madrob commented on a change in pull request #1358: SOLR-14256: replaced EMPTY
with empty() to fix deadlock
URL: https://github.com/apache/lucene-solr/pull/1358#discussion_r394061330
##########
File path: solr/core/src/java/org/apache/solr/search/DocSet.java
##########
@@ -36,6 +36,21 @@
assert this instanceof BitDocSet || this instanceof SortedIntDocSet;
}
+ // we can't simply use a trivial static initializer "= new SortedIntDocSet"
because it can lead to classloader deadlock
+ private static DocSet EMPTY;
+
+ /** An empty instance. */
+ public static DocSet empty() {
Review comment:
I'll refer to the much wiser than myself, Josh Bloch (emphasis mine) -
> In summary, you should initialize most fields normally, not lazily. If you
must initialize a field lazily in order to... break a harmful initialization
circularity, then use the appropriate lazy initialization technique. For
instance fields, it is the double-check idiom; **for static fields, the lazy
initialization holder class idiom**. For instance fields that can tolerate
repeated initialization, you may also consider the single-check idiom.
There's actually a lot more nuance in the surrounding paragraphs in the
book, most of which I don't understand, but the summary appears to suggest that
this is the fastest, simplest way.
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