Get file mtime with millisecond resolution from Java

问题

When I read the mtime of a file from Java using Files.getLastModifiedTime, the return value is truncated to whole seconds. I know this works on other systems to get mtimes with millisecond resolution, so what could be different about mine?

Here is a complete standalone test which compiles and runs:

import java.nio.file.attribute.FileTime;
import java.nio.file.Files;
import java.nio.file.Paths;
public class Test {
  public static void main(String[] args) throws java.io.IOException {
    FileTime timestamp = Files.getLastModifiedTime(Paths.get("/tmp/test"));
    System.out.println(timestamp.toMillis());
  }
}

The output is (with my particular test file) 1405602038000, whereas ls shows:

$ ls --full-time /tmp/test                                                                                                                                                                                                    
-rw-rw-r-- 1 daniel daniel 0 2014-07-17 16:00:38.413008992 +0300 /tmp/test

I would expect the Java output to be 1405602038413.

I'm running on Linux with ext4. I tried both openjdk 1.7 and Oracle jdk 1.8.

回答1:

The ability to get file timestamps on *nix systems with higher precision was added in Java 8 by this commit, however, on the native side, it requires POSIX 2008 compliance:

#if (_POSIX_C_SOURCE >= 200809L) || defined(__solaris__)
    (*env)->SetLongField(env, attrs, attrs_st_atime_nsec, (jlong)buf->st_atim.tv_nsec);
    (*env)->SetLongField(env, attrs, attrs_st_mtime_nsec, (jlong)buf->st_mtim.tv_nsec);
    (*env)->SetLongField(env, attrs, attrs_st_ctime_nsec, (jlong)buf->st_ctim.tv_nsec);
#endif

And, apparently, the Java build you are using doesn't set it, so nanoseconds part of the timestamp is not available and stays zero.

回答2:

I looked at the source code:

In the Java 7 case, the method gives the last modified timestamp with 1 second precision:

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7u40-b43/sun/nio/fs/UnixFileAttributes.java#UnixFileAttributes.lastModifiedTime%28%29

In the Java 8 case, it looks like it should give microsecond precision:

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8-b132/sun/nio/fs/UnixFileAttributes.java#UnixFileAttributes.lastModifiedTime%28%29

But in either case, the code doesn't seem to provide a way to get the timestamp with a different precision.

回答3:

It seems to be fixed in java 9

Oracle jdk 9.0.4 - Files.getLastModifiedTime gives millisecond resolution
Oracle jdk 1.8.162 - Files.getLastModifiedTime gives second resolution

回答4:

You can use the simple date format to display time is milliseconds:

java.nio.file.attribute.FileTime time = java.nio.file.Files.getLastModifiedTime(java.nio.file.Paths.get("/tmp/test"))
java.text.SimpleDateFormat dateformat = new java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
System.out.println(dateformat.format(time.toMillis()));

来源：https://stackoverflow.com/questions/24804618/get-file-mtime-with-millisecond-resolution-from-java

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