Database Reference
In-Depth Information
try
{
List
<
String
>
children
=
zk
.
getChildren
(
path
,
false
);
if
(
children
.
isEmpty
()) {
System
.
out
.
printf
(
"No members in group %s\n"
,
groupName
);
System
.
exit
(
1
);
}
for
(
String child
:
children
) {
System
.
out
.
println
(
child
);
}
}
catch
(
KeeperException
.
NoNodeException
e
) {
System
.
out
.
printf
(
"Group %s does not exist\n"
,
groupName
);
System
.
exit
(
1
);
}
}
public static
void
main
(
String
[]
args
)
throws
Exception
{
ListGroup listGroup
=
new
ListGroup
();
listGroup
.
connect
(
args
[
0
]);
listGroup
.
list
(
args
[
1
]);
listGroup
.
close
();
}
}
In the
list()
method, we call
getChildren()
with a znode path and a watch flag to
retrieve a list of child paths for the znode, which we print out. Placing a watch on a znode
causes the registered
Watcher
to be triggered if the znode changes state. Although we're
not using it here, watching a znode's children would permit a program to get notifications
of members joining or leaving the group, or of the group being deleted.
We catch
KeeperException.NoNodeException
, which is thrown in the case
when the group's znode does not exist.
Let's see
ListGroup
in action. As expected, the
zoo
group is empty, since we haven't
added any members yet:
%
java ListGroup localhost zoo
No members in group zoo
We can use
JoinGroup
to add some members. We launch them as background pro-
cesses, since they don't terminate on their own (due to the sleep statement):
%
java JoinGroup localhost zoo duck &
%
java JoinGroup localhost zoo cow &
%
java JoinGroup localhost zoo goat &
%
goat_pid=$!
