In this post we’ll see how to write a file asynchronously in Java using java.nio.channels.AsynchronousFileChannel class added in Java 7. Using AsynchronousFileChannel class you can create an asynchronous channel for reading, writing, and manipulating a file.
To see how to read a file asynchronously in Java, refer this post- Read File Asynchronously Java Program
Opening an Asynchronous channel
Whether you are reading or writing a file asynchronously first thing you need to do is to create an asynchronous channel. For that you
need to use static open() method of the AsynchronousFileChannel class which opens or creates a file for reading or writing, returning an
asynchronous file channel to access the file.
Following code snippet shows how you can create an asynchronous file channel for writing to a file.
Path path = Paths.get("F:\\netjs\\WriteFile.txt");
AsynchronousFileChannel asyncFileChannel = AsynchronousFileChannel.open(path, StandardOpenOption.WRITE)
Writing file asynchronously using AsynchronousFileChannel class
For writing a file asynchronously there are two versions of write() method in the AsynchronousFileChannel class.
- write() method that writes the passed buffer to the file and returns a
Future representing the result of the write operation.
- write() method where you pass CompletionHandler instance as an argument.
We’ll see examples of of both of these ways to write a file asynchronously to have a better idea.
Writing to a file asynchronously in Java
1. In the first Java program we’ll use the write method that returns a Future.
Future<Integer> write(ByteBuffer src, long position)- This method writes a sequence of bytes to this channel from the passed
buffer, starting at the given file position.
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousFileChannel;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
public class AsyncFileWrite {
public static void main(String[] args) {
Path path = Paths.get("F:\\netjs\\test.txt");
// increase the buffer size if required
ByteBuffer buffer = ByteBuffer.allocate(1024);
// Write Data to buffer
buffer.put("This will be written to a file.".getBytes());
buffer.flip();
try(AsynchronousFileChannel asyncChannel = AsynchronousFileChannel.open(path, StandardOpenOption.WRITE)){
// Write to async channel from buffer
// starting from position 0
Future<Integer> future = asyncChannel.write(buffer, 0);
while(!future.isDone()) {
System.out.println("Waiting for async write operation... ");
// You can do other processing
}
buffer.clear();
System.out.println("Total bytes written- " + future.get());
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ExecutionException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
2. In this Java program for writing file asynchronously we’ll use the write() method where CompletionHandler instance is passed as an argument.
CompletionHandler interface defines two callback methods which you need to implement.
- completed(V result, A attachment)- Invoked when an operation has completed.
- failed(Throwable exc, A attachment)- Invoked when an operation fails.
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousFileChannel;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.nio.channels.CompletionHandler;
public class AsyncFileWrite {
public static void main(String[] args) throws IOException {
Path path = Paths.get("F:\\netjs\\test.txt");
if(!Files.exists(path)){
Files.createFile(path);
}
// increase the buffer size if required
ByteBuffer buffer = ByteBuffer.allocate(1024);
// Write Data to buffer
buffer.put("This will be written to a file.".getBytes());
buffer.flip();
try(AsynchronousFileChannel asyncChannel = AsynchronousFileChannel.open(path, StandardOpenOption.WRITE)){
// Write to channel from buffer, start from position 0
asyncChannel.write(buffer, 0, buffer, new CompletionHandler<Integer, ByteBuffer>() {
@Override
public void completed(Integer result, ByteBuffer attachment) {
System.out.println("Total bytes written- " + result);
}
@Override
public void failed(Throwable ex, ByteBuffer attachment) {
System.out.println("Write operation failed- " + ex.getMessage());
}
});
}
}
}
That's all for this topic Writing a File Asynchronously Java Program. If you have any doubt or any suggestions to make please drop a comment. Thanks!
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