openJDK上有完整的JDK源代码,JDK源代码由C++、Java、C、汇编 这四种语言组成。JVM主体是C++写的,JNI部分是C,工具类是Java写的,JVM里混有汇编代码。路径:openjdk-7-fcs-src-b147\jdk\src\share\native\java\lang\System.c 找到这个
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/* Only register the performance-critical methods */
static JNINativeMethod methods[] = {
{ "currentTimeMillis" , "()J" , ( void *)&JVM_CurrentTimeMillis},
{ "nanoTime" , "()J" , ( void *)&JVM_NanoTime},
{ "arraycopy" , "(" OBJ "I" OBJ "II)V" , ( void *)&JVM_ArrayCopy},
};
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方法的实现应该在JVM部分,路径openjdk-7-fcs-src-b147\hotspot\src\share,不同的操作系统实现不一样,应该是在对应的操作系统的包下,我在windows目录下找到了纳秒的实现
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jlong os::javaTimeNanos() {
if (!has_performance_count) {
return javaTimeMillis() * NANOS_PER_MILLISEC; // the best we can do.
} else {
LARGE_INTEGER current_count;
QueryPerformanceCounter(¤t_count);
double current = as_long(current_count);
double freq = performance_frequency;
jlong time = (jlong)((current/freq) * NANOS_PER_SEC);
return time ;
}
}
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这个就是windows下的C API函数。