com.spotify.zoltar.loaders.ModelMemoizer Maven / Gradle / Ivy

Go to download

Show more of this group Show more artifacts with this name
Show all versions of zoltar-core Show documentation

There is a newer version: 0.6.0

/*-
 * -\-\-
 * zoltar-core
 * --
 * Copyright (C) 2016 - 2018 Spotify AB
 * --
 * 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.spotify.zoltar.loaders;

import com.spotify.zoltar.Model;
import com.spotify.zoltar.ModelLoader;
import java.util.Objects;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.atomic.AtomicReference;

/**
 * Memoizes the result of the supplied {@link ModelLoader}.
 *
 * @param  Model instance type.
 */
@FunctionalInterface
public interface ModelMemoizer> extends ModelLoader {

  /**
   * Creates a memoized model loader.
   *
   * @param loader ModelLoader to be memoized.
   * @param  Model instance type.
   * @return Memoized loader.
   */
  static > ModelMemoizer memoize(final ModelLoader loader) {
    // AtomicReference can be updated atomically and offers volatile write/read semantics.
    // However, in this case it's just being used as a container for the CompletionStage.
    final AtomicReference> value = new AtomicReference<>();
    return () -> {
      CompletionStage val = value.get();
      if (val == null) {
        // we want to avoid .get() being called several times from the different threads
        // because it can be very expensive.
        synchronized (value) {
          val = value.get();
          if (val == null) {
            val = Objects.requireNonNull(loader.get());
            value.set(val);
          }
        }
      }

      return val;
    };
  }

}