Merge pull request #76 from olivergierke/hypermedia

Added hypermedia support in the form of a DiscoveredResource abstraction

spring-cloud-commons/pom.xml

@@ -48,6 +48,16 @@
 			<artifactId>spring-integration-jmx</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-hateoas</artifactId>
+			<optional>true</optional>
+		</dependency>
+		<dependency>
+			<groupId>com.jayway.jsonpath</groupId>
+			<artifactId>json-path</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>org.projectlombok</groupId>
 			<artifactId>lombok</artifactId>

spring-cloud-commons/src/main/java/org/springframework/cloud/client/hypermedia/CloudHypermediaAutoConfiguration.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import lombok.Data;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.context.properties.ConfigurationProperties;
+import org.springframework.boot.context.properties.EnableConfigurationProperties;
+import org.springframework.cloud.client.hypermedia.CloudHypermediaAutoConfiguration.CloudHypermediaProperties;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * Registers a default {@link RemoteResourceRefresher} if at least one {@link RemoteResource} is declared in the system
+ * and applies verification timings defined in the application properties.
+ * 
+ * @author Oliver Gierke
+ */
+@Configuration
+@ConditionalOnBean(type = "org.springframework.cloud.client.hypermedia.RemoteResource")
+@EnableConfigurationProperties(CloudHypermediaProperties.class)
+public class CloudHypermediaAutoConfiguration {
+
+	@Autowired(required = false) List<RemoteResource> discoveredResources = Collections.emptyList();
+	@Autowired CloudHypermediaProperties properties;
+
+	@Bean
+	@ConditionalOnMissingBean
+	public RemoteResourceRefresher discoveredResourceRefresher() {
+		return new RemoteResourceRefresher(discoveredResources, properties.getRefresh().getFixedDelay(),
+				properties.getRefresh().getInitialDelay());
+	}
+
+	@Data
+	@ConfigurationProperties(prefix = "spring.cloud.hypermedia")
+	public static class CloudHypermediaProperties {
+
+		private Refresh refresh = new Refresh();
+
+		@Data
+		public static class Refresh {
+
+			private int fixedDelay = 5000;
+			private int initialDelay = 10000;
+		}
+	}
+}

spring-cloud-commons/src/main/java/org/springframework/cloud/client/hypermedia/DiscoveredResource.java

@@ -0,0 +1,121 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import lombok.Getter;
+import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
+
+import java.net.URI;
+
+import org.springframework.cloud.client.ServiceInstance;
+import org.springframework.hateoas.Link;
+import org.springframework.hateoas.MediaTypes;
+import org.springframework.hateoas.client.Traverson;
+import org.springframework.util.Assert;
+import org.springframework.web.client.RestClientException;
+import org.springframework.web.client.RestOperations;
+import org.springframework.web.client.RestTemplate;
+
+/**
+ * A REST resource that is defined by a service reference and a traversal operation within that service.
+ *
+ * @author Oliver Gierke
+ */
+@Slf4j
+@RequiredArgsConstructor
+public class DiscoveredResource implements RemoteResource {
+
+	private final ServiceInstanceProvider provider;
+	private final TraversalDefinition traversal;
+
+	private RestOperations restOperations = new RestTemplate();
+	private @Getter Link link = null;
+
+	/**
+	 * Configures the {@link RestOperations} to use to execute the traversal and verifying HEAD calls.
+	 * 
+	 * @param restOperations can be {@literal null}, resorting to a default {@link RestTemplate} in that case.
+	 */
+	public void setRestOperations(RestOperations restOperations) {
+		this.restOperations = restOperations == null ? new RestTemplate() : restOperations;
+	}
+
+	/**
+	 * Verifies the link to the current
+	 */
+	public void verifyOrDiscover() {
+		this.link = link == null ? discoverLink() : verify(link);
+	}
+
+	/**
+	 * Verifies the given {@link Link} by issuing an HTTP HEAD request to the resource.
+	 * 
+	 * @param link must not be {@literal null}.
+	 * @return
+	 */
+	private Link verify(Link link) {
+
+		Assert.notNull(link, "Link must not be null!");
+
+		try {
+
+			String uri = link.expand().getHref();
+
+			log.debug("Verifying link pointing to {}…", uri);
+			restOperations.headForHeaders(uri);
+			log.debug("Successfully verified link!");
+
+			return link;
+
+		} catch (RestClientException o_O) {
+
+			log.debug("Verification failed, marking as outdated!");
+			return null;
+		}
+	}
+
+	private Link discoverLink() {
+
+		try {
+
+			ServiceInstance service = provider.getServiceInstance();
+
+			if (service == null) {
+				return null;
+			}
+
+			URI uri = service.getUri();
+			String serviceId = service.getServiceId();
+
+			log.debug("Discovered {} system at {}. Discovering resource…", serviceId, uri);
+
+			Traverson traverson = new Traverson(uri, MediaTypes.HAL_JSON);
+			Link link = traversal.buildTraversal(traverson).asTemplatedLink();
+
+			log.debug("Found link pointing to {}.", link.getHref());
+
+			return link;
+
+		} catch (RuntimeException o_O) {
+
+			this.link = null;
+			log.debug("Target system unavailable. Got: ", o_O.getMessage());
+
+			return null;
+		}
+	}
+}

spring-cloud-commons/src/main/java/org/springframework/cloud/client/hypermedia/DynamicServiceInstanceProvider.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import lombok.RequiredArgsConstructor;
+
+import java.util.List;
+
+import org.springframework.cloud.client.ServiceInstance;
+import org.springframework.cloud.client.discovery.DiscoveryClient;
+
+/**
+ * {@link ServiceInstanceProvider} to work with a {@link DiscoveryClient} to lookup a service by name. Will pick the
+ * first one returned by the configured {@link DiscoveryClient}.
+ * 
+ * @author Oliver Gierke
+ */
+@RequiredArgsConstructor
+public class DynamicServiceInstanceProvider implements ServiceInstanceProvider {
+
+	private final DiscoveryClient client;
+	private final String serviceName;
+
+	/* 
+	 * (non-Javadoc)
+	 * @see example.customers.integration.ServiceInstanceProvider#getServiceInstance()
+	 */
+	@Override
+	public ServiceInstance getServiceInstance() {
+
+		try {
+
+			List<ServiceInstance> instances = client.getInstances(serviceName);
+			return instances.isEmpty() ? null : instances.get(0);
+
+		} catch (RuntimeException o_O) {
+			return null;
+		}
+	}
+}

spring-cloud-commons/src/main/java/org/springframework/cloud/client/hypermedia/RemoteResource.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import org.springframework.hateoas.Link;
+
+/**
+ * A REST resource that can be discovered and can be either gone or available.
+ * 
+ * @author Oliver Gierke
+ */
+public interface RemoteResource {
+
+	/**
+	 * Returns the {@link Link} to the resource in case it is available or {@literal null} in case it's gone, i.e. either
+	 * generally unavailable or can't be discovered.
+	 * 
+	 * @return
+	 */
+	Link getLink();
+
+	/**
+	 * Discovers the the resource in case it hasn't been yet or became unavailable. In case a link has been discovered
+	 * previously, it is verified and either confirmed or the link is removed to indicate it's not available anymore.
+	 */
+	void verifyOrDiscover();
+}

spring-cloud-commons/src/main/java/org/springframework/cloud/client/hypermedia/RemoteResourceRefresher.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import lombok.RequiredArgsConstructor;
+
+import java.util.List;
+
+import org.springframework.scheduling.config.ContextLifecycleScheduledTaskRegistrar;
+import org.springframework.scheduling.config.IntervalTask;
+import org.springframework.scheduling.config.ScheduledTaskRegistrar;
+
+/**
+ * A {@link ScheduledTaskRegistrar} that verifies all {@link DiscoveredResource} instances in the system based
+ * on the given timing configuration.
+ * 
+ * @author Oliver Gierke
+ */
+@RequiredArgsConstructor
+public class RemoteResourceRefresher extends ContextLifecycleScheduledTaskRegistrar {
+
+	private final List<RemoteResource> discoveredResources;
+	private final int fixedDelay, initialDelay;
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.scheduling.config.ContextLifecycleScheduledTaskRegistrar#afterPropertiesSet()
+	 */
+	@Override
+	public void afterPropertiesSet() {
+
+		for (final RemoteResource resource : discoveredResources) {
+			addFixedDelayTask(new IntervalTask(new Runnable() {
+
+				@Override
+				public void run() {
+					resource.verifyOrDiscover();
+				}
+			}, fixedDelay, initialDelay));
+		}
+
+		super.afterPropertiesSet();
+	}
+}

spring-cloud-commons/src/main/java/org/springframework/cloud/client/hypermedia/ServiceInstanceProvider.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import org.springframework.cloud.client.ServiceInstance;
+
+/**
+ * A component that will provide a {@link ServiceInstance} or can express the absence of one by returning
+ * {@literal null}.
+ * 
+ * @author Oliver Gierke
+ */
+public interface ServiceInstanceProvider {
+
+	/**
+	 * Returns the service instance or {@literal null} in case the service is currently unavailable.
+	 * 
+	 * @return the service instance or {@literal null} in case the service is currently unavailable.
+	 */
+	ServiceInstance getServiceInstance();
+}

spring-cloud-commons/src/main/java/org/springframework/cloud/client/hypermedia/StaticServiceInstanceProvider.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import lombok.RequiredArgsConstructor;
+
+import org.springframework.cloud.client.ServiceInstance;
+
+/**
+ * A {@link ServiceInstanceProvider} that will always return the configured {@link ServiceInstance}.
+ * 
+ * @author Oliver Gierke
+ */
+@RequiredArgsConstructor
+public class StaticServiceInstanceProvider implements ServiceInstanceProvider {
+
+	private final ServiceInstance instance;
+
+	/* 
+	 * (non-Javadoc)
+	 * @see example.customers.integration.ServiceInstanceProvider#getServiceInstance()
+	 */
+	@Override
+	public ServiceInstance getServiceInstance() {
+		return instance;
+	}
+}

spring-cloud-commons/src/main/java/org/springframework/cloud/client/hypermedia/TraversalDefinition.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import org.springframework.hateoas.client.Traverson;
+import org.springframework.hateoas.client.Traverson.TraversalBuilder;
+
+/**
+ * Callback to define the traversal to a resource.
+ * 
+ * @author Oliver Gierke
+ */
+public interface TraversalDefinition {
+
+	/**
+	 * @param traverson the Traverson instance to run the traversal on.
+	 * @return
+	 */
+	TraversalBuilder buildTraversal(Traverson traverson);
+}

spring-cloud-commons/src/main/resources/META-INF/spring.factories

@@ -2,6 +2,7 @@
 org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
 org.springframework.cloud.client.CommonsClientAutoConfiguration,\
 org.springframework.cloud.client.discovery.noop.NoopDiscoveryClientAutoConfiguration,\
+org.springframework.cloud.client.hypermedia.CloudHypermediaAutoConfiguration,\
 org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration,\
 org.springframework.cloud.util.UtilAutoConfiguration
 

spring-cloud-commons/src/test/java/org/springframework/cloud/client/hypermedia/CloudHypermediaAutoConfigurationIntegrationTests.java

@@ -0,0 +1,105 @@
+/*
+ * Copyright 2015-2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import static org.hamcrest.Matchers.*;
+import static org.junit.Assert.*;
+
+import org.junit.Test;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.cloud.client.DefaultServiceInstance;
+import org.springframework.cloud.client.hypermedia.CloudHypermediaAutoConfiguration.CloudHypermediaProperties;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.hateoas.client.Traverson;
+import org.springframework.hateoas.client.Traverson.TraversalBuilder;
+
+/**
+ * Integration tests for {@link CloudHypermediaAutoConfiguration}.
+ * 
+ * @author Oliver Gierke
+ */
+public class CloudHypermediaAutoConfigurationIntegrationTests {
+
+	@Test
+	public void picksUpHypermediaProperties() {
+
+		try (ConfigurableApplicationContext context = getApplicationContext(ConfigWithRemoteResource.class)) {
+
+			CloudHypermediaProperties properties = context.getBean(CloudHypermediaProperties.class);
+
+			assertThat(properties.getRefresh().getInitialDelay(), is(50000));
+			assertThat(properties.getRefresh().getFixedDelay(), is(10000));
+		}
+	}
+
+	@Test
+	public void doesNotCreateCloudHypermediaPropertiesifNotActive() {
+
+		try (ConfigurableApplicationContext context = getApplicationContext(Config.class)) {
+			assertThat(context.getBeanNamesForType(CloudHypermediaProperties.class), is(arrayWithSize(0)));
+		}
+	}
+
+	@Test
+	public void doesNotRegisterResourceRefresherIfNoDiscoveredResourceIsDefined() {
+
+		try (ConfigurableApplicationContext context = getApplicationContext(Config.class)) {
+
+			assertThat(context.getBeansOfType(RemoteResource.class).values(), hasSize(0));
+			assertThat(context.getBeanNamesForType(RemoteResourceRefresher.class), is(arrayWithSize(0)));
+		}
+	}
+
+	@Test
+	public void registersResourceRefresherIfDiscoverredResourceIsDefined() {
+
+		try (ConfigurableApplicationContext context = getApplicationContext(ConfigWithRemoteResource.class)) {
+
+			assertThat(context.getBeansOfType(RemoteResource.class).values(), hasSize(1));
+			assertThat(context.getBean(RemoteResourceRefresher.class), is(notNullValue()));
+		}
+	}
+
+	private static ConfigurableApplicationContext getApplicationContext(Class<?> configuration) {
+		return SpringApplication.run(configuration, new String[0]);
+	}
+
+	@Configuration
+	@EnableAutoConfiguration
+	static class Config {}
+
+	@Configuration
+	@EnableAutoConfiguration
+	static class ConfigWithRemoteResource {
+
+		@Bean
+		public RemoteResource resource() {
+
+			ServiceInstanceProvider provider = new StaticServiceInstanceProvider(
+					new DefaultServiceInstance("service", "localhost", 80, false));
+			return new DiscoveredResource(provider, new TraversalDefinition() {
+
+				@Override
+				public TraversalBuilder buildTraversal(Traverson traverson) {
+					return traverson.follow("rel");
+				}
+			});
+		}
+	}
+}

spring-cloud-commons/src/test/java/org/springframework/cloud/client/hypermedia/DiscoveredResourceUnitTests.java

@@ -0,0 +1,108 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import static org.hamcrest.CoreMatchers.*;
+import static org.junit.Assert.*;
+import static org.mockito.Matchers.*;
+import static org.mockito.Mockito.*;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Matchers;
+import org.mockito.Mock;
+import org.mockito.runners.MockitoJUnitRunner;
+import org.springframework.cloud.client.DefaultServiceInstance;
+import org.springframework.hateoas.Link;
+import org.springframework.hateoas.client.Traverson;
+import org.springframework.hateoas.client.Traverson.TraversalBuilder;
+import org.springframework.web.client.RestClientException;
+import org.springframework.web.client.RestOperations;
+
+/**
+ * @author Oliver Gierke
+ */
+@RunWith(MockitoJUnitRunner.class)
+public class DiscoveredResourceUnitTests {
+
+	@Mock ServiceInstanceProvider provider;
+	@Mock TraversalDefinition traversal;
+	@Mock TraversalBuilder builder;
+	@Mock RestOperations operations;
+
+	DiscoveredResource resource;
+
+	@Before
+	public void setUp() {
+		when(traversal.buildTraversal(Matchers.any(Traverson.class))).thenReturn(builder);
+
+		this.resource = new DiscoveredResource(provider, traversal);
+		this.resource.setRestOperations(operations);
+	}
+
+	@Test
+	public void isUndiscoveredByDefault() {
+		assertThat(resource.getLink(), is(nullValue()));
+	}
+
+	@Test
+	public void verificationTriggersDiscovery() {
+
+		Link link = new Link("target", "rel");
+
+		when(provider.getServiceInstance()).thenReturn(new DefaultServiceInstance("service", "localhost", 8080, false));
+		when(builder.asTemplatedLink()).thenReturn(link);
+
+		resource.verifyOrDiscover();
+
+		assertThat(resource.getLink(), is(link));
+		verify(provider, times(1)).getServiceInstance();
+		verify(traversal, times(1)).buildTraversal(Matchers.any(Traverson.class));
+	}
+
+	@Test
+	public void triggersVerificationOnSubsequentCall() {
+
+		verificationTriggersDiscovery();
+
+		resource.verifyOrDiscover();
+
+		assertThat(resource.getLink(), is(notNullValue()));
+		verify(operations, times(1)).headForHeaders(anyString());
+	}
+
+	@Test
+	public void resetsLinkOnFailedVerification() {
+
+		verificationTriggersDiscovery();
+
+		doThrow(RestClientException.class).when(operations).headForHeaders(anyString());
+		resource.verifyOrDiscover();
+
+		assertThat(resource.getLink(), is(nullValue()));
+	}
+
+	@Test
+	public void failedDiscoveryTraversalCausesLinkToStayNull() {
+
+		doThrow(RuntimeException.class).when(provider).getServiceInstance();
+
+		resource.verifyOrDiscover();
+
+		assertThat(resource.getLink(), is(nullValue()));
+	}
+}

spring-cloud-commons/src/test/java/org/springframework/cloud/client/hypermedia/DynamicServiceInstanceProviderUnitTests.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.cloud.client.hypermedia;
+
+import static org.hamcrest.CoreMatchers.*;
+import static org.junit.Assert.*;
+import static org.mockito.Matchers.*;
+import static org.mockito.Mockito.*;
+
+import java.util.Arrays;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.runners.MockitoJUnitRunner;
+import org.springframework.cloud.client.ServiceInstance;
+import org.springframework.cloud.client.discovery.DiscoveryClient;
+
+/**
+ * @author Oliver Gierke
+ */
+@RunWith(MockitoJUnitRunner.class)
+public class DynamicServiceInstanceProviderUnitTests {
+
+	@Mock DiscoveryClient client;
+
+	@Test
+	public void returnsNoServiceInCaseNoneIsAvailable() {
+		assertThat(new DynamicServiceInstanceProvider(client, "service").getServiceInstance(), is(nullValue()));
+	}
+
+	@Test
+	public void returnsFirstServiceInCaseMultipleOnesAreAvailable() {
+
+		ServiceInstance first = mock(ServiceInstance.class);
+		ServiceInstance second = mock(ServiceInstance.class);
+
+		when(client.getInstances(anyString())).thenReturn(Arrays.asList(first, second));
+
+		assertThat(new DynamicServiceInstanceProvider(client, "service").getServiceInstance(), is(first));
+	}
+}

spring-cloud-commons/src/test/resources/application.properties

@@ -2,4 +2,7 @@ message: Hello scope!
 delay: 0
 debug: true
 #logging.level.org.springframework.web: DEBUG
-#logging.level.org.springframework.context.annotation: DEBUG
+#logging.level.org.springframework.context.annotation: DEBUG
+
+spring.cloud.hypermedia.refresh.initial-delay=50000
+spring.cloud.hypermedia.refresh.fixed-delay=10000