环境信息
[caoxudong@localhost ~]$ java -version
java version "1.8.0_65"
Java(TM) SE Runtime Environment (build 1.8.0_65-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.65-b01, mixed mode)
[caoxudong@localhost ~]$ uname -a
Linux localhost 2.6.32-573.18.1.el6.toav2.x86_64 #1 SMP Sun Jul 17 12:44:29 CST 2016 x86_64 x86_64 x86_64 GNU/Linux
方法说明
方法声明
/**
* Returns the current time in milliseconds. Note that
* while the unit of time of the return value is a millisecond,
* the granularity of the value depends on the underlying
* operating system and may be larger. For example, many
* operating systems measure time in units of tens of
* milliseconds.
*
* <p> See the description of the class <code>Date</code> for
* a discussion of slight discrepancies that may arise between
* "computer time" and coordinated universal time (UTC).
*
* @return the difference, measured in milliseconds, between
* the current time and midnight, January 1, 1970 UTC.
* @see java.util.Date
*/
public static native long currentTimeMillis();
currentTimeMillis方法用于获取当前的时间戳，单位为毫秒，但获取数据的精度无法确定，依赖于当前操作系统的具体实现。
本地方法的声明，参见jvm.cpp
JVM_LEAF(jlong, JVM_CurrentTimeMillis(JNIEnv *env, jclass ignored))
JVMWrapper("JVM_CurrentTimeMillis");
return os::javaTimeMillis();
JVM_END
在各个操作系统下的实现
aix，参见os_aix.cpp
jlong os::javaTimeMillis() {
timeval time;
int status = gettimeofday(&time, NULL);
assert(status != -1, "aix error at gettimeofday()");
return jlong(time.tv_sec) * 1000 + jlong(time.tv_usec / 1000);
}
bsd，参见os_bsd.cpp
jlong os::javaTimeMillis() {
timeval time;
int status = gettimeofday(&time, NULL);
assert(status != -1, "bsd error");
return jlong(time.tv_sec) * 1000 + jlong(time.tv_usec / 1000);
}
linux，参见os_linux.cpp
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);
}
solaris，参见os_solaris.cpp
// Must return millis since Jan 1 1970 for JVM_CurrentTimeMillis
jlong os::javaTimeMillis() {
timeval t;
if (gettimeofday(&t, NULL) == -1) {
fatal("os::javaTimeMillis: gettimeofday (%s)", os::strerror(errno));
}
return jlong(t.tv_sec) * 1000 + jlong(t.tv_usec) / 1000;
}
windows，参见os_windows.cpp
jlong os::javaTimeMillis() {
if (UseFakeTimers) {
return fake_time++;
} else {
FILETIME wt;
GetSystemTimeAsFileTime(&wt);
return windows_to_java_time(wt);
}
}
代码实验
以linux平台为例，测试gettimeofday方法
Test.java
public class Test {
public static void main(String[] args) {
long result = 0L;
for (int i=0; i<10000; i++) {
result += System.currentTimeMillis();
}
System.out.println(result);
}
}
test.c
#include <stdio.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>
int main(int argc, char* argv[])
{
for (int i=0; i<10000; i++)
{
struct timeval tv;
gettimeofday (&tv, NULL);
}
}
运行结果
[caoxudong@localhost ~]$ gcc -std=c99 test.c
[caoxudong@localhost ~]$ javac Test.java
[caoxudong@localhost ~]$ ltrace -c ./a.out
% time seconds usecs/call calls function
------ ----------- ----------- --------- --------------------
51.44 2.830242 2830242 1 __libc_start_main
48.56 2.671857 267 10000 gettimeofday
0.00 0.000101 101 1
0.00 0.000094 94 1 SYS_exit_group
------ ----------- ----------- --------- --------------------
100.00 5.502294 10003 total
[caoxudong@localhost ~]$ ltrace -c java -cp . Test
15046761926697740
% time seconds usecs/call calls function
------ ----------- ----------- --------- --------------------
33.99 10.181938 181820 56 __libc_start_main
33.96 10.174553 181688 56 JLI_Launch
32.04 9.598370 174515 55 SYS_clone
0.01 0.002999 2999 1 SYS_exit_group
0.00 0.000301 100 3
------ ----------- ----------- --------- --------------------
100.00 29.958161 171 total
这里使用ltrace是因为linux支持VDSO之后，gettimeofday属于快速系统调用，使用strace是看不到执行结果的。
如下所示： gettimeofday.c
#include <sys/time.h>
#ifdef SHARED
# include <dl-vdso.h>
# include <errno.h>
static int
__gettimeofday_syscall (struct timeval *tv, struct timezone *tz)
{
return INLINE_SYSCALL (gettimeofday, 2, tv, tz);
}
void *gettimeofday_ifunc (void) __asm__ ("__gettimeofday");
void *
gettimeofday_ifunc (void)
{
PREPARE_VERSION_KNOWN (linux26, LINUX_2_6);
/* If the vDSO is not available we fall back to syscall. */
return (_dl_vdso_vsym ("__vdso_gettimeofday", &linux26)
?: (void*) (&__gettimeofday_syscall));
}
asm (".type __gettimeofday, %gnu_indirect_function");
libc_ifunc_hidden_def(__gettimeofday)
#else
# include <sysdep.h>
# include <errno.h>
int
__gettimeofday (struct timeval *tv, struct timezone *tz)
{
return INLINE_SYSCALL (gettimeofday, 2, tv, tz);
}
libc_hidden_def (__gettimeofday)
#endif
weak_alias (__gettimeofday, gettimeofday)
libc_hidden_weak (gettimeofday)
这里面值得注意的是，从上面的运行结果看到，运行c程序的时候，调用了10000次gettimeofday系统调用，而运行java程序的时候，却没有调用gettimeofday。原因待查。