Read part of an InputStream twice

Question

I have a ServletInputStream that can be very big and I want to extract the first X bytes of the InputStream and then let the stream in it initial state.

What I have done for now is markSupported(), mark() and reset() but the markSupported return false so I need to implement an other way of doing it.

A solution is described here to read an input stream twice, but the problem is that my stream can be very big in size and I can't have all of it in memory (moreover i am not sure that the max array size will be enough).

Is there a way to just read a small number of bytes and then put the stream in it initial state. The workaround will be to consume the X bytes I want to read an then let the stream consumed pass X bytes in addition to the following process (which I want to avoid).

What about reading all the `ServletInputStream` and saving it to a file on the local host? — Abra, Mar 13 '20 at 14:31
Can you wrap the input stream in a BufferedInputStream? Then you could mark the the stream for resets. — Ralf, Mar 13 '20 at 14:31
Abra that is not an option. And Ralf thanks, I have done that — bloub, Mar 16 '20 at 09:44

score 2 · Accepted Answer · answered Mar 13 '20 at 14:54

2

BufferedInputStream.markSupported() returns true (see Javadoc). Simply wrap your stream with BufferedInputStream and set a mark limit bigger than X.

answered Mar 13 '20 at 14:54

Pino

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score 0 · Answer 2 · answered Mar 13 '20 at 15:22

Have you looked at java.io.PushbackInputStream?

If I am understanding you correctly, it seems like a good fit for what you want to achieve, especially if the bytes you want to examine are at the beginning of the stream.

byte[] peekBuffer = new byte[n];
PushbackInputStream pis = new PushbackInputStream(yourStream, peekBuffer.length);

pis.read(peekBuffer);
// Examine peekBuffer

// Reinsert the peeked bytes.
pis.unread(peekBuffer);

Read part of an InputStream twice

2 Answers2