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POHow to properly extend Java FilterOutputStream class?
 

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  1. POHow to properly extend Java FilterOutputStream class?
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    <p>I would like to roughly monitor the progress of a file upload. I know that I can override the MultipartEntity and make the writeTo(OutputStream out) method write to a FilterOutputStream class that I created to wrap the default InputStream. For full details on how I did that, see my answer <a href="https://stackoverflow.com/questions/254719/file-upload-with-java-with-progress-bar/3154929#3154929">here</a>. </p> <p>However, upon closer inspection this counts every byte sent twice! I went to the documentation to see what was up. Looks like FilterOutputStream's <a href="http://java.sun.com/j2se/1.4.2/docs/api/java/io/FilterOutputStream.html#write(byte[],%20int,%20int)" rel="nofollow noreferrer">write(byte[], int, int)</a> method simply calls the FilterOutputStream's write(byte) method in a loop. It recommends subclasses to provide a more efficient method. I would assume this to involve calling the underlying OutputStream's write(byte[], int, int) and hoping that the underlying OutputStream has a better method of pushing bytes onto the stream (doc's recommend subclasses of OutputStream override this method and do a better job than simply looping over the OutputStream#write(byte) method). </p> <p>This is where I find myself in a predicament. I can't guarantee that the MultipartEntity#writeTo(OutputStream) will always result in a call to OutputStream.write(byte[],int,int), so if I count the bytes sent there then I might miss some that are sent using the write(byte) method. However, I can't count in the write(byte) method, because the OutputStream.write(byte[],int,int) method may never call the write(byte) method. </p> <p>One answer is to call super.write(byte[],int,int) inside of my subclass's write(byte[],int,int) method. Then, I know that this will simply loop over the write(byte) method, writing one byte at a time. I can then count all bytes written inside of the write(byte) method. However, this is inefficient, and the docs directly recommend against it. I am sure that some of the subclasses of OutputStream manage to write multiple bytes to the stream at once, so it's silly not to use that advantage. </p> <p>So, how do I properly override FilterOutputStream to be both efficient and count all bytes sent? </p> <p>Sorry if this is long, I have made it a wiki in case anyone can describe the issue better than I. </p>
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    1. COhttp://www.karlin.mff.cuni.cz/network/prirucky/javatut/java/io/writingFiltered.html
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