern(original);
}
/* visible for testing */
int getShardId(String original) {
int hashCode = original.hashCode();
return (hashCode ^ (hashCode >> SHARD_BITS)) & SHARD_MASK;
}
private static class Shard {
private final WeakHashMap<String, WeakReference<String>> map =
new WeakHashMap<String, WeakReference<String>>();
private synchronized String intern(String original) {
WeakReference<String> ref = map.get(original);
String interned = ref == null ? null : ref.get();
if (interned == null) {
ref = new WeakReference<String>(original);
map.put(original, ref);
return original;
} else {
return interned;
}
}
}
}