On 12/1/2014 9:16 PM, John wrote:
Hi:
I am reading this
article(http://shipilev.net/blog/2014/safe-public-construction/). It
says the following code is GOOD:
public class SafeDCLFactory {
private volatile Singleton instance;
public Singleton get() {
if (instance == null) { // check 1
synchronized(this) {
if (instance == null) { // check 2
instance = new Singleton();
}
}
}
return instance;
}
}
I feel disagree, by learning from this
article(http://en.wikipedia.org/wiki/Double-checked_locking).
Brian Goetz agrees that this code is incorrect, calling it
"a commonly suggested nonfix." He explains that although the
accesses to `instance' will be consistent because `volatile'
ensures it, any accesses to the member variables of the new
Singleton are *not* consistent (unless they are `volatile', too).
You could get a sequence like this:
Thread T1 finds `instance' null, obtains the lock, finds
that `instance' is still null, and calls the constructor.
The constructor (running in T1) stores initial values in
the member variables of the new Singleton. We presume
that at least some of these variables are not `volatile'.
The constructor finishes, and now T1 stores the new
reference to `instance'. Because `instance' is `volatile',
T1 ensures that the new value is actually flushed from
store buffers and write caches and so on, and appears in
stable memory.
Thread T2 now finds `instance' non-null, and starts using
it to refer to the Singleton's member variables (either
directly or by calling the Singleton's methods).
Unfortunately, the values stored by Singleton's constructor
may still be sitting in caches and what-not, and may not yet
have been flushed to stable memory. Even if the constructor
running in T1 stored 42 in some member variable, T2 may
read the value as zero.
... because there is no "happens-before" between T1's storing
of the value and T2's reading of it.
In short, making sure that `instance' is safe is not sufficient;
you also need to worry about everything `instance' refers to, directly
or indirectly.
http://www.javaworld.com/article/2074979/java-concurrency/double-checked-locking--clever--but-broken.html