NumberGauge.java

/*
 * Copyright 2014 Netflix, Inc.
 *
 * 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.netflix.servo.monitor;

import com.netflix.servo.annotations.DataSourceType;
import com.netflix.servo.util.Preconditions;

import java.lang.ref.WeakReference;

/**
 * A {@link Gauge} that returns the value stored in {@link Number}.
 */
public class NumberGauge extends AbstractMonitor<Number> implements Gauge<Number> {
  private WeakReference<Number> numberRef;

  /**
   * Construct a gauge that will store a weak reference to a number. The number
   * should be set by the subclass immediately after the call to super(config);
   * using the {@link #setBackingNumber(Number)} method.
   */
  protected NumberGauge(MonitorConfig config) {
    super(config.withAdditionalTag(DataSourceType.GAUGE));
  }

  /**
   * Construct a gauge that will store a weak reference to the number. The value returned
   * by the monitor will be the value stored in {@code number} or {@code Double.NaN} in case
   * the referred Number has been garbage collected.
   */
  public NumberGauge(MonitorConfig config, Number number) {
    super(config.withAdditionalTag(DataSourceType.GAUGE));
    Preconditions.checkNotNull(number, "number");
    this.numberRef = new WeakReference<>(number);
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public Number getValue(int pollerIdx) {
    Number n = numberRef.get();
    return n != null ? n : Double.NaN;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public boolean equals(Object o) {
    if (this == o) {
      return true;
    }

    if (!(o instanceof NumberGauge)) {
      return false;
    }

    final NumberGauge that = (NumberGauge) o;
    return config.equals(that.config) && getValue().equals(that.getValue());
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public int hashCode() {
    int result = getConfig().hashCode();
    result = 31 * result + getValue(0).hashCode();
    return result;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public String toString() {
    return "NumberGauge{config=" + config + ", number=" + numberRef.get() + '}';
  }

  /**
   * Returns the {@link Number} hold or null if it has been garbage collected.
   */
  protected Number getBackingNumber() {
    return numberRef.get();
  }

  /**
   * Sets a new {@link Number} to hold. We keep a week reference to this value
   * allowing it to be GC.
   */
  protected void setBackingNumber(Number number) {
    numberRef = new WeakReference<>(number);
  }
}