一文看懂：StopWatch 源码解读

@Controller
@RequestMapping("/app")
@Slf4j
public class TestController extends BaseController{

    @Autowired
    private SystemService systemService;

    @RequestMapping("/test")
    @ResponseBody
    public String testDemo() throws Exception {
        StopWatch clock = new StopWatch("stop watch test");
        clock.start("test1");
        systemService.step1();
        clock.stop();

        clock.start("test1");
        systemService.step2();
        clock.stop();

        clock.start("test3");
        systemService.step3();
        clock.stop();

        Thread.sleep(1000);
        log.debug(MessageFormat.format("log:{0}", clock.prettyPrint()));
        return "Hello World!";
    }
}

StopWatch '': running time = 12292625 ns
---------------------------------------------
ns         %     Task name
---------------------------------------------
012145833  099%  test1
000101042  001%  test2
000045750  000%  test3

  /**
   * Construct a new {@code StopWatch}.
   * <p>Does not start any task.
   */
  public StopWatch() {
    this("");
  }

  /**
   * Construct a new {@code StopWatch} with the given ID.
   * <p>The ID is handy when we have output from multiple stop watches and need
   * to distinguish between them.
   * <p>Does not start any task.
   * @param id identifier for this stop watch
   */
  public StopWatch(String id) {
    this.id = id;
  }

  /**
   * Start a named task.
   * <p>The results are undefined if {@link #stop()} or timing methods are
   * called without invoking this method first.
   * @param taskName the name of the task to start
   * @see #start()
   * @see #stop()
   */
  public void start(String taskName) throws IllegalStateException {
    if (this.currentTaskName != null) {
      throw new IllegalStateException("Can't start StopWatch: it's already running");
    }
    this.currentTaskName = taskName;
    this.startTimeNanos = System.nanoTime();
  }

  /**
   * Stop the current task.
   * <p>The results are undefined if timing methods are called without invoking
   * at least one pair of {@code start()} / {@code stop()} methods.
   * @see #start()
   * @see #start(String)
   */
  public void stop() throws IllegalStateException {
    if (this.currentTaskName == null) {
      throw new IllegalStateException("Can't stop StopWatch: it's not running");
    }
    long lastTime = System.nanoTime() - this.startTimeNanos;
    this.totalTimeNanos += lastTime;
    this.lastTaskInfo = new TaskInfo(this.currentTaskName, lastTime);
    if (this.keepTaskList) {
      this.taskList.add(this.lastTaskInfo);
    }
    ++this.taskCount;
    this.currentTaskName = null;
  }

// TaskInfo 静态内部类（多实例共享一个 TaskInfo 对象）
public static final class TaskInfo {
    private final String taskName;
    private final long timeNanos;

    TaskInfo(String taskName, long timeNanos) {
      this.taskName = taskName;
      this.timeNanos = timeNanos;
    }

    /**
     * Get the name of this task.
     */
    public String getTaskName() {
      return this.taskName;
    }

    /**
     * Get the time in nanoseconds this task took.
     * @since 5.2
     * @see #getTimeMillis()
     * @see #getTimeSeconds()
     */
    public long getTimeNanos() {
      return this.timeNanos;
    }

    /**
     * Get the time in milliseconds this task took.
     * @see #getTimeNanos()
     * @see #getTimeSeconds()
     */
    public long getTimeMillis() {
      return nanosToMillis(this.timeNanos);
    }

    /**
     * Get the time in seconds this task took.
     * @see #getTimeMillis()
     * @see #getTimeNanos()
     */
    public double getTimeSeconds() {
      return nanosToSeconds(this.timeNanos);
    }

  }

  /**
   * Generate a string with a table describing all tasks performed.
   * <p>For custom reporting, call {@link #getTaskInfo()} and use the task info
   * directly.
   */
  public String prettyPrint() {
    StringBuilder sb = new StringBuilder(shortSummary());
    sb.append('\n');
    if (!this.keepTaskList) {
      sb.append("No task info kept");
    }
    else {
      sb.append("---------------------------------------------\n");
      sb.append("ns         %     Task name\n");
      sb.append("---------------------------------------------\n");
      NumberFormat nf = NumberFormat.getNumberInstance();
      nf.setMinimumIntegerDigits(9);
      nf.setGroupingUsed(false);
      NumberFormat pf = NumberFormat.getPercentInstance();
      pf.setMinimumIntegerDigits(3);
      pf.setGroupingUsed(false);
      for (TaskInfo task : getTaskInfo()) {
        sb.append(nf.format(task.getTimeNanos())).append("  ");
        sb.append(pf.format((double) task.getTimeNanos() / getTotalTimeNanos())).append("  ");
        sb.append(task.getTaskName()).append("\n");
      }
    }
    return sb.toString();
  }