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

Question

When I read the source code from java.io.BufferedInputStream.getInIfOpen(), I am confused about why it wrote code like this:

/**
 * Check to make sure that underlying input stream has not been
 * nulled out due to close; if not return it;
 */
private InputStream getInIfOpen() throws IOException {
    InputStream input = in;
    if (input == null)
        throw new IOException("Stream closed");
    return input;
}

Why does it use the alias instead of use the field variable in directly like below:

/**
 * Check to make sure that underlying input stream has not been
 * nulled out due to close; if not return it;
 */
private InputStream getInIfOpen() throws IOException {
    if (in == null)
        throw new IOException("Stream closed");
    return in;
}

Can someone give a reasonable explanation?

Answer 1

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.