How to clone an InputStream

Question

I have a InputStream that I pass to a method to do some processing  I will use the same InputStream in other method  but after the first processing  the InputStream appears be closed inside the method   How I can clone the InputStream to send to the method that closes him  There is another solution   EDIT  the methods that closes the InputStream is an external method from a lib  I dont have control about closing or not   private String getContent HttpURLConnection con        InputStream content   null      String charset           try           content   con getInputStream            CloseShieldInputStream csContent   new CloseShieldInputStream content           charset   getCharset csContent                       return  IOUtils toString content charset         catch  Exception e            System out println  Error downloading page      e           return null           private String getCharset InputStream content        try           Source parser   new Source content           return parser getEncoding          catch  Exception e            System out println  Error determining charset      e           return  UTF-8

User · Answer

UPD  Check the comment before  It isn t exactly what was asked  If you are using apache commons you may copy streams using IOUtils   You can use following code  InputStream   IOUtils toBufferedInputStream toCopy    Here is the full example suitable for your situation  public void cloneStream   throws IOException      InputStream toCopy IOUtils toInputStream  quot aaa quot        InputStream dest  null      dest IOUtils toBufferedInputStream toCopy       toCopy close        String result   new String IOUtils toByteArray dest        System out println result      This code requires some dependencies  MAVEN  lt dependency gt       lt groupId gt commons-io lt  groupId gt       lt artifactId gt commons-io lt  artifactId gt       lt version gt 2 4 lt  version gt   lt  dependency gt   GRADLE  commons-io commons-io 2 4   Here is the DOC reference for this method   Fetches entire contents of an InputStream and represent same data as result InputStream  This method is useful where  Source InputStream is slow  It has network resources associated  so we cannot keep it open for long time  It has network timeout associated   You can find more about IOUtils here  http   commons apache org proper commons-io javadocs api-2 4 org apache commons io IOUtils html toBufferedInputStream java io InputStream

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You want to use Apache s CloseShieldInputStream    This is a wrapper that will prevent the stream from being closed  You d do something like this   InputStream is   null   is   getStream      obtain the stream  CloseShieldInputStream csis   new CloseShieldInputStream is       call the bad function that does things it shouldn t badFunction csis       happiness follows  do something with the original input stream is read

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If the data read from the stream is large  I would recommend using a TeeInputStream from Apache Commons IO  That way you can essentially replicate the input and pass a t d pipe as your clone

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This might not work in all situations  but here is what I did  I extended the FilterInputStream class and do the required processing of the bytes as the external lib reads the data     public class StreamBytesWithExtraProcessingInputStream extends FilterInputStream        protected StreamBytesWithExtraProcessingInputStream InputStream in            super in               Override     public int read   throws IOException           int readByte   super read            processByte readByte           return readByte              Override     public int read byte   buffer  int offset  int count  throws IOException           int readBytes   super read buffer  offset  count           processBytes buffer  offset  readBytes           return readBytes             private void processBytes byte   buffer  int offset  int readBytes           for  int i   0  i  lt  readBytes  i                 processByte buffer i   offset                        private void processByte int readByte              TODO do processing here            Then you simply pass an instance of StreamBytesWithExtraProcessingInputStream where you would have passed in the input stream  With the original input stream as constructor parameter   It should be noted that this works byte for byte  so don t use this if high performance is a requirement

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Below is the solution with Kotlin   You can copy your InputStream into ByteArray  val inputStream        val byteOutputStream   ByteArrayOutputStream   inputStream use   input - gt      byteOutputStream use   output - gt          input copyTo output           val byteInputStream   ByteArrayInputStream byteOutputStream toByteArray      If you need to read the byteInputStream multiple times  call byteInputStream reset   before reading again   https   code luasoftware com tutorials kotlin how-to-clone-inputstream

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If all you want to do is read the same information more than once  and the input data is small enough to fit into memory  you can copy the data from your InputStream to a ByteArrayOutputStream  Then you can obtain the associated array of bytes and open as many  quot cloned quot  ByteArrayInputStreams as you like  ByteArrayOutputStream baos   new ByteArrayOutputStream        Code simulating the copy    You could alternatively use NIO    And please  unlike me  do something about the Exceptions  D byte   buffer   new byte 1024   int len  while   len   input read buffer    gt  -1         baos write buffer  0  len     baos flush            Open new InputStreams using recorded bytes    Can be repeated as many times as you wish InputStream is1   new ByteArrayInputStream baos toByteArray      InputStream is2   new ByteArrayInputStream baos toByteArray       But if you really need to keep the original stream open to receive new data  then you will need to track the external call to close    You will need to prevent close   from being called somehow  UPDATE  2019   Since Java 9 the the middle bits can be replaced with InputStream transferTo  ByteArrayOutputStream baos   new ByteArrayOutputStream    input transferTo baos   InputStream firstClone   new ByteArrayInputStream baos toByteArray      InputStream secondClone   new ByteArrayInputStream baos toByteArray

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You can t clone it  and how you are going to solve your problem depends on what the source of the data is    One solution is to read all data from the InputStream into a byte array  and then create a ByteArrayInputStream around that byte array  and pass that input stream into your method   Edit 1  That is  if the other method also needs to read the same data  I e you want to  reset  the stream

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The class below should do the trick  Just create an instance  call the  multiply  method  and provide the source input stream and the amount of duplicates you need   Important  you must consume all cloned streams simultaneously in separate threads   package foo bar   import java io IOException  import java io InputStream  import java io PipedInputStream  import java io PipedOutputStream  import java util concurrent ExecutorService  import java util concurrent Executors   public class InputStreamMultiplier       protected static final int BUFFER SIZE   1024      private ExecutorService executorService   Executors newCachedThreadPool         public InputStream   multiply final InputStream source  int count  throws IOException           PipedInputStream   ins   new PipedInputStream count           final PipedOutputStream   outs   new PipedOutputStream count            for  int i   0  i  lt  count  i                          ins i    new PipedInputStream                outs i    new PipedOutputStream ins i                       executorService execute new Runnable                 public void run                     try                       copy source  outs                     catch  IOException e                        e printStackTrace                                                         return ins             protected void copy final InputStream source  final PipedOutputStream   outs  throws IOException           byte   buffer   new byte BUFFER SIZE           int n   0          try               while  -1     n   source read buffer                        write each chunk to all output streams                 for  PipedOutputStream out   outs                        out write buffer  0  n                                             finally                 close all output streams             for  PipedOutputStream out   outs                    try                       out close                      catch  IOException e                        e printStackTrace

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Cloning an input stream might not be a good idea  because this requires deep knowledge about the details of the input stream being cloned  A workaround for this is to create a new input stream that reads from the same source again   So using some Java 8 features this would look like this   public class Foo        private Supplier lt InputStream gt  inputStreamSupplier       public void bar             procesDataThisWay inputStreamSupplier get             procesDataTheOtherWay inputStreamSupplier get                private void procesDataThisWay InputStream                              private void procesDataTheOtherWay InputStream                             This method has the positive effect that it will reuse code that is already in place - the creation of the input stream encapsulated in inputStreamSupplier  And there is no need to maintain a second code path for the cloning of the stream   On the other hand  if reading from the stream is expensive  because a it s done over a low bandwith connection   then this method will double the costs  This could be circumvented by using a specific supplier that will store the stream content locally first and provide an InputStream for that now local resource

[java] How to clone an InputStream?

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