Java FileLock for Reading and Writing
I have a process that will be called rather frequently from cron to read a file that has certain move related commands in it. My process needs to read and write to this data file - and keep it locked to prevent other processes from touching it during this time. A completely separate process can be executed by a user to (potential) write/append to this same data file. I want these two processes to play nice and only access the file one at a time.
The nio FileLock seemed to be what I needed (short of writing my own semaphore type files), but I'm having trouble locking it for reading. I can lock and write just fine, but when attempting to create lock when reading I get a NonWritableChannelException. Is it even possible to lock a file for reading? Seems like a RandomAccessFile is closer to what I need, but I don't see how to implement that.
Here is the code that fails:
FileInputStream fin = new FileInputStream(f);
FileLock fl = fin.getChannel().tryLock();
if(fl != null)
{
System.out.println("Locked File");
BufferedReader in = new BufferedReader(new InputStreamReader(fin));
System.out.println(in.readLine());
...
The exception is thrown on the FileLock line.
java.nio.channels.NonWritableChannelException
at sun.nio.ch.FileChannelImpl.tryLock(Unknown Source)
at java.nio.channels.FileChannel.tryLock(Unknown Source)
at Mover.run(Mover.java:74)
at java.lang.Thread.run(Unknown Source)
Looking at the JavaDocs, it says
Unchecked开发者_运维知识库 exception thrown when an attempt is made to write to a channel that was not originally opened for writing.
But I don't necessarily need to write to it. When I try creating a FileOutpuStream, etc. for writing purposes it is happy until I try to open a FileInputStream on the same file.
- Are you aware that locking the file won't keep other processes from touching it unless they also use locks?
- You have to lock via a writable channel. Get the lock via a
RandomAccessFile
in "rw" mode and then open yourFileInputStream
. Make sure to close both!
It would be better if you created the lock using tryLock(0L, Long.MAX_VALUE, true)
.
This creates a shared lock which is the right thing to do for reading.
tryLock()
is a shorthand for tryLock(0L, Long.MAX_VALUE, false)
, i.e. it requests an exclusive write-lock.
I wrote a test program and bash commands to confirm the effectivity of the file lock:
import java.io.File;
import java.io.RandomAccessFile;
import java.nio.channels.FileLock;
public class FileWriterTest
{
public static void main(String[] args) throws Exception
{
if (args.length != 4)
{
System.out.println("Usage: FileWriterTest <filename> <string> <sleep ms> <enable lock>");
System.exit(1);
}
String filename = args[0];
String data = args[1];
int sleep = Integer.parseInt(args[2]);
boolean enableLock = Boolean.parseBoolean(args[3]);
try (RandomAccessFile raFile = new RandomAccessFile(new File(filename), "rw"))
{
FileLock lock = null;
if (enableLock)
{
lock = raFile.getChannel().lock();
}
Thread.sleep(sleep);
raFile.seek(raFile.length());
System.out.println("writing " + data + " in a new line; current pointer = " + raFile.getFilePointer());
raFile.write((data+"\n").getBytes());
if (lock != null)
{
lock.release();
}
}
}
}
Run with this bash command to check it works:
for i in {1..1000}
do
java FileWriterTest test.txt $i 10 true &
done
You should see the writing only happening once every 10ms (from the outputs), and in the end all numbers to be present in the file.
Output:
/tmp wc -l test.txt
1000 test.txt
/tmp
The same test without the lock shows data being lost:
for i in {1..1000}
do
java FileWriterTest test.txt $i 10 false &
done
Output:
/tmp wc -l test.txt
764 test.txt
/tmp
It should be easy to modify it to test the tryLock instead.
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