Standard concise way to copy a file in Java

Question

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

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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

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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    gt  0                    out write buf  0  len                           finally               out close                    finally           in close

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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

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Three possible problems with the above code     If getChannel throws an exception  you might leak an open stream   For large files  you might be trying to transfer more at once than the OS can handle    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

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Google s Guava library also has a copy method    public static void copy File nbsp from                          File nbsp 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

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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    gt  0                        p    dp                      finally           try                       if  out    null  out close                      finally               if  in    null  in close

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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

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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

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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

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A little late to the party  but here is a comparison of the time taken to copy a file using various file copy methods  I looped in through the methods for 10 times and took an average  File transfer using IO streams seem to be the worst candidate     Here are the methods   private static long fileCopyUsingFileStreams File fileToCopy  File newFile  throws IOException       FileInputStream input   new FileInputStream fileToCopy       FileOutputStream output   new FileOutputStream newFile       byte   buf   new byte 1024       int bytesRead      long start   System currentTimeMillis        while   bytesRead   input read buf    gt  0                output write buf  0  bytesRead             long end   System currentTimeMillis         input close        output close         return  end-start      private static long fileCopyUsingNIOChannelClass File fileToCopy  File newFile  throws IOException       FileInputStream inputStream   new FileInputStream fileToCopy       FileChannel inChannel   inputStream getChannel         FileOutputStream outputStream   new FileOutputStream newFile       FileChannel outChannel   outputStream getChannel         long start   System currentTimeMillis        inChannel transferTo 0  fileToCopy length    outChannel       long end   System currentTimeMillis         inputStream close        outputStream close         return  end-start      private static long fileCopyUsingApacheCommons File fileToCopy  File newFile  throws IOException       long start   System currentTimeMillis        FileUtils copyFile fileToCopy  newFile       long end   System currentTimeMillis        return  end-start      private static long fileCopyUsingNIOFilesClass File fileToCopy  File newFile  throws IOException       Path source   Paths get fileToCopy getPath         Path destination   Paths get newFile getPath         long start   System currentTimeMillis        Files copy source  destination  StandardCopyOption REPLACE EXISTING       long end   System currentTimeMillis         return  end-start       The only drawback what I can see while using NIO channel class is that I still can t seem to find a way to show intermediate file copy progress

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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

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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

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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

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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 lt FileVisitOption gt  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 lt Path gt             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  amp  amp  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   lt filepath1 gt    toPath                      new java io File   lt filepath2 gt    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   lt filepath1 gt    toPath                      new java io File   lt filepath2 gt    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   lt filepath1 gt    toPath                      new java io File   lt filepath2 gt    toPath                     java nio file StandardCopyOption REPLACE EXISTING                   java nio file StandardCopyOption COPY ATTRIBUTES                  java nio file LinkOption NOFOLLOW LINKS

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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

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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

[java] Standard concise way to copy a file in Java?

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