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.lang.management.ManagementFactory;
  import java.lang.management.ThreadMXBean;
  
  /**
   * An abstraction for how time passes. It is passed to {@link Timer} to track timing.
   */
  public abstract class Clock {
      /**
       * Returns the current time tick.
       *
       * @return time tick in nanoseconds
       */
      public abstract long getTick();
  
      /**
       * Returns the current time in milliseconds.
       *
       * @return time in milliseconds
       */
      public long getTime() {
          return System.currentTimeMillis();
      }
  
      private static final Clock DEFAULT = new UserTimeClock();
  
      /**
       * The default clock to use.
       *
       * @return the default {@link Clock} instance
       *
       * @see Clock.UserTimeClock
       */
      public static Clock defaultClock() {
          return DEFAULT;
      }
  
      /**
       * A clock implementation which returns the current time in epoch nanoseconds.
       */
      public static class UserTimeClock extends Clock {
          @Override
          public long getTick() {
              return System.nanoTime();
          }
      }
  
      /**
       * A clock implementation which returns the current thread's CPU time.
       */
      public static class CpuTimeClock extends Clock {
          private static final ThreadMXBean THREAD_MX_BEAN = ManagementFactory.getThreadMXBean();
  
          @Override
          public long getTick() {
              return THREAD_MX_BEAN.getCurrentThreadCpuTime();
          }
      }
  }
  