JVM 源码分析之 System.currentTimeMillis 及 nanoTime 原理详解

地址：https://juejin.im/entry/5860c0fe570c35006946af25

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

[/code]
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

[/code]
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;
}
}

[/code]
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__

[/code]
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;
}
}

[/code]
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;
}

[/code]

_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");
}

[/code]
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)

[/code]
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.  */

[/code]
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);
}

[/code]
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(>od->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(>od->lock, seq)));
vset_normalized_timespec(ts, secs, ns);
return 0;
}

[/code]
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);
}

[/code]
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());
}

[/code]
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);
}

[/code]
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