return null for choose() rather than throw an illegal argument exception.

spring-cloud-netflix-core/src/main/java/org/springframework/cloud/netflix/ribbon/RibbonLoadBalancerClient.java

@@ -53,7 +53,11 @@ public class RibbonLoadBalancerClient implements LoadBalancerClient {
 
 	@Override
 	public ServiceInstance choose(String serviceId) {
-		return new RibbonServer(serviceId, getServer(serviceId));
+		Server server = getServer(serviceId);
+		if (server == null) {
+			return null;
+		}
+		return new RibbonServer(serviceId, server);
 	}
 
 	@Override
@@ -61,7 +65,7 @@ public class RibbonLoadBalancerClient implements LoadBalancerClient {
 		ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
 		RibbonLoadBalancerContext context = this.clientFactory
 				.getLoadBalancerContext(serviceId);
-		Server server = getServer(serviceId, loadBalancer);
+		Server server = getServer(loadBalancer);
 		RibbonServer ribbonServer = new RibbonServer(serviceId, server);
 
 		ServerStats serverStats = context.getServerStats(server);
@@ -88,16 +92,14 @@ public class RibbonLoadBalancerClient implements LoadBalancerClient {
 	}
 
 	protected Server getServer(String serviceId) {
-		return getServer(serviceId, getLoadBalancer(serviceId));
+		return getServer(getLoadBalancer(serviceId));
 	}
 
-	protected Server getServer(String serviceId, ILoadBalancer loadBalancer) {
-		Server server = loadBalancer.chooseServer("default");
-		if (server == null) {
-			throw new IllegalStateException(
-					"Unable to locate ILoadBalancer for service: " + serviceId);
+	protected Server getServer(ILoadBalancer loadBalancer) {
+		if (loadBalancer == null) {
+			return null;
 		}
-		return server;
+		return loadBalancer.chooseServer("default"); //TODO: better handling of key
 	}
 
 	protected ILoadBalancer getLoadBalancer(String serviceId) {

spring-cloud-netflix-core/src/test/java/org/springframework/cloud/netflix/ribbon/RibbonLoadBalancerClientTests.java

@@ -35,6 +35,7 @@ import com.netflix.loadbalancer.ServerStats;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.fail;
+import static org.junit.Assert.assertNull;
 import static org.mockito.BDDMockito.given;
 import static org.mockito.Matchers.anyDouble;
 import static org.mockito.Matchers.anyString;
@@ -84,6 +85,16 @@ public class RibbonLoadBalancerClientTests {
 	}
 
 	@Test
+	public void testChooseMissing() {
+		given(this.clientFactory.getLoadBalancer(this.loadBalancer.getName()))
+				.willReturn(null);
+		given(this.loadBalancer.getName()).willReturn("missingservice");
+		RibbonLoadBalancerClient client = new RibbonLoadBalancerClient(this.clientFactory);
+		ServiceInstance instance = client.choose("missingservice");
+		assertNull("instance wasn't null", instance);
+	}
+
+	@Test
 	public void testExecute() {
 		final RibbonServer server = getRibbonServer();
 		RibbonLoadBalancerClient client = getRibbonLoadBalancerClient(server);