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;
  
  public abstract class CachedMetricSet implements MetricSet {
  
      protected static long DEFAULT_DATA_TTL = 5000;
  
      // The time (in milli-seconds) to live of cached data
      protected long dataTTL;
  
      // The last collect time
      protected AtomicLong lastCollectTime;
  
      // The clock used to calculate time
      protected Clock clock;
  
      // The lock used to collect metric
      private final Object collectLock = new Object();
  
      public CachedMetricSet() {
         this(DEFAULT_DATA_TTL, TimeUnit.MILLISECONDS, Clock.defaultClock());
      }
  
      public CachedMetricSet(long dataTTL, TimeUnit unit) {
          this(dataTTL, unit, Clock.defaultClock());
      }
  
      public CachedMetricSet(long dataTTL, TimeUnit unit, Clock clock) {
          this.dataTTL = unit.toMillis(dataTTL);
          this.clock = clock;
          this.lastCollectTime = new AtomicLong(0);
      }
  
      /**
       * Do not collect data if our cached copy of data is valid.
       * The purpose is to minimize the cost to collect system metric.
       */
      public void refreshIfNecessary() {
          if (clock.getTime() - lastCollectTime.get() > dataTTL) {
              synchronized (collectLock) {
                  // double check, in case other thread has already entered.
                  if (clock.getTime() - lastCollectTime.get() > dataTTL) {
                      getValueInternal();
                      // update the last collect time stamp
                      lastCollectTime.set(clock.getTime());
                  }
              }
          }
      }
  
      @Override
      public long lastUpdateTime() {
          return lastCollectTime.get();
      }
  
      protected abstract void getValueInternal();
  }
  