Read input stream twice

Question

How do you read the same inputstream twice  Is it possible to copy it somehow  I need to get a image from web  save it locally and then return the saved image  I just thought it would be faster to use the same stream instead of starting a new stream to the downloaded content and then read it again

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If you are using an implementation of InputStream  you can check the result of InputStream markSupported   that tell you whether or not you can use the method mark     reset      If you can mark the stream when you read  then call reset   to go back to begin   If you can t you ll have to open a stream again   Another solution would be to convert InputStream to byte array  then iterate over the array as many time as you need  You can find several solutions in this post Convert InputStream to byte array in Java using 3rd party libs or not  Caution  if the read content is too big you might experience some memory troubles   Finally  if your need is to read image  then use    BufferedImage image   ImageIO read new URL  http   www example com images toto jpg       Using ImageIO read java net URL  also allows you to use cache

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In case anyone is running in a Spring Boot app  and you want to read the response body of a RestTemplate  which is why I want to read a stream twice   there is a clean er  way of doing this   First of all  you need to use Spring s StreamUtils to copy the stream to a String   String text   StreamUtils copyToString response getBody    Charset defaultCharset       But that s not all  You also need to use a request factory that can buffer the stream for you  like so   ClientHttpRequestFactory factory   new BufferingClientHttpRequestFactory new SimpleClientHttpRequestFactory     RestTemplate restTemplate   new RestTemplate factory     Or  if you re using the factory bean  then  this is Kotlin but nevertheless     Bean  Scope ConfigurableBeanFactory SCOPE PROTOTYPE  fun createRestTemplate    RestTemplate   RestTemplateBuilder      requestFactory   BufferingClientHttpRequestFactory SimpleClientHttpRequestFactory         additionalInterceptors loggingInterceptor     build     Source  https   objectpartners com 2018 03 01 log-your-resttemplate-request-and-response-without-destroying-the-body

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ByteArrayInputStream ins   new ByteArrayInputStream  quot Hello quot  getBytes     System out println  quot ins available   at begining    quot    ins available     ins mark 0      Read input stream for some operations System out println  quot ins available   after reading     quot    ins available         ins reset        System out println  quot ins available   after resetting     quot    ins available            ins is ready for reading once again

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If you are using RestTemplate to make http calls Simply add an interceptor   Response body is cached by the implementation of ClientHttpResponse   Now  inputstream can be retrieved from respose as many times as we need  ClientHttpRequestInterceptor interceptor    new ClientHttpRequestInterceptor                   Override             public ClientHttpResponse intercept HttpRequest request  byte   body                      ClientHttpRequestExecution execution  throws IOException                   ClientHttpResponse  response   execution execute request  body                         additional work before returning response                   return response                                   Add the interceptor to RestTemplate Instance            restTemplate getInterceptors   add interceptor

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Convert inputstream into bytes and then pass it to savefile function where you assemble the same into inputstream  Also in original function use bytes to use for other tasks

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For splitting an InputStream in two  while avoiding to load all data in memory  and then process them independently    Create a couple of OutputStream  precisely  PipedOutputStream Connect each PipedOutputStream with a PipedInputStream  these PipedInputStream are the returned InputStream  Connect the sourcing InputStream with just created OutputStream  So  everything read it from the sourcing InputStream  would be written in both OutputStream  There is not need to implement that  because it is done already in TeeInputStream  commons io   Within a separated thread read the whole sourcing inputStream  and implicitly the input data is transferred to the target inputStreams          public static final List lt InputStream gt  splitInputStream InputStream input       throws IOException         Objects requireNonNull input              PipedOutputStream pipedOut01   new PipedOutputStream        PipedOutputStream pipedOut02   new PipedOutputStream         List lt InputStream gt  inputStreamList   new ArrayList lt  gt         inputStreamList add new PipedInputStream pipedOut01        inputStreamList add new PipedInputStream pipedOut02         TeeOutputStream tout   new TeeOutputStream pipedOut01  pipedOut02        TeeInputStream tin   new TeeInputStream input  tout  true        Executors newSingleThreadExecutor   submit tin  readAllBytes          return Collections unmodifiableList inputStreamList        Be aware to close the inputStreams after being consumed  and close the thread that runs  TeeInputStream readAllBytes    In case  you need to split it into multiple InputStream  instead of just two  Replace in the previous fragment of code the class TeeOutputStream for your own implementation  which would encapsulate a List lt OutputStream gt  and override the OutputStream interface   public final class TeeListOutputStream extends OutputStream       private final List lt   extends OutputStream gt  branchList       public TeeListOutputStream final List lt   extends OutputStream gt  branchList            Objects requireNonNull branchList           this branchList   branchList              Override     public synchronized void write final int b  throws IOException           for  OutputStream branch   branchList                branch write b                         Override     public void flush   throws IOException           for  OutputStream branch   branchList                branch flush                          Override     public void close   throws IOException           for  OutputStream branch   branchList                branch close

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Depending on where the InputStream is coming from  you might not be able to reset it  You can check if mark   and reset   are supported using markSupported     If it is  you can call reset   on the InputStream to return to the beginning  If not  you need to read the InputStream from the source again

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You can use org apache commons io IOUtils copy to copy the contents of the InputStream to a byte array  and then repeatedly read from the byte array using a ByteArrayInputStream  E g    ByteArrayOutputStream baos   new ByteArrayOutputStream    org apache commons io IOUtils copy in  baos   byte   bytes   baos toByteArray        either while  needToReadAgain        ByteArrayInputStream bais   new ByteArrayInputStream bytes       yourReadMethodHere bais         or ByteArrayInputStream bais   new ByteArrayInputStream bytes   while  needToReadAgain        bais reset        yourReadMethodHere bais

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You can wrap input stream with PushbackInputStream  PushbackInputStream allows to unread   write back   bytes which were already read  so you can do like this   public class StreamTest     public static void main String   args  throws IOException       byte   bytes   new byte     1  2  3  4  5  6  7  8  9         InputStream originalStream   new ByteArrayInputStream bytes        byte   readBytes   getBytes originalStream  3       printBytes readBytes      prints  1 2 3      readBytes   getBytes originalStream  3       printBytes readBytes      prints  4 5 6         now let s wrap it with PushBackInputStream      originalStream   new ByteArrayInputStream bytes        InputStream wrappedStream   new PushbackInputStream originalStream  10      10 means that maximnum 10 characters can be  written back  to the stream      readBytes   getBytes wrappedStream  3       printBytes readBytes      prints 1 2 3        PushbackInputStream  wrappedStream  unread readBytes  0  readBytes length        readBytes   getBytes wrappedStream  3       printBytes readBytes      prints 1 2 3          private static byte   getBytes InputStream is  int howManyBytes  throws IOException       System out print  Reading stream           byte   buf   new byte howManyBytes        int next   0      for  int i   0  i  lt  howManyBytes  i            next   is read          if  next  gt  0            buf i     byte  next                    return buf         private static void printBytes byte   buffer  throws IOException       System out print  Reading stream           for  int i   0  i  lt  buffer length  i            System out print buffer i                    System out println              Please note that PushbackInputStream stores internal buffer of bytes so it really creates a buffer in memory which holds bytes  written back    Knowing this approach we can go further and combine it with FilterInputStream  FilterInputStream stores original input stream as a delegate  This allows to create new class definition which allows to  unread  original data automatically  The definition of this class is following   public class TryReadInputStream extends FilterInputStream     private final int maxPushbackBufferSize             Creates a  lt code gt FilterInputStream lt  code gt      by assigning the  argument  lt code gt in lt  code gt      to the field  lt code gt this in lt  code gt  so as     to remember it for later use           param in the underlying input stream  or  lt code gt null lt  code gt  if               this instance is to be created without an underlying stream         public TryReadInputStream InputStream in  int maxPushbackBufferSize        super new PushbackInputStream in  maxPushbackBufferSize        this maxPushbackBufferSize   maxPushbackBufferSize                  Reads from input stream the  lt code gt length lt  code gt  of bytes to given buffer  The read bytes are still avilable      in the stream            param buffer the destination buffer to which read the data       param offset  the start offset in the destination  lt code gt buffer lt  code gt        aram length how many bytes to read from the stream to buff  Length needs to be less than              lt code gt maxPushbackBufferSize lt  code gt  or IOException will be thrown            return number of bytes read       throws java io IOException in case length is         public int tryRead byte   buffer  int offset  int length  throws IOException       validateMaxLength length           NOTE  below reading byte by byte instead of  int bytesRead   is read firstBytes  0  maxBytesOfResponseToLog           because read   guarantees to read a byte      int bytesRead   0       int nextByte   0       for  int i   0   i  lt  length   amp  amp   nextByte  gt   0   i            nextByte   read          if  nextByte  gt   0            buffer offset   bytesRead       byte  nextByte                     if  bytesRead  gt  0            PushbackInputStream  in  unread buffer  offset  bytesRead              return bytesRead          public byte   tryRead int maxBytesToRead  throws IOException       validateMaxLength maxBytesToRead        ByteArrayOutputStream baos   new ByteArrayOutputStream       as ByteArrayOutputStream to dynamically allocate internal bytes array instead of allocating possibly large buffer  if maxBytesToRead is large          NOTE  below reading byte by byte instead of  int bytesRead   is read firstBytes  0  maxBytesOfResponseToLog           because read   guarantees to read a byte      int nextByte   0       for  int i   0   i  lt  maxBytesToRead   amp  amp   nextByte  gt   0   i            nextByte   read          if  nextByte  gt   0            baos write  byte  nextByte                      byte   buffer   baos toByteArray         if  buffer length  gt  0            PushbackInputStream  in  unread buffer  0  buffer length              return buffer          private void validateMaxLength int length  throws IOException       if  length  gt  maxPushbackBufferSize          throw new IOException           Trying to read more bytes than maxBytesToRead  Max bytes      maxPushbackBufferSize      Trying to read              length                  This class has two methods  One for reading into existing buffer  defintion is analogous  to calling public int read byte b    int off  int len  of InputStream class   Second which returns new buffer  this may be more effective if the size of buffer to read is unknown     Now let s see our class in action   public class StreamTest2     public static void main String   args  throws IOException       byte   bytes   new byte     1  2  3  4  5  6  7  8  9         InputStream originalStream   new ByteArrayInputStream bytes        byte   readBytes   getBytes originalStream  3       printBytes readBytes      prints  1 2 3      readBytes   getBytes originalStream  3       printBytes readBytes      prints  4 5 6         now let s use our TryReadInputStream      originalStream   new ByteArrayInputStream bytes        InputStream wrappedStream   new TryReadInputStream originalStream  10        readBytes     TryReadInputStream  wrappedStream  tryRead 3      NOTE  no manual call to  unread     because TryReadInputStream handles this internally     printBytes readBytes      prints 1 2 3      readBytes     TryReadInputStream  wrappedStream  tryRead 3        printBytes readBytes      prints 1 2 3      readBytes     TryReadInputStream  wrappedStream  tryRead 3       printBytes readBytes      prints 1 2 3         we can also call normal read which will actually read the bytes without  writing them back      readBytes   getBytes wrappedStream  3       printBytes readBytes      prints 1 2 3      readBytes   getBytes wrappedStream  3       printBytes readBytes      prints 4 5 6      readBytes     TryReadInputStream  wrappedStream  tryRead 3      now we can try read next bytes     printBytes readBytes      prints 7 8 9      readBytes     TryReadInputStream  wrappedStream  tryRead 3        printBytes readBytes      prints 7 8 9

User · Answer

if your InputStream support using mark  then you can mark   your inputStream and then reset   it   if your InputStrem doesn t support mark then you can use the class java io BufferedInputStream so you can embed your stream inside a BufferedInputStream like this      InputStream bufferdInputStream   new BufferedInputStream yourInputStream       bufferdInputStream mark some value         read your bufferdInputStream      bufferdInputStream reset          read it again

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How about   if  stream markSupported      false                lets replace the stream object         ByteArrayOutputStream baos   new ByteArrayOutputStream            IOUtils copy stream  baos           stream close            stream   new ByteArrayInputStream baos toByteArray                now the stream should support  mark  and  reset

[java] Read input stream twice

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