ZooKeeper - Hadoop: The Definitive Guide - page 761

KeeperException {
byte [] data = zk . getData ( path , watcher , null /*stat*/ );
return new String ( data , CHARSET );
}
The getData() method of ZooKeeper takes the path, a Watcher , and a Stat object.
The Stat object is filled in with values by getData() and is used to pass information
back to the caller. In this way, the caller can get both the data and the metadata for a
znode, although in this case, we pass a null Stat because we are not interested in the
metadata.
As a consumer of the service, ConfigWatcher (see Example 21-7 ) creates an Act-
iveKeyValueStore and, after starting, calls the store's read() method (in its dis-
playConfig() method) to pass a reference to itself as the watcher. It displays the ini-
tial value of the configuration that it reads.
Example 21-7. An application that watches for updates of a property in ZooKeeper and
prints them to the console
public class ConfigWatcher implements Watcher {
private ActiveKeyValueStore store ;
public ConfigWatcher ( String hosts ) throws IOException ,
InterruptedException {
store = new ActiveKeyValueStore ();
store . connect ( hosts );
}
public void displayConfig () throws InterruptedException ,
KeeperException {
String value = store . read ( ConfigUpdater . PATH , this );
System . out . printf ( "Read %s as %s\n" , ConfigUpdater . PATH , value );
}
@Override
public void process ( WatchedEvent event ) {
if ( event . getType () == EventType . NodeDataChanged ) {
try {
displayConfig ();
} catch ( InterruptedException e ) {
System . err . println ( "Interrupted. Exiting." );
Thread . currentThread (). interrupt ();
} catch ( KeeperException e ) {
System . err . printf ( "KeeperException: %s. Exiting.\n" , e );
}