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(); // requires API Level 17
083 *        }
084 *      });
085 * }</pre>
086 *
087 * @author Kevin Bourrillion
088 * @since 10.0
089 */
090@GwtCompatible(emulated = true)
091@SuppressWarnings("GoodTime") // lots of violations
092@ElementTypesAreNonnullByDefault
093public final class Stopwatch {
094  private final Ticker ticker;
095  private boolean isRunning;
096  private long elapsedNanos;
097  private long startTick;
098
099  /**
100   * Creates (but does not start) a new stopwatch using {@link System#nanoTime} as its time source.
101   *
102   * @since 15.0
103   */
104  public static Stopwatch createUnstarted() {
105    return new Stopwatch();
106  }
107
108  /**
109   * Creates (but does not start) a new stopwatch, using the specified time source.
110   *
111   * @since 15.0
112   */
113  public static Stopwatch createUnstarted(Ticker ticker) {
114    return new Stopwatch(ticker);
115  }
116
117  /**
118   * Creates (and starts) a new stopwatch using {@link System#nanoTime} as its time source.
119   *
120   * @since 15.0
121   */
122  public static Stopwatch createStarted() {
123    return new Stopwatch().start();
124  }
125
126  /**
127   * Creates (and starts) a new stopwatch, using the specified time source.
128   *
129   * @since 15.0
130   */
131  public static Stopwatch createStarted(Ticker ticker) {
132    return new Stopwatch(ticker).start();
133  }
134
135  Stopwatch() {
136    this.ticker = Ticker.systemTicker();
137  }
138
139  Stopwatch(Ticker ticker) {
140    this.ticker = checkNotNull(ticker, "ticker");
141  }
142
143  /**
144   * Returns {@code true} if {@link #start()} has been called on this stopwatch, and {@link #stop()}
145   * has not been called since the last call to {@code start()}.
146   */
147  public boolean isRunning() {
148    return isRunning;
149  }
150
151  /**
152   * Starts the stopwatch.
153   *
154   * @return this {@code Stopwatch} instance
155   * @throws IllegalStateException if the stopwatch is already running.
156   */
157  @CanIgnoreReturnValue
158  public Stopwatch start() {
159    checkState(!isRunning, "This stopwatch is already running.");
160    isRunning = true;
161    startTick = ticker.read();
162    return this;
163  }
164
165  /**
166   * Stops the stopwatch. Future reads will return the fixed duration that had elapsed up to this
167   * point.
168   *
169   * @return this {@code Stopwatch} instance
170   * @throws IllegalStateException if the stopwatch is already stopped.
171   */
172  @CanIgnoreReturnValue
173  public Stopwatch stop() {
174    long tick = ticker.read();
175    checkState(isRunning, "This stopwatch is already stopped.");
176    isRunning = false;
177    elapsedNanos += tick - startTick;
178    return this;
179  }
180
181  /**
182   * Sets the elapsed time for this stopwatch to zero, and places it in a stopped state.
183   *
184   * @return this {@code Stopwatch} instance
185   */
186  @CanIgnoreReturnValue
187  public Stopwatch reset() {
188    elapsedNanos = 0;
189    isRunning = false;
190    return this;
191  }
192
193  private long elapsedNanos() {
194    return isRunning ? ticker.read() - startTick + elapsedNanos : elapsedNanos;
195  }
196
197  /**
198   * Returns the current elapsed time shown on this stopwatch, expressed in the desired time unit,
199   * with any fraction rounded down.
200   *
201   * <p><b>Note:</b> the overhead of measurement can be more than a microsecond, so it is generally
202   * not useful to specify {@link TimeUnit#NANOSECONDS} precision here.
203   *
204   * <p>It is generally not a good idea to use an ambiguous, unitless {@code long} to represent
205   * elapsed time. Therefore, we recommend using {@link #elapsed()} instead, which returns a
206   * strongly-typed {@code Duration} instance.
207   *
208   * @since 14.0 (since 10.0 as {@code elapsedTime()})
209   */
210  public long elapsed(TimeUnit desiredUnit) {
211    return desiredUnit.convert(elapsedNanos(), NANOSECONDS);
212  }
213
214  /**
215   * Returns the current elapsed time shown on this stopwatch as a {@link Duration}. Unlike {@link
216   * #elapsed(TimeUnit)}, this method does not lose any precision due to rounding.
217   *
218   * @since 22.0
219   */
220  @GwtIncompatible
221  @J2ObjCIncompatible
222  public Duration elapsed() {
223    return Duration.ofNanos(elapsedNanos());
224  }
225
226  /** Returns a string representation of the current elapsed time. */
227  @Override
228  public String toString() {
229    long nanos = elapsedNanos();
230
231    TimeUnit unit = chooseUnit(nanos);
232    double value = (double) nanos / NANOSECONDS.convert(1, unit);
233
234    // Too bad this functionality is not exposed as a regular method call
235    return Platform.formatCompact4Digits(value) + " " + abbreviate(unit);
236  }
237
238  private static TimeUnit chooseUnit(long nanos) {
239    if (DAYS.convert(nanos, NANOSECONDS) > 0) {
240      return DAYS;
241    }
242    if (HOURS.convert(nanos, NANOSECONDS) > 0) {
243      return HOURS;
244    }
245    if (MINUTES.convert(nanos, NANOSECONDS) > 0) {
246      return MINUTES;
247    }
248    if (SECONDS.convert(nanos, NANOSECONDS) > 0) {
249      return SECONDS;
250    }
251    if (MILLISECONDS.convert(nanos, NANOSECONDS) > 0) {
252      return MILLISECONDS;
253    }
254    if (MICROSECONDS.convert(nanos, NANOSECONDS) > 0) {
255      return MICROSECONDS;
256    }
257    return NANOSECONDS;
258  }
259
260  private static String abbreviate(TimeUnit unit) {
261    switch (unit) {
262      case NANOSECONDS:
263        return "ns";
264      case MICROSECONDS:
265        return "\u03bcs"; // μs
266      case MILLISECONDS:
267        return "ms";
268      case SECONDS:
269        return "s";
270      case MINUTES:
271        return "min";
272      case HOURS:
273        return "h";
274      case DAYS:
275        return "d";
276      default:
277        throw new AssertionError();
278    }
279  }
280}