Re: Mutable Objects and Thread Boundaries
On Wed, 21 Jul 2010 01:22:11 -0700, markspace wrote:
Peter Duniho wrote:
markspace wrote:
Joshua Cranmer wrote:
Immutability is the easiest way to guarantee safe publication: final
fields guarantee that they can be used by any thread safely.
Note quite. Final fields by themselves don't guarantee immutability
or safe publication. An immutable object must:
* All fields are final.
* It is properly constructed. (No "this escapes." That bit is
important.)
* Its state cannot be modified after construction.
Those three things must hold for an object to be treated as immutable
by the JVM. Mess up any one, and poof, no more immutability.
I would be interested in seeing an example where your third point is
violated but the first point is not.
class Mutable {
private final List<String> names;
public Mutable() {
names = new List<String>();
names.add("Joe");
names.add("Jim");
}
public List<String> getNames() {
return names;
}
}
This class would be immutable except for the fact that the private final
variable "names" escapes and could be modified.
That, and the fact that it won't compile. :-)
Try this version for immutable goodness:
public class Immutable {
private final List<String> names;
public Mutable() {
List<String> n = new LinkedList<String>();
n.add("Joe");
n.add("Jim");
names = Collections.unmodifiableList(n);
}
public List<String> getNames() {
return names;
}
}