#21 Bad singleton patern

[Gabarren]

Hello,

In XMLUtils.java we found

public static XMLUtils getInstance() {
if (self == null) {
synchronized(XMLUtils.class) {
self = new XMLUtils();
}
}
return self;
}

instead of the right singleton patern (with double check)

public static XMLUtils getInstance() {
if (self == null) {
synchronized(XMLUtils.class) {
if (self == null) {
self = new XMLUtils();
}
}
}
return self;
}

Without this double-check, if a 2nd thread (or more) is
waiting on synchronize during the 1st thread (the
instancier's one), it will instance an other class (on
more).

Have fun and thanks for your good job,
Jef

[David McReynolds]

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user_id=683632

I wouldn't waste my time with DCL.

"There's only one problem with it -- it doesn't work. Why
not? The most obvious reason is that the writes which
initialize instance and the write to the instance field can
be reordered by the compiler or the cache, which would have
the effect of returning what appears to be a partially
constructed Something. The result would be that we read an
uninitialized object. There are lots of other reasons why
this is wrong, and why algorithmic corrections to it are
wrong. There is no way to fix it using the old Java memory
model."
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html