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Tip of the Day
Language: Java
Expertise: Beginner
Apr 5, 1999

Unix select Equivalent

Question:
Is there any Java facility like the Unix select() system call that will wait until there is something to read from a file descriptor before the programmer does the read?

Answer:
The closest things to the select system call in Java are the InputStream.available() and Reader.ready(). The available() method returns the number of bytes that can be read from the stream without blocking. However, it is not always implemented correctly in subclasses making it unreliable. The ready() method returns true if the next read won't block, but says nothing about how much can be read. The problem with these methods is that they only operate on a single input source. The select system call simultaneously tests many file descriptors. To simulate select would require a chain of if statements, which might not be the most efficient thing. For a small number of streams it is often desirable to do the I/O in separate threads. After all, select is used to multiplex I/O. That's what threads are good for.

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