Read entire file in Scala

Question

What s a simple and canonical way to read an entire file into memory in Scala    Ideally  with control over character encoding    The best I can come up with is   scala io Source fromPath  file txt   getLines reduceLeft        or am I supposed to use one of Java s god-awful idioms  the best of which  without using an external library  seems to be   import java util Scanner import java io File new Scanner new File  file txt    useDelimiter    Z   next     From reading mailing list discussions  it s not clear to me that scala io Source is even supposed to be the canonical I O library   I don t understand what its intended purpose is  exactly       I d like something dead-simple and easy to remember   For example  in these languages it s very hard to forget the idiom      Ruby    open  file txt   read Ruby    File read  file txt   Python  open  file txt   read

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If you don t mind a third-party dependency  you should consider using my OS-Lib library  This makes reading writing files and working with the filesystem very convenient      Make sure working directory exists and is empty val wd   os pwd  out   splash  os remove all wd  os makeDir all wd      Read write files os write wd  file txt    hello   os read wd  file txt      gt   hello      Perform filesystem operations os copy wd  file txt   wd  copied txt   os list wd     gt  Seq wd  copied txt   wd  file txt     with one-line helpers for reading bytes  reading chunks  reading lines  and many other useful common operations

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Java 8  import java nio charset StandardCharsets import java nio file  Files  Paths   val path   Paths get  quot file txt quot   new String Files readAllBytes path   StandardCharsets UTF 8   Java 11  import java nio charset StandardCharsets import java nio file  Files  Path   val path   Path of  quot file txt quot   Files readString path  StandardCharsets UTF 8   These offer control over character encoding  and no resources to clean up  It s also faster than other patterns  e g  getLines   mkString  quot  n quot    due to more efficient allocation patterns

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You can use Source fromFile fileName  getLines   mkString  however it should be noticed that getLines   removes all new line characters  If you want save formatting you should use Source fromFile fileName  iter mkString

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print every line  like use Java BufferedReader read ervery line  and print it   scala io Source fromFile  test txt    foreach   print      equivalent   scala io Source fromFile  test txt    foreach  x   gt  print x

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import scala io source object ReadLine  def main args Array String    if  args length gt 0   for  line  lt - Source fromLine args 0   getLine    println line        in arguments you can give file path and it will return all lines

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For faster overall reading   uploading a  large  file  consider increasing the size of bufferSize  Source DefaultBufSize set to 2048   for instance as follows   val file   new java io File  myFilename   io Source fromFile file  bufferSize   Source DefaultBufSize   2    Note Source scala  For further discussion see Scala fast text file read and upload to memory

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I ve been told that Source fromFile is problematic  Personally  I have had problems opening large files with Source fromFile and have had to resort to Java InputStreams   Another interesting solution is using scalax  Here s an example  of some well commented code that opens a log file using ManagedResource to open a file with scalax helpers  http   pastie org pastes 420714

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You do not need to parse every single line and then concatenate them again     Source fromFile path  Codec UTF8  mkString   I prefer to use this   import scala io  BufferedSource  Codec  Source  import scala util Try  def readFileUtf8 path  String   Try String    Try     val source  BufferedSource   Source fromFile path  Codec UTF8    val content   source mkString   source close     content

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Just like in Java  using CommonsIO library   FileUtils readFileToString file  StandardCharsets UTF 8    Also  many answers here forget Charset  It s better to always provide it explicitly  or it will hit one day

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as a few people mentioned scala io Source is best to be avoided due to connection leaks    Probably scalax and pure java libs like commons-io are the best options until the new incubator project  ie scala-io  gets merged

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For emulating Ruby syntax  and convey the semantics  of opening and reading a file  consider this implicit class  Scala 2 10 and upper    import java io File  def open filename  String    new File filename   implicit class RichFile val file  File  extends AnyVal     def read   io Source fromFile file  getLines mkString   n       In this way   open  file txt   read

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EDIT  This does not work in scala 2 9 and maybe not 2 8 either   Use trunk   scala gt  io File   etc passwd   slurp res0  String         User Database        etc

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for file with utf-8 encoding val lines   scala io Source fromFile  file txt    utf-8   getLines mkString

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Using getLines   on scala io Source discards what characters were used for line terminators   n   r   r n  etc    The following should preserve it character-for-character  and doesn t do excessive string concatenation  performance problems    def fileToString file  File  encoding  String        val inStream   new FileInputStream file    val outStream   new ByteArrayOutputStream   try       var reading   true     while   reading           inStream read   match           case -1   gt  reading   false         case c   gt  outStream write c                    outStream flush         finally       inStream close         new String outStream toByteArray    encoding

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val lines   scala io Source fromFile  file txt   mkString   By the way   scala   isn t really necessary  as it s always in scope anyway  and you can  of course  import io s contents  fully or partially  and avoid having to prepend  io   too   The above leaves the file open  however  To avoid problems  you should close it like this   val source   scala io Source fromFile  file txt   val lines   try source mkString finally source close     Another problem with the code above is that it is horrible slow due to its implementation nature  For larger files one should use   source getLines mkString   n

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One more  https   github com pathikrit better-files streams-and-codecs  Various ways to slurp a file without loading the contents into memory   val bytes    Iterator Byte               file bytes val chars    Iterator Char               file chars val lines    Iterator String             file lines val source   scala io BufferedSource     file content    You can supply your own codec too for anything that does a read write  it assumes scala io Codec default if you don t provide one    val content  String   file contentAsString     default codec    custom codec  import scala io Codec file contentAsString Codec ISO8859    or import scala io Codec string2codec file write  hello world   codec    US-ASCII

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you can also use Path from scala io to read and process files   import scalax file Path   Now you can get file path using this -   val filePath   Path  path of file to b read        val lines   file lines includeTerminator   true    You can also Include terminators but by default it is set to false

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The obvious question being  why do you want to read in the entire file   This is obviously not a scalable solution if your files get very large  The scala io Source gives you back an Iterator String  from the getLines method  which is very useful and concise   It s not much of a job to come up with an implicit conversion using the underlying java IO utilities to convert a File  a Reader or an InputStream to a String  I think that the lack of scalability means that they are correct not to add this to the standard API

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Just to expand on Daniel s solution  you can shorten things up tremendously by inserting the following import into any file which requires file manipulation   import scala io Source     With this  you can now do   val lines   fromFile  file txt   getLines   I would be wary of reading an entire file into a single String   It s a very bad habit  one which will bite you sooner and harder than you think   The getLines method returns a value of type Iterator String    It s effectively a lazy cursor into the file  allowing you to examine just the data you need without risking memory glut   Oh  and to answer your implied question about Source  yes  it is the canonical I O library   Most code ends up using java io due to its lower-level interface and better compatibility with existing frameworks  but any code which has a choice should be using Source  particularly for simple file manipulation

[scala] Read entire file in Scala?

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