Why does BufferedInputStream copy a field to a local variable rather than use the field directly

If you look at this code out of context there is no good explanation for that “alias”. It is simply redundant code or poor code style.

But the context is that BufferedInputStream is a class that can be subclassed, and that it needs to work in a multi-threaded context.

The clue is that in is declared in FilterInputStream is protected volatile. That means that there is a chance that a subclass could reach in and assign null to in. Given that possibility, the “alias” is actually there to prevent a race condition.

Consider the code without the “alias”

private InputStream getInIfOpen() throws IOException {
    if (in == null)
        throw new IOException("Stream closed");
    return in;
}

1. Thread A calls getInIfOpen()
2. Thread A evaluates in == null and sees that in is not null.
3. Thread B assigns null to in.
4. Thread A executes return in. Which returns null because a is a volatile.

The “alias” prevents this. Now in is read just once by thread A. If thread B assigns null after thread A has in it doesn’t matter. Thread A will either throw an exception or return a (guaranteed) non-null value.