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Standard concise way to copy a file in Java?

Posted by: admin November 2, 2017 Leave a comment

Questions:

It has always bothered me that the only way to copy a file in Java involves opening streams, declaring a buffer, reading in one file, looping through it, and writing it out to the other steam. The web is littered with similar, yet still slightly different implementations of this type of solution.

Is there a better way that stays within the bounds of the Java language (meaning does not involve exec-ing OS specific commands)? Perhaps in some reliable open source utility package, that would at least obscure this underlying implementation and provide a one line solution?

Answers:

As toolkit mentions above, Apache Commons IO is the way to go, specifically FileUtils.copyFile(); it handles all the heavy lifting for you.

And as a postscript, note that recent versions of FileUtils (such as the 2.0.1 release) have added the use of NIO for copying files; NIO can significantly increase file-copying performance, in a large part because the NIO routines defer copying directly to the OS/filesystem rather than handle it by reading and writing bytes through the Java layer. So if you’re looking for performance, it might be worth checking that you are using a recent version of FileUtils.

Questions:
Answers:

I would avoid the use of a mega api like apache commons. This is a simplistic operation and its built into the JDK in the new NIO package. It was kind of already linked to in a previous answer, but the key method in the NIO api are the new functions “transferTo” and “transferFrom”.

http://java.sun.com/javase/6/docs/api/java/nio/channels/FileChannel.html#transferTo(long,%20long,%20java.nio.channels.WritableByteChannel)

One of the linked articles shows a great way on how to integrate this function into your code, using the transferFrom:

public static void copyFile(File sourceFile, File destFile) throws IOException {
    if(!destFile.exists()) {
        destFile.createNewFile();
    }

    FileChannel source = null;
    FileChannel destination = null;

    try {
        source = new FileInputStream(sourceFile).getChannel();
        destination = new FileOutputStream(destFile).getChannel();
        destination.transferFrom(source, 0, source.size());
    }
    finally {
        if(source != null) {
            source.close();
        }
        if(destination != null) {
            destination.close();
        }
    }
}

Learning NIO can be a little tricky, so you might want to just trust in this mechanic before going off and trying to learn NIO overnight. From personal experience it can be a very hard thing to learn if you don’t have the experience and were introduced to IO via the java.io streams.

Questions:
Answers:

Now with Java 7, you can use the following try-with-resource syntax:

public static void copyFile( File from, File to ) throws IOException {

    if ( !to.exists() ) { to.createNewFile(); }

    try (
        FileChannel in = new FileInputStream( from ).getChannel();
        FileChannel out = new FileOutputStream( to ).getChannel() ) {

        out.transferFrom( in, 0, in.size() );
    }
}

Or, better yet, this can also be accomplished using the new Files class introduced in Java 7:

public static void copyFile( File from, File to ) throws IOException {
    Files.copy( from.toPath(), to.toPath() );
}

Pretty snazzy, eh?

Questions:
Answers:
  • These methods are performance-engineered (they integrate with operating system native I/O).
  • These methods work with files, directories and links.
  • Each of the options supplied may be left out – they are optional.

The utility class

package com.yourcompany.nio;

class Files {

    static int copyRecursive(Path source, Path target, boolean prompt, CopyOptions options...) {
        CopyVisitor copyVisitor = new CopyVisitor(source, target, options).copy();
        EnumSet<FileVisitOption> fileVisitOpts;
        if (Arrays.toList(options).contains(java.nio.file.LinkOption.NOFOLLOW_LINKS) {
            fileVisitOpts = EnumSet.noneOf(FileVisitOption.class) 
        } else {
            fileVisitOpts = EnumSet.of(FileVisitOption.FOLLOW_LINKS);
        }
        Files.walkFileTree(source[i], fileVisitOpts, Integer.MAX_VALUE, copyVisitor);
    }

    private class CopyVisitor implements FileVisitor<Path>  {
        final Path source;
        final Path target;
        final CopyOptions[] options;

        CopyVisitor(Path source, Path target, CopyOptions options...) {
             this.source = source;  this.target = target;  this.options = options;
        };

        @Override
        FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {
        // before visiting entries in a directory we copy the directory
        // (okay if directory already exists).
        Path newdir = target.resolve(source.relativize(dir));
        try {
            Files.copy(dir, newdir, options);
        } catch (FileAlreadyExistsException x) {
            // ignore
        } catch (IOException x) {
            System.err.format("Unable to create: %s: %s%n", newdir, x);
            return SKIP_SUBTREE;
        }
        return CONTINUE;
    }

    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
        Path newfile= target.resolve(source.relativize(file));
        try {
            Files.copy(file, newfile, options);
        } catch (IOException x) {
            System.err.format("Unable to copy: %s: %s%n", source, x);
        }
        return CONTINUE;
    }

    @Override
    public FileVisitResult postVisitDirectory(Path dir, IOException exc) {
        // fix up modification time of directory when done
        if (exc == null && Arrays.toList(options).contains(COPY_ATTRIBUTES)) {
            Path newdir = target.resolve(source.relativize(dir));
            try {
                FileTime time = Files.getLastModifiedTime(dir);
                Files.setLastModifiedTime(newdir, time);
            } catch (IOException x) {
                System.err.format("Unable to copy all attributes to: %s: %s%n", newdir, x);
            }
        }
        return CONTINUE;
    }

    @Override
    public FileVisitResult visitFileFailed(Path file, IOException exc) {
        if (exc instanceof FileSystemLoopException) {
            System.err.println("cycle detected: " + file);
        } else {
            System.err.format("Unable to copy: %s: %s%n", file, exc);
        }
        return CONTINUE;
    }
}

Copying a directory or file

long bytes = java.nio.file.Files.copy( 
                 new java.io.File("<filepath1>").toPath(), 
                 new java.io.File("<filepath2>").toPath(),
                 java.nio.file.StandardCopyOption.REPLACE_EXISTING,
                 java.nio.file.StandardCopyOption.COPY_ATTRIBUTES,
                 java.nio.file.LinkOption.NOFOLLOW_LINKS);

Moving a directory or file

long bytes = java.nio.file.Files.move( 
                 new java.io.File("<filepath1>").toPath(), 
                 new java.io.File("<filepath2>").toPath(),
                 java.nio.file.StandardCopyOption.ATOMIC_MOVE,
                 java.nio.file.StandardCopyOption.REPLACE_EXISTING);

Copying a directory or file recursively

long bytes = com.yourcompany.nio.Files.copyRecursive( 
                 new java.io.File("<filepath1>").toPath(), 
                 new java.io.File("<filepath2>").toPath(),
                 java.nio.file.StandardCopyOption.REPLACE_EXISTING,
                 java.nio.file.StandardCopyOption.COPY_ATTRIBUTES
                 java.nio.file.LinkOption.NOFOLLOW_LINKS );

Questions:
Answers:

In Java 7 it is easy…

File src = new File("original.txt");
File target = new File("copy.txt");

Files.copy(src.toPath(), target.toPath(), StandardCopyOption.REPLACE_EXISTING);

Questions:
Answers:

Note that all of these mechanisms only copy the contents of the file, not the metadata such as permissions. So if you were to copy or move an executable .sh file on linux the new file would not be executable.

In order to truly a copy or move a file, ie to get the same result as copying from a command line, you actually need to use a native tool. Either a shell script or JNI.

Apparently, this might be fixed in java 7 – http://today.java.net/pub/a/today/2008/07/03/jsr-203-new-file-apis.html. Fingers crossed!

Questions:
Answers:

To copy a file and save it to your destination path you can use the method below.

public void copy(File src, File dst) throws IOException {
    InputStream in = new FileInputStream(src);
    try {
        OutputStream out = new FileOutputStream(dst);
        try {
            // Transfer bytes from in to out
            byte[] buf = new byte[1024];
            int len;
            while ((len = in.read(buf)) > 0) {
                out.write(buf, 0, len);
            }
        } finally {
            out.close();
        }
    } finally {
        in.close();
    }
}

Questions:
Answers:

Google’s Guava library also has a copy method:


public static void copy(File from,
File to)
throws IOException


Copies all the bytes from one file to another.

Warning: If to represents an existing file, that file
will be overwritten with the contents of from. If to and
from refer to the same file, the contents of that file
will be deleted.



Parameters:from – the source fileto – the destination file


Throws:
IOException – if an I/O error occurs
IllegalArgumentException – if from.equals(to)



Questions:
Answers:

Available as standard in Java 7, path.copyTo:
http://openjdk.java.net/projects/nio/javadoc/java/nio/file/Path.html
http://java.sun.com/docs/books/tutorial/essential/io/copy.html

I can’t believe it took them so long to standardise something so common and simple as file copying 🙁

Questions:
Answers:

Here is three ways that you can easily copy files with single line of code!

Java7:

java.nio.file.Files#copy

private static void copyFileUsingJava7Files(File source, File dest) throws IOException {
    Files.copy(source.toPath(), dest.toPath());
}

Appache Commons IO:

FileUtils#copyFile

private static void copyFileUsingApacheCommonsIO(File source, File dest) throws IOException {
    FileUtils.copyFile(source, dest);
}

Guava :

Files#copy

private static void copyFileUsingGuava(File source,File dest) throws IOException{
    Files.copy(source,dest);          
}

Questions:
Answers:

Three possible problems with the above code:

  1. If getChannel throws an exception, you might leak an open stream.
  2. For large files, you might be trying to transfer more at once than the OS can handle.
  3. You are ignoring the return value of transferFrom, so it might be copying just part of the file.

This is why org.apache.tools.ant.util.ResourceUtils.copyResource is so complicated. Also note that while transferFrom is OK, transferTo breaks on JDK 1.4 on Linux (see Bug ID:5056395) – Jesse Glick Jan

Questions:
Answers:

If you are in a web application which already uses Spring and if you do not want to include Apache Commons IO for simple file copying, you can use FileCopyUtils of the Spring framework.

Questions:
Answers:
public static void copyFile(File src, File dst) throws IOException
{
    long p = 0, dp, size;
    FileChannel in = null, out = null;

    try
    {
        if (!dst.exists()) dst.createNewFile();

        in = new FileInputStream(src).getChannel();
        out = new FileOutputStream(dst).getChannel();
        size = in.size();

        while ((dp = out.transferFrom(in, p, size)) > 0)
        {
            p += dp;
        }
    }
    finally {
        try
        {
            if (out != null) out.close();
        }
        finally {
            if (in != null) in.close();
        }
    }
}

Questions:
Answers:

Fast and work with all the versions of Java also Android:

private void copy(final File f1, final File f2) throws IOException {
    f2.createNewFile();

    final RandomAccessFile file1 = new RandomAccessFile(f1, "r");
    final RandomAccessFile file2 = new RandomAccessFile(f2, "rw");

    file2.getChannel().write(file1.getChannel().map(FileChannel.MapMode.READ_ONLY, 0, f1.length()));

    file1.close();
    file2.close();
}

Questions:
Answers:

NIO copy with a buffer is the fastest according to my test. See the working code below from a test project of mine at https://github.com/mhisoft/fastcopy

import java.io.Closeable;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.text.DecimalFormat;


public class test {

private static final int BUFFER = 4096*16;
static final DecimalFormat df = new DecimalFormat("#,###.##");
public static void nioBufferCopy(final File source, final File target )  {
    FileChannel in = null;
    FileChannel out = null;
    double  size=0;
    long overallT1 =  System.currentTimeMillis();

    try {
        in = new FileInputStream(source).getChannel();
        out = new FileOutputStream(target).getChannel();
        size = in.size();
        double size2InKB = size / 1024 ;
        ByteBuffer buffer = ByteBuffer.allocateDirect(BUFFER);

        while (in.read(buffer) != -1) {
            buffer.flip();

            while(buffer.hasRemaining()){
                out.write(buffer);
            }

            buffer.clear();
        }
        long overallT2 =  System.currentTimeMillis();
        System.out.println(String.format("Copied %s KB in %s millisecs", df.format(size2InKB),  (overallT2 - overallT1)));
    }
    catch (IOException e) {
        e.printStackTrace();
    }

    finally {
        close(in);
        close(out);
    }
}

private static void close(Closeable closable)  {
    if (closable != null) {
        try {
            closable.close();
        } catch (IOException e) {
            if (FastCopy.debug)
                e.printStackTrace();
        }    
    }
}

}