概述

上周@望陶问了我一个现象很诡异的问题，说JDK7和JDK8下的System.nanoTime()输出完全不一样，而且差距还非常大，是不是两个版本里的实现不一样，之前我也没注意过这个细节，觉得非常奇怪，于是自己也在本地mac机器上马上测试了一下，得到如下输出：

~/Documents/workspace/Test/src ᐅ /Library/Java/JavaVirtualMachines/jdk1.7.0_79.jdk/Contents/Home/bin/java NanosTest
1480265318432558000
~/Documents/workspace/Test/src ᐅ /Library/Java/JavaVirtualMachines/jdk1.8.0_101.jdk/Contents/Home/bin/java NanosTest
1188453233877

还真不一样，于是我再到linux下跑了一把，发现两个版本下的值基本上差不多的，也就是主要是mac下的实现可能不一样

于是我又调用System.currentTimeMillis()，发现其输出结果和System.nanoTime()也完全不是1000000倍的比例

~/Documents/workspace/Test/src ᐅ /Library/Java/JavaVirtualMachines/jdk1.8.0_101.jdk/Contents/Home/bin/java NanosTest
1563115443175
1480265707257

另外System.nanoTime()输出的到底是什么东西，这个数字好奇怪

这三个小细节平时没有留意，好奇心作祟，于是马上想一查究竟

再列下主要想理清楚的三个问题

• 在mac下发现System.nanoTime()在JDK7和JDK8下输出的值怎么完全不一样
• System.nanoTime()的值很奇怪，究竟是怎么算出来的
• System.currentTimeMillis()为何不是System.nanoTime()的1000000倍

MAC不同JDK版本下nanoTime实现异同

在mac下，首先看JDK7的nanoTime实现

jlong os::javaTimeNanos() {
  if (Bsd::supports_monotonic_clock()) {
    struct timespec tp;
    int status = Bsd::clock_gettime(CLOCK_MONOTONIC, &tp);
    assert(status == 0, "gettime error");
    jlong result = jlong(tp.tv_sec) * (1000 * 1000 * 1000) + jlong(tp.tv_nsec);
    return result;
  } else {
    timeval time;
    int status = gettimeofday(&time, NULL);
    assert(status != -1, "bsd error");
    jlong usecs = jlong(time.tv_sec) * (1000 * 1000) + jlong(time.tv_usec);
    return 1000 * usecs;
  }
}

再来看JDK8下的实现

#ifdef __APPLE__

jlong os::javaTimeNanos() {
    const uint64_t tm = mach_absolute_time();
    const uint64_t now = (tm * Bsd::_timebase_info.numer) / Bsd::_timebase_info.denom;
    const uint64_t prev = Bsd::_max_abstime;
    if (now <= prev) {
      return prev;   // same or retrograde time;
    }
    const uint64_t obsv = Atomic::cmpxchg(now, (volatile jlong*)&Bsd::_max_abstime, prev);
    assert(obsv >= prev, "invariant");   // Monotonicity
    // If the CAS succeeded then we're done and return "now".
    // If the CAS failed and the observed value "obsv" is >= now then
    // we should return "obsv".  If the CAS failed and now > obsv > prv then
    // some other thread raced this thread and installed a new value, in which case
    // we could either (a) retry the entire operation, (b) retry trying to install now
    // or (c) just return obsv.  We use (c).   No loop is required although in some cases
    // we might discard a higher "now" value in deference to a slightly lower but freshly
    // installed obsv value.   That's entirely benign -- it admits no new orderings compared
    // to (a) or (b) -- and greatly reduces coherence traffic.
    // We might also condition (c) on the magnitude of the delta between obsv and now.
    // Avoiding excessive CAS operations to hot RW locations is critical.
    // See https://blogs.oracle.com/dave/entry/cas_and_cache_trivia_invalidate
    return (prev == obsv) ? now : obsv;
}

#else // __APPLE__

果然发现JDK8下多了一个__APPLE__宏下定义的实现，和JDK7及之前的版本的实现是不一样的，不过其他BSD系统是一样的，只是macos有点不一样，因为平时咱们主要使用的环境还是Linux为主，因此对于macos下具体异同就不做过多解释了，有兴趣的自己去研究一下。

Linux下nanoTime的实现

在linux下JDK7和JDK8的实现都是一样的

jlong os::javaTimeNanos() {
  if (Linux::supports_monotonic_clock()) {
    struct timespec tp;
    int status = Linux::clock_gettime(CLOCK_MONOTONIC, &tp);
    assert(status == 0, "gettime error");
    jlong result = jlong(tp.tv_sec) * (1000 * 1000 * 1000) + jlong(tp.tv_nsec);
    return result;
  } else {
    timeval time;
    int status = gettimeofday(&time, NULL);
    assert(status != -1, "linux error");
    jlong usecs = jlong(time.tv_sec) * (1000 * 1000) + jlong(time.tv_usec);
    return 1000 * usecs;
  }
}

而Linux::supports_monotonic_clock决定了走哪个具体的分支

static inline bool supports_monotonic_clock() {
    return _clock_gettime != NULL;
}

_clock_gettime的定义在

void os::Linux::clock_init() {
  // we do dlopen's in this particular order due to bug in linux
  // dynamical loader (see 6348968) leading to crash on exit
  void* handle = dlopen("librt.so.1", RTLD_LAZY);
  if (handle == NULL) {
    handle = dlopen("librt.so", RTLD_LAZY);
  }

  if (handle) {
    int (*clock_getres_func)(clockid_t, struct timespec*) =
           (int(*)(clockid_t, struct timespec*))dlsym(handle, "clock_getres");
    int (*clock_gettime_func)(clockid_t, struct timespec*) =
           (int(*)(clockid_t, struct timespec*))dlsym(handle, "clock_gettime");
    if (clock_getres_func && clock_gettime_func) {
      // See if monotonic clock is supported by the kernel. Note that some
      // early implementations simply return kernel jiffies (updated every
      // 1/100 or 1/1000 second). It would be bad to use such a low res clock
      // for nano time (though the monotonic property is still nice to have).
      // It's fixed in newer kernels, however clock_getres() still returns
      // 1/HZ. We check if clock_getres() works, but will ignore its reported
      // resolution for now. Hopefully as people move to new kernels, this
      // won't be a problem.
      struct timespec res;
      struct timespec tp;
      if (clock_getres_func (CLOCK_MONOTONIC, &res) == 0 &&
          clock_gettime_func(CLOCK_MONOTONIC, &tp)  == 0) {
        // yes, monotonic clock is supported
        _clock_gettime = clock_gettime_func;
        return;
      } else {
        // close librt if there is no monotonic clock
        dlclose(handle);
      }
    }
  }
  warning("No monotonic clock was available - timed services may " \
          "be adversely affected if the time-of-day clock changes");
}

说白了，其实就是看librt.so.1或者librt.so中是否定义了clock_gettime函数，如果定义了，就直接调用这个函数来获取时间，注意下上面的传给clock_gettime的一个参数是CLOCK_MONOTONIC，至于这个参数的作用后面会说，这个函数在glibc中有定义

/* Get current value of CLOCK and store it in TP.  */
int
__clock_gettime (clockid_t clock_id, struct timespec *tp)
{
  int retval = -1;

  switch (clock_id)
    {
#ifdef SYSDEP_GETTIME
      SYSDEP_GETTIME;
#endif

#ifndef HANDLED_REALTIME
    case CLOCK_REALTIME:
      {
    struct timeval tv;
    retval = gettimeofday (&tv, NULL);
    if (retval == 0)
      TIMEVAL_TO_TIMESPEC (&tv, tp);
      }
      break;
#endif

    default:
#ifdef SYSDEP_GETTIME_CPU
      SYSDEP_GETTIME_CPU (clock_id, tp);
#endif
#if HP_TIMING_AVAIL
      if ((clock_id & ((1 << CLOCK_IDFIELD_SIZE) - 1))
      == CLOCK_THREAD_CPUTIME_ID)
    retval = hp_timing_gettime (clock_id, tp);
      else
#endif
    __set_errno (EINVAL);
      break;

#if HP_TIMING_AVAIL && !defined HANDLED_CPUTIME
    case CLOCK_PROCESS_CPUTIME_ID:
      retval = hp_timing_gettime (clock_id, tp);
      break;
#endif
    }

  return retval;
}
weak_alias (__clock_gettime, clock_gettime)
libc_hidden_def (__clock_gettime)

而对应的宏SYSDEP_GETTIME定义如下：

#define SYSDEP_GETTIME \
  SYSDEP_GETTIME_CPUTIME;                              \
  case CLOCK_REALTIME:                                  \
  case CLOCK_MONOTONIC:                                  \
    retval = INLINE_VSYSCALL (clock_gettime, 2, clock_id, tp);              \
    break

/* We handled the REALTIME clock here.  */
#define HANDLED_REALTIME    1
#define HANDLED_CPUTIME    1

#define SYSDEP_GETTIME_CPU(clock_id, tp) \
  retval = INLINE_VSYSCALL (clock_gettime, 2, clock_id, tp); \
  break
#define SYSDEP_GETTIME_CPUTIME    /* Default catches them too.  */

最终是调用的clock_gettime系统调用：

int clock_gettime(clockid_t, struct timespec *)
    __attribute__((weak, alias("__vdso_clock_gettime")));
    
    
notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
{
    if (likely(gtod->sysctl_enabled))
        switch (clock) {
        case CLOCK_REALTIME:
            if (likely(gtod->clock.vread))
                return do_realtime(ts);
            break;
        case CLOCK_MONOTONIC:
            if (likely(gtod->clock.vread))
                return do_monotonic(ts);
            break;
        case CLOCK_REALTIME_COARSE:
            return do_realtime_coarse(ts);
        case CLOCK_MONOTONIC_COARSE:
            return do_monotonic_coarse(ts);
        }
    return vdso_fallback_gettime(clock, ts);
}    

而我们JVM里取纳秒数时传入的是CLOCK_MONOTONIC这个参数，因此会调用如下的方法

notrace static noinline int do_monotonic(struct timespec *ts)
{
    unsigned long seq, ns, secs;
    do {
        seq = read_seqbegin(&gtod->lock);
        secs = gtod->wall_time_sec;
        ns = gtod->wall_time_nsec + vgetns();
        secs += gtod->wall_to_monotonic.tv_sec;
        ns += gtod->wall_to_monotonic.tv_nsec;
    } while (unlikely(read_seqretry(&gtod->lock, seq)));
    vset_normalized_timespec(ts, secs, ns);
    return 0;
}

上面的wall_to_monotonic的tv_sec以及tv_nsec都是负数，在系统启动初始化的时候设置，记录了启动的时间

void __init timekeeping_init(void)
{
    struct clocksource *clock;
    unsigned long flags;
    struct timespec now, boot;

    read_persistent_clock(&now);
    read_boot_clock(&boot);

    write_seqlock_irqsave(&xtime_lock, flags);

    ntp_init();

    clock = clocksource_default_clock();
    if (clock->enable)
        clock->enable(clock);
    timekeeper_setup_internals(clock);

    xtime.tv_sec = now.tv_sec;
    xtime.tv_nsec = now.tv_nsec;
    raw_time.tv_sec = 0;
    raw_time.tv_nsec = 0;
    if (boot.tv_sec == 0 && boot.tv_nsec == 0) {
        boot.tv_sec = xtime.tv_sec;
        boot.tv_nsec = xtime.tv_nsec;
    }
    set_normalized_timespec(&wall_to_monotonic,
                -boot.tv_sec, -boot.tv_nsec);
    total_sleep_time.tv_sec = 0;
    total_sleep_time.tv_nsec = 0;
    write_sequnlock_irqrestore(&xtime_lock, flags);
}

因此nanoTime其实算出来的是一个相对的时间，相对于系统启动的时候的时间

Java里currentTimeMillis的实现

我们其实可以写一个简单的例子从侧面来验证currentTimeMillis返回的到底是什么值

    public static void main(String args[]) {
        System.out.println(new Date().getTime()-new Date(0).getTime());
        System.out.println(System.currentTimeMillis());
    }

你将看到输出结果会是两个一样的值，这说明了什么？另外new Date(0).getTime()其实就是1970/01/01 08:00:00,而new Date().getTime()是返回的当前时间，两个日期一减，其实就是当前时间距离1970/01/01 08:00:00有多少毫秒，而System.currentTimeMillis()返回的正好是这个值，也就是说System.currentTimeMillis()就是返回的当前时间距离1970/01/01 08:00:00的毫秒数。

就实现上来说，currentTimeMillis其实是通过gettimeofday来实现的

jlong os::javaTimeMillis() {
  timeval time;
  int status = gettimeofday(&time, NULL);
  assert(status != -1, "linux error");
  return jlong(time.tv_sec) * 1000  +  jlong(time.tv_usec / 1000);
}

至此应该大家也清楚了，为什么currentTimeMillis返回的值并不是nanoTime返回的值的1000000倍左右了，因为两个值的参照不一样，所以没有可比性