Guava 中的 Stopwatch 是个什么鬼？

Stopwatch stopwatch = Stopwatch.createStarted();
   doSomething();
 stopwatch.stop(); // optional

   long millis = stopwatch.elapsed(MILLISECONDS);
// formatted string like "12.3 ms"}

log.info("time: " + stopwatch);

Stopwatch.createStarted(
         new Ticker() {
           public long read() {
             return android.os.SystemClock.elapsedRealtime();
            }
          });}

   long startTime = System.currentTimeMillis();

        try {
            // 模拟业务逻辑
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println(System.currentTimeMillis() - startTime);

  //时间源 一般和Stopwatch一起使用，而不是单独使用
  private final Ticker ticker;
  private boolean isRunning;
  private long elapsedNanos;
  private long startTick;

  public static Ticker systemTicker() {
    return SYSTEM_TICKER;
  }

  private static final Ticker SYSTEM_TICKER =
      new Ticker() {
        @Override
        public long read() {
          // 实际上就是System.nanoTime();
          return Platform.systemNanoTime();
        }
      };
   
   // 子类重写   
   public abstract long read();

  public static Stopwatch createUnstarted() {
    return new Stopwatch();
  }

  /**
   * Creates (but does not start) a new stopwatch, using the specified time source.
   *
   * @since 15.0
   */
  public static Stopwatch createUnstarted(Ticker ticker) {
    return new Stopwatch(ticker);
  }

  /**
   * Creates (and starts) a new stopwatch using {@link System#nanoTime} as its time source.
   *
   * @since 15.0
   */
  public static Stopwatch createStarted() {
    return new Stopwatch().start();
  }
  
 Stopwatch() {
    this.ticker = Ticker.systemTicker();
  }

  Stopwatch(Ticker ticker) {
    this.ticker = checkNotNull(ticker, "ticker");
  }

 public Stopwatch start() {
    // 先判断是否处于执行状态
    checkState(!isRunning, "This stopwatch is already running.");
    isRunning = true;
    // 初始化 当前的纳秒时间
    startTick = ticker.read();
    return this;
  }

public Stopwatch stop() {
    long tick = ticker.read();
    checkState(isRunning, "This stopwatch is already stopped.");
    isRunning = false;
    elapsedNanos += tick - startTick;
    return this;
  }
  
  public Stopwatch reset() {
    elapsedNanos = 0;
    isRunning = false;
    return this;
  }

  // 计算纳秒
  private long elapsedNanos() {
    return isRunning ? ticker.read() - startTick + elapsedNanos : elapsedNanos;
  }

  // 转换其他单位
  public long elapsed(TimeUnit desiredUnit) {
    return desiredUnit.convert(elapsedNanos(), NANOSECONDS);
  }

还有一些单位转换和toString方法，就不分析了