Data can be read from a file, or written to a file in following ways:
– File read using Files.lines method
– File read using Files.readAllLines method
– File read using Files.readAllBytes method
– Write bytes or multiple lines using Files.writeFile method
– Lock file for write
Example 1: File read using Files.lines method
Files.lines method reads all lines in a file as a Stream.
Stream is populated lazily as consumed, hence it is also suitable for large files.
import java.io.IOException;
|import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.charset.StandardCharsets;
import java.util.stream.Collectors;
private void readFileAsString()
{
try ( Stream<String> stream = Files.lines( Paths.get( “F:/work/test/sample.txt” ), StandardCharsets.UTF_8 ) )
{
String content = stream.collect( Collectors.joining( System.lineSeparator() ) );
System.out.printf( “content: %s”, content );
}
catch ( IOException e )
{
e.printStackTrace();
}
}
Output:
content: Line1
Line2
Line3
Example 2: File read using Files.readAllLines method
Files.readAllLines method reads a file and returns a list of string.
It reads all data into memory, hence it is not recommended for large files.
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.charset.StandardCharsets;
import java.util.stream.Collectors;
private void readFileAsString2()
{
try
{
List<String> str = Files.readAllLines( Paths.get( “F:/work/test/sample.txt” ), StandardCharsets.UTF_8 );
String content = str.stream().collect( Collectors.joining( System.lineSeparator() ) ); System.out.printf( “content: %s”, content );
}
catch( IOException e )
{
e.printStackTrace();
}
}
Example 3: File read using Files.readAllBytes method
Files.readAllBytes method reads a file and returns a byte array.
It reads all data into memory, hence it is not recommended for large files.
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
private byte[] readFileAsBytes()
{
byte[] bytes = null;
try
{
bytes = Files.readAllBytes( Paths.get( “F:/work/test/sample.txt” ) );
}
catch( IOException e )
{
e.printStackTrace();
}
return bytes;
}
Example 4: Write bytes or multiple lines using Files.writeFile method
Files.writeFile method is used to write byte array into a file.
CREATE option creates file is it doesn’t exist. Use APPEND option to append bytes to an existing file, else data will get overwritten.
If a string has to be written, then convert it into byte array before passing it into this method.
System.getProperty(“line.separator”) has to be added where necessary.
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.Arrays;
import java.util.List;
private void writeMultipleLinesInFile()
{
List<String> data = Arrays.asList( “Line1” + System.getProperty(“line.separator”),
“Line2” + System.getProperty(“line.separator”), “Line3” );
data.forEach( str ->
{
try
{
Files.write( Paths.get( “F:/work/test/write_multiple_lines.txt” ), str.getBytes( StandardCharsets.UTF_8 ), StandardOpenOption.CREATE, StandardOpenOption.APPEND );
}
catch( IOException e )
{
e.printStackTrace();
}
});
}
Output
File created with the below content:
Line1
Line2
Line3
Example 5: Lock file for write
In this example, file is exclusively locked, and released only after writing all lines to the file.
CREATE option creates file is it doesn’t exist. Use APPEND option to append bytes to an existing file, else data will get overwritten.
System.getProperty(“line.separator”) has to be added where necessary.
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
private void lockFileAndWrite()
{
ByteBuffer[] byteBuffers = { ByteBuffer.wrap( ( “Line1” + System.getProperty(“line.separator”) ).getBytes() ), ByteBuffer.wrap( ( “Line2” + System.getProperty(“line.separator”) ).getBytes() ), ByteBuffer.wrap( ( “Line3” + System.getProperty(“line.separator”) ).getBytes() ) };
try ( FileChannel fileChannel = FileChannel.open( Paths.get( “F:/work/test/sample_lock.txt” ), StandardOpenOption.CREATE, StandardOpenOption.APPEND );
FileLock fileLock = fileChannel.lock() )
{
fileChannel.write( byteBuffers );
}
catch ( IOException e )
{
e.printStackTrace();
}
}
Output
File created with the below content:
Line1
Line2
Line3