com.google.common.base

Class Stopwatch

java.lang.Object

com.google.common.base.Stopwatch

@GwtCompatible(emulated=true)
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.

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 Summary

Modifier and Type	Method and Description
static Stopwatch	createStarted() Creates (and starts) a new stopwatch using System.nanoTime() as its time source.
static Stopwatch	createStarted(Ticker ticker) Creates (and starts) a new stopwatch, using the specified time source.
static Stopwatch	createUnstarted() Creates (but does not start) a new stopwatch using System.nanoTime() as its time source.
static Stopwatch	createUnstarted(Ticker ticker) Creates (but does not start) a new stopwatch, using the specified time source.
Duration	elapsed() Returns the current elapsed time shown on this stopwatch as a Duration.
long	elapsed(TimeUnit desiredUnit) Returns the current elapsed time shown on this stopwatch, expressed in the desired time unit, with any fraction rounded down.
boolean	isRunning() Returns true if start() has been called on this stopwatch, and stop() has not been called since the last call to start().
Stopwatch	reset() Sets the elapsed time for this stopwatch to zero, and places it in a stopped state.
Stopwatch	start() Starts the stopwatch.
Stopwatch	stop() Stops the stopwatch.
String	toString() Returns a string representation of the current elapsed time.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Method Detail

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
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.

Since:

22.0

toString

public String toString()

Returns a string representation of the current elapsed time.

Overrides:

toString in class Object

Returns:

a string representation of the object.