如何刷新'RandomAccessFile'（java）？

Question

I'm using RandomAccessFile in java:

file = new RandomAccessFile(filename, "rw");
...
file.writeBytes(...);

How can I ensure that this data is flushed to the Operating System? There is no file.flush() method. (Note that I don't actually expect it to be physically written, I'm content with it being flushed to the operating system, so that the data will survive a tomcat crash but not necessarily an unexpected server power loss).

I'm using tomcat6 on Linux.

Answer 1

The only classes that provide a .flush() method are those that actually maintain their own buffers. As java.io.RandomAccessFile does not itself maintain a buffer, it does not need to be flushed.

Answer 2

Have a look carefully at RandomAccessFile constructor javadoc:

The "rws" and "rwd" modes work much like the force(boolean) method of the FileChannel class, passing arguments of true and false, respectively, except that they always apply to every I/O operation and are therefore often more efficient. If the file resides on a local storage device then when an invocation of a method of this class returns it is guaranteed that all changes made to the file by that invocation will have been written to that device . This is useful for ensuring that critical information is not lost in the event of a system crash. If the file does not reside on a local device then no such guarantee is made.

Answer 3

您可以使用getFD().sync()方法。

Answer 4

here's what i do in my app:

rf.close();
rf = new RandomAccessFile("mydata", "rw");

this is give 3-4times gain in performance compared to getFd().sync() and 5-7 times compared to "rws' mode

deoes exactly what the original question proposed: passes on unsaved data to the OS and out of JVM. Doesn't physically write to disk, and therefore introduces no annoying delays

Answer 5

I reached here with the very same curiosity.

And I really can't figure what need to flush on OS and not necessarily need to flush to Disk part means.

In my opinion,

The best thing matches to the concept of a managed flushing is getFD().sync() , as @AVD said,

try(RandomAccessFile raw = new RandomAccessFile(file, "rw")) {
    raw.write...
    raw.write...
    raw.getFD().sync();
    raw.wirte...
}

which looks like, by its documentation, it works very much like what FileChannel#force(boolean) does with true .

Now "rws" and "rwd" are look like they work as if specifying StandardOpenOption#SYNC and StandardOpenOption#DSYNC respectively while a FileChannel is open .

try(RandomAccessFile raw = new RandomAccessFile(file, "rws")) {
    raw.write...
    raw.write...
    raw.wirte...
    // don't worry be happy, woo~ hoo~ hoo~
}

Answer 6

I learned that you can't..

Some related links here: http://www.cs.usfca.edu/~parrt/course/601/lectures/io.html and here: http://tutorials.jenkov.com/java-io/bufferedwriter.html