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// Copyright (C) 2018 The Android Open Source Project
//
// 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.gerrit.server.cache;
import static com.google.common.base.Preconditions.checkState;
import com.google.common.base.Objects;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Maps;
import com.google.gerrit.common.Nullable;
import java.util.Map;
import java.util.function.Supplier;
/**
* Caches object instances for a request as {@link ThreadLocal} in the serving thread.
*
* <p>This class is intended to cache objects that have a high instantiation cost, are specific to
* the current request and potentially need to be instantiated multiple times while serving a
* request.
*
* <p>This is different from the key-value storage in {@code CurrentUser}: {@code CurrentUser}
* offers a key-value storage by providing thread-safe {@code get} and {@code put} methods. Once the
* value is retrieved through {@code get} there is not thread-safety anymore - apart from the
* retrieved object guarantees. Depending on the implementation of {@code CurrentUser}, it might be
* shared between the request serving thread as well as sub- or background treads.
*
* <p>In comparison to that, this class guarantees thread safety even on non-thread-safe objects as
* its cache is tied to the serving thread only. While allowing to cache non-thread-safe objects, it
* has the downside of not sharing any objects with background threads or executors.
*
* <p>Lastly, this class offers a cache, that requires callers to also provide a {@code Supplier} in
* case the object is not present in the cache, while {@code CurrentUser} provides a storage where
* just retrieving stored values is a valid operation.
*/
public class PerThreadCache implements AutoCloseable {
private static final ThreadLocal<PerThreadCache> CACHE = new ThreadLocal<>();
/**
* Unique key for key-value mappings stored in PerThreadCache. The key is based on the value's
* class and a list of identifiers that in combination uniquely set the object apart form others
* of the same class.
*/
public static final class Key<T> {
private final Class<T> clazz;
private final ImmutableList<Object> identifiers;
/**
* Returns a key based on the value's class and an identifier that uniquely identify the value.
* The identifier needs to implement {@code equals()} and {@code hashCode()}.
*/
public static <T> Key<T> create(Class<T> clazz, Object identifier) {
return new Key<>(clazz, ImmutableList.of(identifier));
}
/**
* Returns a key based on the value's class and a set of identifiers that uniquely identify the
* value. Identifiers need to implement {@code equals()} and {@code hashCode()}.
*/
public static <T> Key<T> create(Class<T> clazz, Object... identifiers) {
return new Key<>(clazz, ImmutableList.copyOf(identifiers));
}
private Key(Class<T> clazz, ImmutableList<Object> identifiers) {
this.clazz = clazz;
this.identifiers = identifiers;
}
@Override
public int hashCode() {
return Objects.hashCode(clazz, identifiers);
}
@Override
public boolean equals(Object o) {
if (!(o instanceof Key)) {
return false;
}
Key<?> other = (Key<?>) o;
return this.clazz == other.clazz && this.identifiers.equals(other.identifiers);
}
}
public static PerThreadCache create() {
checkState(CACHE.get() == null, "called create() twice on the same request");
PerThreadCache cache = new PerThreadCache();
CACHE.set(cache);
return cache;
}
@Nullable
public static PerThreadCache get() {
return CACHE.get();
}
public static <T> T getOrCompute(Key<T> key, Supplier<T> loader) {
PerThreadCache cache = get();
return cache != null ? cache.get(key, loader) : loader.get();
}
private final Map<Key<?>, Object> cache = Maps.newHashMap();
private PerThreadCache() {}
/**
* Returns an instance of {@code T} that was either loaded from the cache or obtained from the
* provided {@link Supplier}.
*/
public <T> T get(Key<T> key, Supplier<T> loader) {
@SuppressWarnings("unchecked")
T value = (T) cache.get(key);
if (value == null) {
value = loader.get();
cache.put(key, value);
}
return value;
}
@Override
public void close() {
CACHE.remove();
}
}