Class Stopwatch

java.lang.Object
com.google.common.base.Stopwatch
@GwtCompatible public final class Stopwatch extends Object
An object that accurately measures elapsed time: the measured duration between two successive readings of "now" in the same process.

In contrast, wall time is a reading of "now" as given by a method like System.currentTimeMillis(), best represented as an Instant. Such values can be subtracted to obtain a Duration (such as by Duration.between), but doing so does not give a reliable measurement of elapsed time, because wall time readings are inherently approximate, routinely affected by periodic clock corrections. Because this class (by default) uses System.nanoTime(), it is unaffected by these changes.

Use this class instead of direct calls to System.nanoTime() for two reasons:

  • The raw long values returned by nanoTime are meaningless and unsafe to use in any other way than how Stopwatch uses them.
  • An alternative source of nanosecond ticks can be substituted, for example for testing or performance reasons, without affecting most of your code.

The one downside of Stopwatch relative to System.nanoTime() is that Stopwatch requires object allocation and additional method calls, which can reduce the accuracy of the elapsed times reported. Stopwatch is still suitable for logging and metrics where reasonably accurate values are sufficient. If the uncommon case that you need to maximize accuracy, use System.nanoTime() directly instead.

Basic usage: 

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

Duration duration = stopwatch.elapsed();

log.info("time: " + stopwatch); // formatted string like "12.3 ms"

The state-changing methods are not idempotent; it is an error to start or stop a stopwatch that is already in the desired state.

When testing code that uses this class, use createUnstarted(Ticker) or createStarted(Ticker) to supply a fake or mock ticker. This allows you to simulate any valid behavior of the stopwatch.

Note: This class is not thread-safe.

Warning for Android users: a stopwatch with default behavior may not continue to keep time while the device is asleep. Instead, create one like this: 

Stopwatch.createStarted(
     new Ticker() {
       public long read() {
         return android.os.SystemClock.elapsedRealtimeNanos();
       }
     });
Since:
10.0
Author:
Kevin Bourrillion

  • Method Details

    • createUnstarted

      public static Stopwatch createUnstarted()
      Creates (but does not start) a new stopwatch using System.nanoTime() as its time source.
      Since:
      15.0

    • createUnstarted

      public static Stopwatch createUnstarted(Ticker ticker)
      Creates (but does not start) a new stopwatch, using the specified time source.
      Since:
      15.0

    • createStarted

      public static Stopwatch createStarted()
      Creates (and starts) a new stopwatch using System.nanoTime() as its time source.
      Since:
      15.0

    • createStarted

      public static Stopwatch createStarted(Ticker ticker)
      Creates (and starts) a new stopwatch, using the specified time source.
      Since:
      15.0

    • isRunning

      public boolean isRunning()
      Returns true if start() has been called on this stopwatch, and stop() has not been called since the last call to start().

    • start

      @CanIgnoreReturnValue public Stopwatch start()
      Starts the stopwatch.
      Returns:
      this Stopwatch instance
      Throws:
      IllegalStateException - if the stopwatch is already running.

    • stop

      @CanIgnoreReturnValue public Stopwatch stop()
      Stops the stopwatch. Future reads will return the fixed duration that had elapsed up to this point.
      Returns:
      this Stopwatch instance
      Throws:
      IllegalStateException - if the stopwatch is already stopped.

    • reset

      @CanIgnoreReturnValue public Stopwatch reset()
      Sets the elapsed time for this stopwatch to zero, and places it in a stopped state.
      Returns:
      this Stopwatch instance

    • elapsed

      public long elapsed(TimeUnit desiredUnit)
      Returns the current elapsed time shown on this stopwatch, expressed in the desired time unit, with any fraction rounded down.

      Note: the overhead of measurement can be more than a microsecond, so it is generally not useful to specify TimeUnit.NANOSECONDS precision here.

      It is generally not a good idea to use an ambiguous, unitless long to represent elapsed time. Therefore, we recommend using elapsed() instead, which returns a strongly-typed Duration instance.

      Since:
      14.0 (since 10.0 as elapsedTime())

    • elapsed

      @GwtIncompatible @J2ObjCIncompatible public Duration elapsed()
      Returns the current elapsed time shown on this stopwatch as a Duration. Unlike elapsed(TimeUnit), this method does not lose any precision due to rounding.

      Warning: do not call this method from Android code unless you are on Android API level 26+ or you opt in to library desugaring.

      Since:
      33.4.0 (but since 22.0 in the JRE flavor)

    • toString

      public String toString()
      Returns a string representation of the current elapsed time.
      Overrides:
      toString in class Object