1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
|
/*
* Copyright (C) 2007 The Guava Authors
*
* 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.common.collect;
import static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Equivalence;
import com.google.common.base.Function;
import com.google.common.collect.MapMaker.Dummy;
import com.google.common.collect.MapMakerInternalMap.InternalEntry;
/**
* Contains static methods pertaining to instances of {@link Interner}.
*
* @author Kevin Bourrillion
* @since 3.0
*/
@Beta
@GwtIncompatible
public final class Interners {
private Interners() {}
/**
* Builder for {@link Interner} instances.
*
* @since 21.0
*/
public static class InternerBuilder {
private final MapMaker mapMaker = new MapMaker();
private boolean strong = true;
private InternerBuilder() {}
/**
* Instructs the {@link InternerBuilder} to build a strong interner.
*
* @see Interners#newStrongInterner()
*/
public InternerBuilder strong() {
this.strong = true;
return this;
}
/**
* Instructs the {@link InternerBuilder} to build a weak interner.
*
* @see Interners#newWeakInterner()
*/
@GwtIncompatible("java.lang.ref.WeakReference")
public InternerBuilder weak() {
this.strong = false;
return this;
}
/**
* Sets the concurrency level that will be used by the to-be-built {@link Interner}.
*
* @see MapMaker#concurrencyLevel(int)
*/
public InternerBuilder concurrencyLevel(int concurrencyLevel) {
this.mapMaker.concurrencyLevel(concurrencyLevel);
return this;
}
public <E> Interner<E> build() {
if (!strong) {
mapMaker.weakKeys();
}
return new InternerImpl<E>(mapMaker);
}
}
/** Returns a fresh {@link InternerBuilder} instance. */
public static InternerBuilder newBuilder() {
return new InternerBuilder();
}
/**
* Returns a new thread-safe interner which retains a strong reference to each instance it has
* interned, thus preventing these instances from being garbage-collected. If this retention is
* acceptable, this implementation may perform better than {@link #newWeakInterner}.
*/
public static <E> Interner<E> newStrongInterner() {
return newBuilder().strong().build();
}
/**
* Returns a new thread-safe interner which retains a weak reference to each instance it has
* interned, and so does not prevent these instances from being garbage-collected. This most
* likely does not perform as well as {@link #newStrongInterner}, but is the best alternative when
* the memory usage of that implementation is unacceptable.
*/
@GwtIncompatible("java.lang.ref.WeakReference")
public static <E> Interner<E> newWeakInterner() {
return newBuilder().weak().build();
}
@VisibleForTesting
static final class InternerImpl<E> implements Interner<E> {
// MapMaker is our friend, we know about this type
@VisibleForTesting final MapMakerInternalMap<E, Dummy, ?, ?> map;
private InternerImpl(MapMaker mapMaker) {
this.map =
MapMakerInternalMap.createWithDummyValues(mapMaker.keyEquivalence(Equivalence.equals()));
}
@Override
public E intern(E sample) {
while (true) {
// trying to read the canonical...
InternalEntry<E, Dummy, ?> entry = map.getEntry(sample);
if (entry != null) {
E canonical = entry.getKey();
if (canonical != null) { // only matters if weak/soft keys are used
return canonical;
}
}
// didn't see it, trying to put it instead...
Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE);
if (sneaky == null) {
return sample;
} else {
/* Someone beat us to it! Trying again...
*
* Technically this loop not guaranteed to terminate, so theoretically (extremely
* unlikely) this thread might starve, but even then, there is always going to be another
* thread doing progress here.
*/
}
}
}
}
/**
* Returns a function that delegates to the {@link Interner#intern} method of the given interner.
*
* @since 8.0
*/
public static <E> Function<E, E> asFunction(Interner<E> interner) {
return new InternerFunction<E>(checkNotNull(interner));
}
private static class InternerFunction<E> implements Function<E, E> {
private final Interner<E> interner;
public InternerFunction(Interner<E> interner) {
this.interner = interner;
}
@Override
public E apply(E input) {
return interner.intern(input);
}
@Override
public int hashCode() {
return interner.hashCode();
}
@Override
public boolean equals(Object other) {
if (other instanceof InternerFunction) {
InternerFunction<?> that = (InternerFunction<?>) other;
return interner.equals(that.interner);
}
return false;
}
}
}
|
