Avoid leaking Zookeeper znodes in tryLock

Creating a EPHEMERAL_SEQUENTIAL node was being used to ensure the
Zookeeper connection had been established. These were never explicitly
removed, which led to tryLock effectively leaking zNodes until the
client was terminated. This in turn lead to an extremely large number of
ephemeral nodes being deleted whenever a long running service
terminated!

This fix replaces the creation of a node with a stat of "/" instead.
This has the desired effect of re-establishing the Zookeeper connection,
but is a read-only operation.

* Remove unused import

(cherry picked from commit 47a74ab)

Author: JonathanO

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/lock/ZookeeperLockRegistry.java

@@ -29,7 +29,6 @@
 
 import org.apache.curator.framework.CuratorFramework;
 import org.apache.curator.framework.recipes.locks.InterProcessMutex;
-import org.apache.zookeeper.CreateMode;
 
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.core.task.AsyncTaskExecutor;
@@ -281,28 +280,24 @@ public boolean tryLock() {
 
 		@Override
 		public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
-			Future<String> future = null;
+			Future<Boolean> future = null;
 			try {
 				long startTime = System.currentTimeMillis();
 
-				future = this.mutexTaskExecutor.submit(new Callable<String>() {
+				future = this.mutexTaskExecutor.submit(new Callable<Boolean>() {
 
 					@Override
-					public String call() throws Exception {
-						return ZkLock.this.client.create()
-								.creatingParentContainersIfNeeded()
-								.withProtection()
-								.withMode(CreateMode.EPHEMERAL_SEQUENTIAL)
-								.forPath(ZkLock.this.path);
+					public Boolean call() throws Exception {
+						return ZkLock.this.client.checkExists().forPath("/") != null;
 					}
 
 				});
 
 				long waitTime = unit.toMillis(time);
 
-				String ourPath = future.get(waitTime, TimeUnit.MILLISECONDS);
+				boolean connected = future.get(waitTime, TimeUnit.MILLISECONDS);
 
-				if (ourPath == null) {
+				if (!connected) {
 					future.cancel(true);
 					return false;
 				}