Java – How to lock a file before writing
Java – How to lock a file before writing
In Java, we can combine
RandomAccessFile and FileChannel to lock a file before writing.
LockFileAndWrite.java
package com.mkyong;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.channels.FileLock;
import java.util.concurrent.TimeUnit;
public class LockFileAndWrite {
public static void main(String[] args) {
writeFileWithLock(new File("D:\\server.log"), "mkyong");
}
public static void writeFileWithLock(File file, String content) {
// auto close and release the lock
try (RandomAccessFile reader = new RandomAccessFile(file, "rw");
FileLock lock = reader.getChannel().lock()) {
// Simulate a 10s locked
TimeUnit.SECONDS.sleep(10);
reader.write(content.getBytes());
} catch (IOException | InterruptedException e) {
e.printStackTrace();
}
}
}
While the file is locked, try to read it in another process:
try {
List<String> lines = Files.readAllLines(Paths.get("D:\\server.log"));
lines.forEach(x -> System.out.println(x));
} catch (IOException e) {
e.printStackTrace();
}
Output
java.io.IOException: The process cannot access the file because another process has locked a portion of the file
at java.base/sun.nio.ch.FileDispatcherImpl.read0(Native Method)
at java.base/sun.nio.ch.FileDispatcherImpl.read(FileDispatcherImpl.java:54)
at java.base/sun.nio.ch.IOUtil.readIntoNativeBuffer(IOUtil.java:276)
at java.base/sun.nio.ch.IOUtil.read(IOUtil.java:245)
at java.base/sun.nio.ch.FileChannelImpl.read(FileChannelImpl.java:223)
at java.base/sun.nio.ch.ChannelInputStream.read(ChannelInputStream.java:65)
at java.base/sun.nio.ch.ChannelInputStream.read(ChannelInputStream.java:109)
at java.base/sun.nio.ch.ChannelInputStream.read(ChannelInputStream.java:103)
at java.base/sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
at java.base/sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
at java.base/sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
at java.base/java.io.InputStreamReader.read(InputStreamReader.java:185)
at java.base/java.io.BufferedReader.fill(BufferedReader.java:161)
at java.base/java.io.BufferedReader.readLine(BufferedReader.java:326)
at java.base/java.io.BufferedReader.readLine(BufferedReader.java:392)
at java.base/java.nio.file.Files.readAllLines(Files.java:3405)
at java.base/java.nio.file.Files.readAllLines(Files.java:3442)
at com.mkyong.TestFile.main(TestFile.java:19)
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