nanotime()是一个非常精确的系统实用程序，用于测量执行时间。但是要注意，如果运行在抢占式调度器模式(默认)，这个实用程序实际上测量的是时钟时间，而不是CPU时间。因此，您可能会注意到每次运行的执行时间值是不同的，这取决于系统负载。如果您需要CPU时间，我认为在实时模式下运行您的程序就可以了。您必须使用RT linux。链接:使用Linux进行实时编程

为此，我们使用了AspectJ和Java注释。如果我们需要知道一个方法的执行时间，我们可以简单地注释它。更高级的版本可以使用自己的日志级别，可以在运行时启用和禁用。

public @interface Trace {
  boolean showParameters();
}

@Aspect
public class TraceAspect {
  [...]
  @Around("tracePointcut() && @annotation(trace) && !within(TraceAspect)")
  public Object traceAdvice ( ProceedingJintPoint jP, Trace trace ) {

    Object result;
    // initilize timer

    try { 
      result = jp.procced();
    } finally { 
      // calculate execution time 
    }

    return result;
  }
  [...]
}

在Java 8中引入了一个名为Instant的新类。根据文件:

Instant represents the start of a nanosecond on the time line. This class is useful for generating a time stamp to represent machine time. The range of an instant requires the storage of a number larger than a long. To achieve this, the class stores a long representing epoch-seconds and an int representing nanosecond-of-second, which will always be between 0 and 999,999,999. The epoch-seconds are measured from the standard Java epoch of 1970-01-01T00:00:00Z where instants after the epoch have positive values, and earlier instants have negative values. For both the epoch-second and nanosecond parts, a larger value is always later on the time-line than a smaller value.

这可以用于:

Instant start = Instant.now();
try {
    Thread.sleep(7000);
} catch (InterruptedException e) {
    e.printStackTrace();
}
Instant end = Instant.now();
System.out.println(Duration.between(start, end));

打印pt7.001。

下面是打印好的字符串，格式化后的秒数，类似于谷歌搜索所需的时间:

        long startTime = System.nanoTime();
        //  ... methodToTime();
        long endTime = System.nanoTime();
        long duration = (endTime - startTime);
        long seconds = (duration / 1000) % 60;
        // formatedSeconds = (0.xy seconds)
        String formatedSeconds = String.format("(0.%d seconds)", seconds);
        System.out.println("formatedSeconds = "+ formatedSeconds);
        // i.e actual formatedSeconds = (0.52 seconds)

这是一个简单的类用于简单的函数计时。下面有一个例子。

public class Stopwatch {
    static long startTime;
    static long splitTime;
    static long endTime;

    public Stopwatch() {
        start();
    }

    public void start() {
        startTime = System.currentTimeMillis();
        splitTime = System.currentTimeMillis();
        endTime = System.currentTimeMillis();
    }

    public void split() {
        split("");
    }

    public void split(String tag) {
        endTime = System.currentTimeMillis();
        System.out.println("Split time for [" + tag + "]: " + (endTime - splitTime) + " ms");
        splitTime = endTime;
    }

    public void end() {
        end("");
    }
    public void end(String tag) {
        endTime = System.currentTimeMillis();
        System.out.println("Final time for [" + tag + "]: " + (endTime - startTime) + " ms");
    }
}

使用示例:

public static Schedule getSchedule(Activity activity_context) {
        String scheduleJson = null;
        Schedule schedule = null;
/*->*/  Stopwatch stopwatch = new Stopwatch();

        InputStream scheduleJsonInputStream = activity_context.getResources().openRawResource(R.raw.skating_times);
/*->*/  stopwatch.split("open raw resource");

        scheduleJson = FileToString.convertStreamToString(scheduleJsonInputStream);
/*->*/  stopwatch.split("file to string");

        schedule = new Gson().fromJson(scheduleJson, Schedule.class);
/*->*/  stopwatch.split("parse Json");
/*->*/  stopwatch.end("Method getSchedule"); 
    return schedule;
}

控制台输出示例:

Split time for [file to string]: 672 ms
Split time for [parse Json]: 893 ms
Final time for [get Schedule]: 1565 ms

纯Java SE代码，不需要添加依赖项，使用TimeTracedExecuter:

public static void main(String[] args) {

    Integer square = new TimeTracedExecutor<>(Main::calculateSquare)
                .executeWithInput("calculate square of num",5,logger);

}
public static int calculateSquare(int num){
    return num*num;
}

会产生这样的结果:

信息:计算num的平方需要3毫秒

自定义可重用类:TimeTracedExecutor

import java.text.NumberFormat;
import java.time.Duration;
import java.time.Instant;
import java.util.function.Function;
import java.util.logging.Logger;

public class TimeTracedExecutor<T,R> {
    Function<T,R> methodToExecute;

    public TimeTracedExecutor(Function<T, R> methodToExecute) {
        this.methodToExecute = methodToExecute;
    }

    public R executeWithInput(String taskDescription, T t, Logger logger){
        Instant start = Instant.now();
        R r= methodToExecute.apply(t);
        Instant finish = Instant.now();
        String format = "It took %s milliseconds to "+taskDescription;
        String elapsedTime = NumberFormat.getNumberInstance().format(Duration.between(start, finish).toMillis());
        logger.info(String.format(format, elapsedTime));
        return r;
    }
}