A general usage of closure is to be as an identifier (the application-specific purpose) of item subscriptions. So that the application can distinguish an event received in the callback methods by checking the closure object.

Here this is the example code (modified from example100 MarketPrice_Streaming):

** Calling registerClient using a closure object**

AppClient appClient = new AppClient();
OmmConsumerConfig config = EmaFactory.createOmmConsumerConfig();
consumer  = EmaFactory.createOmmConsumer(config.host("<host>:<port>").username("<username>"));
ReqMsg reqMsg = EmaFactory.createReqMsg();

// equity ric group
consumer.registerClient(reqMsg.clear().serviceName("<serviceName>").name("IBM.N"), appClient, "EQUITY_CLOSURE");
consumer.registerClient(reqMsg.clear().serviceName("<serviceName>").name("TRI.N"), appClient, "EQUITY_CLOSURE");

// currency ric group
consumer.registerClient(reqMsg.clear().serviceName("<serviceName>").name("JPY="), appClient, "CURRENCY_CLOSURE");
consumer.registerClient(reqMsg.clear().serviceName("<serviceName>").name("GBP="), appClient, "CURRENCY_CLOSURE");
consumer.registerClient(reqMsg.clear().serviceName("<serviceName>").name("EUR="), appClient, "CURRENCY_CLOSURE");

// chain ric group
consumer.registerClient(reqMsg.clear().serviceName("<serviceName>").name("0#.SETI"), appClient, "CHAIN_CLOSURE");

// etc ric group
consumer.registerClient(reqMsg.clear().serviceName("<serviceName>").name("THIS_IS_A_NON_EXISTING_RIC"), appClient, "ETC_CLOSURE");

** Printing a closure object in the callback method **

public void onRefreshMsg(RefreshMsg refreshMsg, OmmConsumerEvent event)
{
	System.out.print("onRefreshMsg: \t");
	System.out.print(refreshMsg.name());
	System.out.print("\t");
	System.out.println(event.closure());
}
	
public void onUpdateMsg(UpdateMsg updateMsg, OmmConsumerEvent event) 
{
	System.out.print("onUpdateMsg: \t");
	System.out.print(updateMsg.name());
	System.out.print("\t");
	System.out.println(event.closure());
}


public void onStatusMsg(StatusMsg statusMsg, OmmConsumerEvent event) 
{
	System.out.print("onStatusMsg: \t");
	System.out.print(statusMsg.name());
	System.out.print("\t");
	System.out.println(event.closure());
	}

** Here this is the result **

onRefreshMsg: 	IBM.N	EQUITY_CLOSURE
onRefreshMsg: 	JPY=	CURRENCY_CLOSURE
onRefreshMsg: 	EUR=	CURRENCY_CLOSURE
onRefreshMsg: 	0#.SETI	CHAIN_CLOSURE
onRefreshMsg: 	GBP=	CURRENCY_CLOSURE
onStatusMsg: 	THIS_IS_A_NON_EXISTING_RIC	ETC_CLOSURE
onRefreshMsg: 	TRI.N	EQUITY_CLOSURE
onUpdateMsg: 	JPY=	CURRENCY_CLOSURE
onUpdateMsg: 	JPY=	CURRENCY_CLOSURE
onUpdateMsg: 	JPY=	CURRENCY_CLOSURE
onUpdateMsg: 	JPY=	CURRENCY_CLOSURE
onUpdateMsg: 	EUR=	CURRENCY_CLOSURE
onUpdateMsg: 	EUR=	CURRENCY_CLOSURE

Hope this helps!