What 0 returned by InputStream.read(byte[]) means? How to handle this?

Question

What 0 (number of bytes read) returned by InputStream.read(byte[]) and InputStream.read(byte[], int, int) means? How to handle this situation?

To be clear, I mean read(byte[] b) or read(byte[] b, int off, int len) methods which return number of bytes read.

Tamqrah

Answer 1

The only situation in which a InputStream may return 0 from a call to read(byte[]) is when the byte[] passed in has a length of 0:

 byte[] buf = new byte[0];
 int read = in.read(buf); // read will contain 0

As specified by this part of the JavaDoc:

If the length of b is zero, then no bytes are read and 0 is returned

My guess: you used available() to see how big the buffer should be and it returned 0. Note that this is a misuse of available(). The JavaDoc explicitly states that:

It is never correct to use the return value of this method to allocate a buffer intended to hold all data in this stream.

Arman

I agree: >This method blocks until input data is available, end of file is detected, or an exception is thrown. >If the length of b is zero, then no bytes are read and 0 is returned; otherwise, there is an attempt to read at least one byte. >If no byte is available because the stream is at the end of the file, the value -1 is returned; otherwise, at least one byte is read and stored into b. Thanks! – Tamqrah

Answer 2

Take a look at the implementation of javax.sound.AudioInputStream#read(byte[] b, int off, int len) ... yuck. They completely violated the standard java.io.InputStream semantics and return a read size of 0 if you request fewer than a whole frame of data.

So unfortunately; the common advice (and api spec) should preclude having to deal with return of zero when len > 0 but even for JDK provided classes you can't universally rely on this to be true for InputStreams of arbitrary types.

Again, yuck.

Ragucci

InputStream from UrlConnection.getInputStream() seems also violated the standard java.io.InputStream semantics and return a read size of 0 when handling large file. Can anyone confirm this? – Woolson

^ Yes, thats confirmed, I came to this question when searching for the same exact problem of UrlConnection.getInputStream read returning a zero 0 – Coparcener

Answer 3

According to Java API Doc:

http://java.sun.com/j2se/1.4.2/docs/api/java/io/InputStream.html#read(byte[])

It only can happen if the byte[] you passed has zero items (new byte[0]).

In other situations it must return at least one byte. Or -1 if EOF reached. Or an exception.

Of course: it depends of the actual implementation of the InputStream you are using!!! (it could be a wrong one)

Quinque

Answer 4

I observed the same behavior (reading 0 bytes) when I build a swing console output window and made a reader-thread for stdout and stderr via the following code:

                this.pi = new PipedInputStream();
                po = new PipedOutputStream((PipedInputStream)pi);
                System.setOut(new PrintStream(po, true));

When the 'main' swing application exits, and my console window is still open I read 0 from this.pi.read(). The read data was put on the console window resulting in a race condition some how, just ignoring the result and not updating the console window solved the issue.

Petry