How can I check if an InputStream is empty without reading from it

Question

I want to know if an InputStream is empty  but without using the method read    Is there a way to know if it s empty without reading from it

User · Answer

If the InputStream you re using supports mark reset support  you could also attempt to read the first byte of the stream and then reset it to its original position   input mark 1   final int bytesRead   input read new byte 1    input reset    if  bytesRead    -1          stream not empty   else         stream empty      If you don t control what kind of InputStream you re using  you can use the markSupported   method to check whether mark reset will work on the stream  and fall back to the available   method or the java io PushbackInputStream method otherwise

User · Answer

Based on the suggestion of using the PushbackInputStream  you ll find an exemple implementation here           author Lorber Sebastien  lt i gt  lorber sebastien gmail com  lt  i gt      public class NonEmptyInputStream extends FilterInputStream               Once this stream has been created  do not consume the original InputStream       because there will be one missing byte          param originalInputStream       throws IOException       throws EmptyInputStreamException         public NonEmptyInputStream InputStream originalInputStream  throws IOException  EmptyInputStreamException       super  checkStreamIsNotEmpty originalInputStream                      Permits to check the InputStream is empty or not      Please note that only the returned InputStream must be consummed            see       http   stackoverflow com questions 1524299 how-can-i-check-if-an-inputstream-is-empty-without-reading-from-it            param inputStream       return         private static InputStream checkStreamIsNotEmpty InputStream inputStream  throws IOException  EmptyInputStreamException       Preconditions checkArgument inputStream    null  The InputStream is mandatory        PushbackInputStream pushbackInputStream   new PushbackInputStream inputStream       int b      b   pushbackInputStream read        if   b    -1           throw new EmptyInputStreamException  No byte can be read from stream     inputStream             pushbackInputStream unread b       return pushbackInputStream         public static class EmptyInputStreamException extends RuntimeException       public EmptyInputStreamException String message          super message                  And here are some passing tests      Test expected   EmptyInputStreamException class    public void test check empty input stream raises exception for empty stream   throws IOException       InputStream emptyStream   new ByteArrayInputStream new byte 0        new NonEmptyInputStream emptyStream           Test   public void test check empty input stream ok for non empty stream and returned stream can be consummed fully   throws IOException       String streamContent    HELLoooo   w  rld       InputStream inputStream   IOUtils toInputStream streamContent  StandardCharsets UTF 8       inputStream   new NonEmptyInputStream inputStream       assertThat IOUtils toString inputStream StandardCharsets UTF 8   isEqualTo streamContent

User · Answer

I think you are looking for inputstream available    It does not tell you whether its empty but it can give you an indication as to whether data is there to be read or not

User · Answer

You can use the available   method to ask the stream whether there is any data available at the moment you call it  However  that function isn t guaranteed to work on all types of input streams  That means that you can t use available   to determine whether a call to read   will actually block or not

User · Answer

public void run         byte   buffer   new byte 256         int bytes                             while  true            try               bytes   mmInStream read buffer               mHandler obtainMessage RECIEVE MESSAGE  bytes  -1  buffer  sendToTarget              catch  IOException e                break

User · Answer

How about using inputStreamReader ready   to find out   import java io InputStreamReader            InputStreamReader reader   new InputStreamReader inputStream   if  reader ready             do something

User · Answer

No  you can t  InputStream is designed to work with remote resources  so you can t know if it s there until you actually read from it   You may be able to use a java io PushbackInputStream  however  which allows you to read from the stream to see if there s something there  and then  push it back  up the stream  that s not how it really works  but that s the way it behaves to client code

User · Answer

Without reading you can t do this  But you can use a work around like below   You can use mark   and reset   methods to do this   mark int readlimit  method marks the current position in this input stream   reset   method repositions this stream to the position at the time the mark method was last called on this input stream   Before you can use the mark and reset  you need to test out whether these operations are supported on the inputstream you   re reading off  You can do that with markSupported   The mark method accepts an limit  integer   which denotes the maximum number of bytes that are to be read ahead  If you read more than this limit  you cannot return to this mark   To apply this functionalities for this use case   we need to mark the position as 0 and then read the input stream  There after we need to reset the input stream and the input stream will be reverted to the original one       if  inputStream markSupported                inputStream mark 0             if  inputStream read      -1                   inputStream reset                else                    Inputstream is empty                     Here if the input stream is empty then read   method will return -1

[java] How can I check if an InputStream is empty without reading from it?

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