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 static com.alibaba.metrics.Constants.NOT_AVAILABLE;
  import static java.lang.Double.isInfinite;
  import static java.lang.Double.isNaN;
  
  /**
   * A gauge which measures the ratio of one value to another.
   * <p/>
   * If the denominator is zero, not a number, or infinite, the resulting ratio is not a number.
   *
   * 一种衡量比率的度量器，传入分子和分母，该度量器会自动计算比率
   */
  public abstract class RatioGauge implements Gauge<Double> {
  
      private long lastUpdated = System.currentTimeMillis();
  
      /**
       * A ratio of one quantity to another.
       */
      public static class Ratio {
          /**
           * Creates a new ratio with the given numerator and denominator.
           *
           * @param numerator      the numerator of the ratio
           * @param denominator    the denominator of the ratio
           * @return {@code numerator:denominator}
           */
          public static Ratio of(double numerator, double denominator) {
              return new Ratio(numerator, denominator);
          }
  
          private final double numerator;
          private final double denominator;
  
          private Ratio(double numerator, double denominator) {
              this.numerator = numerator;
              this.denominator = denominator;
          }
  
          /**
           * Returns the ratio, which is either a {@code double} between 0 and 1 (inclusive) or
           * {@code NaN}.
           *
           * @return the ratio
           */
          public double getValue() {
              final double d = denominator;
              if (isNaN(d) || isInfinite(d) || d == 0) {
                  return Double.NaN;
              }
              return numerator / d;
          }
  
          @Override
          public String toString() {
              return numerator + ":" + denominator;
          }
      }
  
      /**
       * Returns the {@link Ratio} which is the gauge's current value.
       *
       * @return the {@link Ratio} which is the gauge's current value
       */
      protected abstract Ratio getRatio();
  
      public Double getValue() {
          try {
              Ratio r = getRatio();
              if (r == null) {
                  return (double)NOT_AVAILABLE;
              }
              double result = r.getValue();
              lastUpdated = System.currentTimeMillis();
              return result;
          } catch (Exception e) {
              return (double)NOT_AVAILABLE;
          }
      }
  
      @Override
      public long lastUpdateTime() {
          return lastUpdated;
      }
  }
  