Pattern: Double checked locking
Issue: -
This rule detects double checked locking, where a lock hint is tested for null before initializing an object within a synchronized block. Double checked locking does not guarantee correctness and is an anti-pattern.
Example of violations:
if (object == null) {
synchronized(this) {
if (object == null) {
// createObject() could be called twice depending
// on the Thread Scheduler.
object = createObject()
}
}
}
// there are several idioms to fix this problem.
def result = object;
if (result == null) {
synchronized(this) {
result = object;
if (result == null)
object = result = createObject()
}
}
// and a better solution for a singleton:
class someClass {
private static class ObjectHolder {
public static Object object = createObject()
}
public static Object getObject() {
return ObjectHolder.object;
}
}