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.

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.

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.

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)

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.