001/*
002 * Copyright (C) 2008 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.base;
016
017import static com.google.common.base.Preconditions.checkNotNull;
018import static com.google.common.base.Preconditions.checkState;
019import static java.util.concurrent.TimeUnit.DAYS;
020import static java.util.concurrent.TimeUnit.HOURS;
021import static java.util.concurrent.TimeUnit.MICROSECONDS;
022import static java.util.concurrent.TimeUnit.MILLISECONDS;
023import static java.util.concurrent.TimeUnit.MINUTES;
024import static java.util.concurrent.TimeUnit.NANOSECONDS;
025import static java.util.concurrent.TimeUnit.SECONDS;
026
027import com.google.common.annotations.GwtCompatible;
028import com.google.common.annotations.GwtIncompatible;
029import com.google.errorprone.annotations.CanIgnoreReturnValue;
030import com.google.j2objc.annotations.J2ObjCIncompatible;
031import java.time.Duration;
032import java.util.concurrent.TimeUnit;
033
034/**
035 * An object that accurately measures <i>elapsed time</i>: the measured duration between two
036 * successive readings of "now" in the same process.
037 *
038 * <p>In contrast, <i>wall time</i> is a reading of "now" as given by a method like
039 * {@link System#currentTimeMillis()}, best represented as an {@link Instant}. Such values
040 * <i>can</i> be subtracted to obtain a {@code Duration} (such as by {@code Duration.between}), but
041 * doing so does <i>not</i> give a reliable measurement of elapsed time, because wall time readings
042 * are inherently approximate, routinely affected by periodic clock corrections. Because this class
043 * (by default) uses {@link System#nanoTime}, it is unaffected by these changes.
044 *
045 * <p>Use this class instead of direct calls to {@link System#nanoTime} for two reasons:
046 *
047 * <ul>
048 *   <li>The raw {@code long} values returned by {@code nanoTime} are meaningless and unsafe to use
049 *       in any other way than how {@code Stopwatch} uses them.
050 *   <li>An alternative source of nanosecond ticks can be substituted, for example for testing or
051 *       performance reasons, without affecting most of your code.
052 * </ul>
053 *
054 * <p>Basic usage:
055 *
056 * <pre>{@code
057 * Stopwatch stopwatch = Stopwatch.createStarted();
058 * doSomething();
059 * stopwatch.stop(); // optional
060 *
061 * Duration duration = stopwatch.elapsed();
062 *
063 * log.info("time: " + stopwatch); // formatted string like "12.3 ms"
064 * }</pre>
065 *
066 * <p>The state-changing methods are not idempotent; it is an error to start or stop a stopwatch
067 * that is already in the desired state.
068 *
069 * <p>When testing code that uses this class, use {@link #createUnstarted(Ticker)} or {@link
070 * #createStarted(Ticker)} to supply a fake or mock ticker. This allows you to simulate any valid
071 * behavior of the stopwatch.
072 *
073 * <p><b>Note:</b> This class is not thread-safe.
074 *
075 * <p><b>Warning for Android users:</b> a stopwatch with default behavior may not continue to keep
076 * time while the device is asleep. Instead, create one like this:
077 *
078 * <pre>{@code
079 * Stopwatch.createStarted(
080 *      new Ticker() {
081 *        public long read() {
082 *          return android.os.SystemClock.elapsedRealtimeNanos();
083 *        }
084 *      });
085 * }</pre>
086 *
087 * @author Kevin Bourrillion
088 * @since 10.0
089 */
090@GwtCompatible(emulated = true)
091@SuppressWarnings("GoodTime") // lots of violations
092public final class Stopwatch {
093  private final Ticker ticker;
094  private boolean isRunning;
095  private long elapsedNanos;
096  private long startTick;
097
098  /**
099   * Creates (but does not start) a new stopwatch using {@link System#nanoTime} as its time source.
100   *
101   * @since 15.0
102   */
103  public static Stopwatch createUnstarted() {
104    return new Stopwatch();
105  }
106
107  /**
108   * Creates (but does not start) a new stopwatch, using the specified time source.
109   *
110   * @since 15.0
111   */
112  public static Stopwatch createUnstarted(Ticker ticker) {
113    return new Stopwatch(ticker);
114  }
115
116  /**
117   * Creates (and starts) a new stopwatch using {@link System#nanoTime} as its time source.
118   *
119   * @since 15.0
120   */
121  public static Stopwatch createStarted() {
122    return new Stopwatch().start();
123  }
124
125  /**
126   * Creates (and starts) a new stopwatch, using the specified time source.
127   *
128   * @since 15.0
129   */
130  public static Stopwatch createStarted(Ticker ticker) {
131    return new Stopwatch(ticker).start();
132  }
133
134  Stopwatch() {
135    this.ticker = Ticker.systemTicker();
136  }
137
138  Stopwatch(Ticker ticker) {
139    this.ticker = checkNotNull(ticker, "ticker");
140  }
141
142  /**
143   * Returns {@code true} if {@link #start()} has been called on this stopwatch, and {@link #stop()}
144   * has not been called since the last call to {@code start()}.
145   */
146  public boolean isRunning() {
147    return isRunning;
148  }
149
150  /**
151   * Starts the stopwatch.
152   *
153   * @return this {@code Stopwatch} instance
154   * @throws IllegalStateException if the stopwatch is already running.
155   */
156  @CanIgnoreReturnValue
157  public Stopwatch start() {
158    checkState(!isRunning, "This stopwatch is already running.");
159    isRunning = true;
160    startTick = ticker.read();
161    return this;
162  }
163
164  /**
165   * Stops the stopwatch. Future reads will return the fixed duration that had elapsed up to this
166   * point.
167   *
168   * @return this {@code Stopwatch} instance
169   * @throws IllegalStateException if the stopwatch is already stopped.
170   */
171  @CanIgnoreReturnValue
172  public Stopwatch stop() {
173    long tick = ticker.read();
174    checkState(isRunning, "This stopwatch is already stopped.");
175    isRunning = false;
176    elapsedNanos += tick - startTick;
177    return this;
178  }
179
180  /**
181   * Sets the elapsed time for this stopwatch to zero, and places it in a stopped state.
182   *
183   * @return this {@code Stopwatch} instance
184   */
185  @CanIgnoreReturnValue
186  public Stopwatch reset() {
187    elapsedNanos = 0;
188    isRunning = false;
189    return this;
190  }
191
192  private long elapsedNanos() {
193    return isRunning ? ticker.read() - startTick + elapsedNanos : elapsedNanos;
194  }
195
196  /**
197   * Returns the current elapsed time shown on this stopwatch, expressed in the desired time unit,
198   * with any fraction rounded down.
199   *
200   * <p><b>Note:</b> the overhead of measurement can be more than a microsecond, so it is generally
201   * not useful to specify {@link TimeUnit#NANOSECONDS} precision here.
202   *
203   * <p>It is generally not a good idea to use an ambiguous, unitless {@code long} to represent
204   * elapsed time. Therefore, we recommend using {@link #elapsed()} instead, which returns a
205   * strongly-typed {@code Duration} instance.
206   *
207   * @since 14.0 (since 10.0 as {@code elapsedTime()})
208   */
209  public long elapsed(TimeUnit desiredUnit) {
210    return desiredUnit.convert(elapsedNanos(), NANOSECONDS);
211  }
212
213  /**
214   * Returns the current elapsed time shown on this stopwatch as a {@link Duration}. Unlike {@link
215   * #elapsed(TimeUnit)}, this method does not lose any precision due to rounding.
216   *
217   * @since 22.0
218   */
219  @GwtIncompatible
220  @J2ObjCIncompatible
221  public Duration elapsed() {
222    return Duration.ofNanos(elapsedNanos());
223  }
224
225  /** Returns a string representation of the current elapsed time. */
226  @Override
227  public String toString() {
228    long nanos = elapsedNanos();
229
230    TimeUnit unit = chooseUnit(nanos);
231    double value = (double) nanos / NANOSECONDS.convert(1, unit);
232
233    // Too bad this functionality is not exposed as a regular method call
234    return Platform.formatCompact4Digits(value) + " " + abbreviate(unit);
235  }
236
237  private static TimeUnit chooseUnit(long nanos) {
238    if (DAYS.convert(nanos, NANOSECONDS) > 0) {
239      return DAYS;
240    }
241    if (HOURS.convert(nanos, NANOSECONDS) > 0) {
242      return HOURS;
243    }
244    if (MINUTES.convert(nanos, NANOSECONDS) > 0) {
245      return MINUTES;
246    }
247    if (SECONDS.convert(nanos, NANOSECONDS) > 0) {
248      return SECONDS;
249    }
250    if (MILLISECONDS.convert(nanos, NANOSECONDS) > 0) {
251      return MILLISECONDS;
252    }
253    if (MICROSECONDS.convert(nanos, NANOSECONDS) > 0) {
254      return MICROSECONDS;
255    }
256    return NANOSECONDS;
257  }
258
259  private static String abbreviate(TimeUnit unit) {
260    switch (unit) {
261      case NANOSECONDS:
262        return "ns";
263      case MICROSECONDS:
264        return "\u03bcs"; // μs
265      case MILLISECONDS:
266        return "ms";
267      case SECONDS:
268        return "s";
269      case MINUTES:
270        return "min";
271      case HOURS:
272        return "h";
273      case DAYS:
274        return "d";
275      default:
276        throw new AssertionError();
277    }
278  }
279}