content: /*
   * 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 com.alibaba.metrics;
  
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicLong;
  
  /**
   * A {@link Gauge} implementation which caches its value for a period of time.
   *
   * @param <T>    the type of the gauge's value
   */
  public abstract class CachedGauge<T> implements Gauge<T> {
      private final Clock clock;
      private final AtomicLong reloadAt;
      private final long timeoutNS;
      private long lastUpdate = System.currentTimeMillis();
  
      private volatile T value;
  
      /**
       * Creates a new cached gauge with the given timeout period.
       *
       * @param timeout        the timeout
       * @param timeoutUnit    the unit of {@code timeout}
       */
      protected CachedGauge(long timeout, TimeUnit timeoutUnit) {
          this(Clock.defaultClock(), timeout, timeoutUnit);
      }
  
      /**
       * Creates a new cached gauge with the given clock and timeout period.
       *
       * @param clock          the clock used to calculate the timeout
       * @param timeout        the timeout
       * @param timeoutUnit    the unit of {@code timeout}
       */
      protected CachedGauge(Clock clock, long timeout, TimeUnit timeoutUnit) {
          this.clock = clock;
          this.reloadAt = new AtomicLong(clock.getTick());
          this.timeoutNS = timeoutUnit.toNanos(timeout);
      }
  
      /**
       * Loads the value and returns it.
       *
       * @return the new value
       */
      protected abstract T loadValue();
  
      public T getValue() {
          if (shouldLoad()) {
              this.value = loadValue();
              lastUpdate = System.currentTimeMillis();
          }
          return value;
      }
  
      @Override
      public long lastUpdateTime() {
          return lastUpdate;
      }
  
      private boolean shouldLoad() {
          for (; ; ) {
              final long currentTick = clock.getTick();
              final long reloadAtTick = reloadAt.get();
              if (currentTick < reloadAtTick) {
                  return false;
              }
              if (reloadAt.compareAndSet(reloadAtTick, currentTick + timeoutNS)) {
                  return true;
              }
          }
      }
  }
  