Including Dependencies for Java Clients

You should include two dependencies in your classpath to start using the Hazelcast client: hazelcast.jar and hazelcast-client.jar.

After adding these dependencies, you can start using the Hazelcast client as if you are using the Hazelcast API. The differences are discussed in the below sections.

If you prefer to use maven, add the following lines to your pom.xml.


Getting Started with Client API

The first step is configuration. You can configure the Java client declaratively or programmatically. We will use the programmatic approach throughout this tutorial. Please refer to the Java Client Declarative Configuration section for details.

ClientConfig clientConfig = new ClientConfig();
clientConfig.getNetworkConfig().addAddress("", "");

The second step is to initialize the HazelcastInstance to be connected to the cluster.

HazelcastInstance client = HazelcastClient.newHazelcastClient(clientConfig);

This client interface is your gateway to access all Hazelcast IMDG distributed objects.

Let's create a map and populate it with some data.

IMap<String, Customer> mapCustomers = client.getMap("customers"); //creates the map proxy

mapCustomers.put("1", new Customer("Joe", "Smith"));
mapCustomers.put("2", new Customer("Ali", "Selam"));
mapCustomers.put("3", new Customer("Avi", "Noyan"));

As a final step, if you are done with your client, you can shut it down as shown below. This will release all the used resources and will close connections to the cluster.


Java Client Operation Modes

The client has two operation modes because of the distributed nature of the data and cluster.

Smart Client: In smart mode, clients connect to each cluster member. Since each data partition uses the well known and consistent hashing algorithm, each client can send an operation to the relevant cluster member, which increases the overall throughput and efficiency. When the relied cluster member dies, the client will transparently switch to another live member. Smart mode is the default mode.

Dummy Client: For some cases, the clients can be required to connect to a single member instead of to each member in the cluster. Firewalls, security, or some custom networking issues can be the reason for these cases.

In dummy client mode, the client will only connect to one of the configured addresses. This single member will behave as a gateway to the other members. For any operation requested from the client, it will redirect the request to the relevant member and return the response back to the client returned from this member.

Handling Failures

There are two main failure cases you should be aware of, and configurations you can perform to achieve proper behavior.

Handling Client Connection Failure

While the client is trying to connect initially to one of the members in the ClientNetworkConfig.addressList, all the members might be not available. Instead of giving up, throwing an exception and stopping the client, the client will retry as many as connectionAttemptLimit times.

You can configure connectionAttemptLimit for the number of times you want the client to retry connecting. Please see Setting Connection Attempt Limit.

The client executes each operation through the already established connection to the cluster. If this connection(s) disconnects or drops, the client will try to reconnect as configured.

Handling Retry-able Operation Failure

While sending the requests to related members, operations can fail due to various reasons. Read-only operations are retried by default. If you want to enable retry for the other operations, set the redoOperation to true. Please see Enabling Redo Operation.

You can set a timeout for retrying the operations sent to a member. This can be provided by using the property hazelcast.client.invocation.timeout.seconds in ClientProperties. The client will retry an operation within this given period, of course, if it is a read-only operation or you enabled the redoOperation as stated in the above paragraph. This timeout value is important when there is a failure resulted by either of the following causes:

  • Member throws an exception.
  • Connection between the client and member is closed.
  • Client's heartbeat requests are timed out.

Please see the System Properties - Client section.

Using Supported Distributed Data Structures

Most of the Distributed Data Structures are supported by the Java client. When you use clients in other languages, you should check for the exceptions.

As a general rule, you configure these data structures on the server side and access them through a proxy on the client side.

Using Map with Java Client

You can use any distributed map object with the client, as shown below.

Imap<Integer, String> map = client.getMap(“myMap”);

map.put(1, “Ali”);
String value= map.get(1);

Locality is ambiguous for the client, so addLocalEntryListener and localKeySet are not supported. Please see the Distributed Map section for more information.

Using MultiMap with Java Client

A MultiMap usage example is shown below.

MultiMap<Integer, String> multiMap = client.getMultiMap("myMultiMap");


Collection<String> values = multiMap.get(1);

addLocalEntryListener, localKeySet and getLocalMultiMapStats are not supported because locality is ambiguous for the client. Please see the Distributed MultiMap section for more information.

Using Queue with Java Client

A sample usage is shown below.

IQueue<String> myQueue = client.getQueue(“theQueue”);

getLocalQueueStats is not supported because locality is ambiguous for the client. Please see the Distributed Queue section for more information.

Using Topic with Java Client

getLocalTopicStats is not supported because locality is ambiguous for the client.

Using Other Supported Distributed Structures

The distributed data structures listed below are also supported by the client. Since their logic is the same in both the member side and client side, you can refer to their sections as listed below.

  • Replicated Map
  • MapReduce
  • List
  • Set
  • IAtomicLong
  • IAtomicReference
  • ICountDownLatch
  • ISemaphore
  • IdGenerator
  • Lock

Using Client Services

Hazelcast provides the services discussed below for some common functionalities on the client side.

Using Distributed Executor Service

The distributed executor service is for distributed computing. It can be used to execute tasks on the cluster on a designated partition or on all the partitions. It can also be used to process entries. Please see the Distributed Executor Service section for more information.

IExecutorService executorService = client.getExecutorService("default");

After getting an instance of IExecutorService, you can use the instance as the interface with the one provided on the server side. Please see the Distributed Computing chapter for detailed usage.

image NOTE: This service is only supported by the Java client.

Listening to Client Connection

If you need to track clients and you want to listen to their connection events, you can use the clientConnected and clientDisconnected methods of the ClientService class. This class must be run on the member side. The following is an example code.

final ClientService clientService = hazelcastInstance.getClientService();
final Collection<Client> connectedClients = clientService.getConnectedClients();

clientService.addClientListener(new ClientListener() {
    public void clientConnected(Client client) {
	//Handle client connected event

    public void clientDisconnected(Client client) {
      //Handle client disconnected event
Finding the Partition of a Key

You use partition service to find the partition of a key. It will return all partitions. See the example code below.

PartitionService partitionService = client.getPartitionService();

//partition of a key
Partition partition = partitionService.getPartition(key);

//all partitions
Set<Partition> partitions = partitionService.getPartitions();
Handling Lifecycle

Lifecycle handling performs the following:

  • checks to see if the client is running,
  • shuts down the client gracefully,
  • terminates the client ungracefully (forced shutdown), and
  • adds/removes lifecycle listeners.
LifecycleService lifecycleService = client.getLifecycleService();

    //it is running

//shutdown client gracefully

Client Listeners

You can configure listeners to listen to various event types on the client side. You can configure global events not relating to any distributed object through Client ListenerConfig. You should configure distributed object listeners like map entry listeners or list item listeners through their proxies. You can refer to the related sections under each distributed data structure in this reference manual.

Client Transactions

Transactional distributed objects are supported on the client side. Please see the Transactions chapter on how to use them.

Async Start and Reconnect Modes

Java client can be configured to connect to a cluster in an async manner during the client start and reconnecting after a cluster disconnect. Both of these options are configured via ClientConnectionStrategyConfig.

Async client start is configured by setting the configuration element async-start to true. This configuration will change the behavior of HazelcastClient.newHazelcastClient() call. It'll return a client instance without waiting to establish a cluster connection. Until the client connects to cluster, it will throw HazelcastClientOfflineException on any network dependent operations hence they won't block. If you want to check or wait the client to complete its cluster connection, you can use the built-in lifecycle listener:

ClientStateListener clientStateListener = new ClientStateListener(clientConfig);
HazelcastInstance client = HazelcastClient.newHazelcastClient(clientConfig);
//Client started but may not be connected to cluster yet.
//check connection status
//blocks until client completes connect to cluster 
if (clientStateListener.awaitConnected()) {
	//connected successfully
} else {
	//client failed to connect to cluster

Java client can also be configured to specify how it reconnects after a cluster disconnection. There are three options:

  • A client can reject to reconnect to the cluster and trigger the client shutdown process.
  • Client can open a connection to the cluster by blocking all waiting invocations.
  • Client can open a connection to the cluster without blocking the waiting invocations. All invocations will receive HazelcastClientOfflineException during the establishment of cluster connection. If cluster connection is failed to connect, then client shutdown will be triggered.

You can refer to the Configuring Client Connection Strategy section to learn how to configure these.