private volatile static Singleton uniqueInstance
In a singleton when using double lock method for synchronization why is the single instance declared as volatile ? Can I achieve the same functionality without declaring it as volatile ?
Without volatile
the code doesn't work correctly with multiple threads.
From Wikipedia's Double-checked locking:
As of J2SE 5.0, this problem has been fixed. The volatile keyword now ensures that multiple threads handle the singleton instance correctly. This new idiom is described in The "Double-Checked Locking is Broken" Declaration:
// Works with acquire/release semantics for volatile
// Broken under Java 1.4 and earlier semantics for volatile
class Foo {
private volatile Helper helper = null;
public Helper getHelper() {
Helper result = helper;
if (result == null) {
synchronized(this) {
result = helper;
if (result == null) {
helper = result = new Helper();
}
}
}
return result;
}
// other functions and members...
}
In general you should avoid double-check locking if possible, as it is difficult to get right and if you get it wrong it can be difficult to find the error. Try this simpler approach instead:
If the helper object is static (one per class loader), an alternative is the initialization on demand holder idiom
// Correct lazy initialization in Java
@ThreadSafe
class Foo {
private static class HelperHolder {
public static Helper helper = new Helper();
}
public static Helper getHelper() {
return HelperHolder.helper;
}
}