Implemented clients eager load

Fixes gh-729

# Conflicts:
#	spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/config/LoadBalancerAutoConfiguration.java
#	spring-cloud-loadbalancer/src/main/resources/META-INF/additional-spring-configuration-metadata.json

docs/src/main/asciidoc/_configprops.adoc

@@ -59,6 +59,8 @@
 |spring.cloud.loadbalancer.sticky-session.instance-id-cookie-name | `+++sc-lb-instance-id+++` | The name of the cookie holding the preferred instance id.
 |spring.cloud.loadbalancer.x-forwarded.enabled | `+++false+++` | To Enable X-Forwarded Headers.
 |spring.cloud.loadbalancer.zone |  | Spring Cloud LoadBalancer zone.
+|spring.cloud.loadbalancer.eager-load.enabled|  | Enables eager load of client context.
+|spring.cloud.loadbalancer.eager-load.clients | |
 |spring.cloud.refresh.additional-property-sources-to-retain |  | Additional property sources to retain during a refresh. Typically only system property sources are retained. This property allows property sources, such as property sources created by EnvironmentPostProcessors to be retained as well.
 |spring.cloud.refresh.enabled | `+++true+++` | Enables autoconfiguration for the refresh scope and associated features.
 |spring.cloud.refresh.extra-refreshable | `+++true+++` | Additional class names for beans to post process into refresh scope.

spring-cloud-commons/src/main/java/org/springframework/cloud/client/loadbalancer/LoadbalancerEagerLoadProperties.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2012-2021 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
+ *
+ *      https://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.loadbalancer;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.springframework.boot.context.properties.ConfigurationProperties;
+
+/**
+ * @author Andrii Bohutskyi
+ */
+@ConfigurationProperties("spring.cloud.loadbalancer.eager-load")
+public class LoadbalancerEagerLoadProperties {
+
+	private List<String> clients = new ArrayList<>();
+
+	public List<String> getClients() {
+		return clients;
+	}
+
+	public void setClients(List<String> clients) {
+		this.clients = clients;
+	}
+}

spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/config/LoadBalancerAutoConfiguration.java

@@ -25,11 +25,13 @@ import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.cloud.client.loadbalancer.LoadBalancerClientsProperties;
+import org.springframework.cloud.client.loadbalancer.LoadbalancerEagerLoadProperties;
 import org.springframework.cloud.client.loadbalancer.reactive.LoadBalancerBeanPostProcessorAutoConfiguration;
 import org.springframework.cloud.client.loadbalancer.reactive.ReactorLoadBalancerClientAutoConfiguration;
 import org.springframework.cloud.loadbalancer.annotation.LoadBalancerClientSpecification;
 import org.springframework.cloud.loadbalancer.annotation.LoadBalancerClients;
 import org.springframework.cloud.loadbalancer.support.LoadBalancerClientFactory;
+import org.springframework.cloud.loadbalancer.support.LoadBalancerEagerContextInitializer;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.env.Environment;
@@ -66,4 +68,10 @@ public class LoadBalancerAutoConfiguration {
 		return clientFactory;
 	}
 
+	@Bean
+	@ConditionalOnProperty(value = "spring.cloud.loadbalancer.eager-load.enabled", havingValue = "true")
+	public LoadBalancerEagerContextInitializer loadBalancerEagerContextInitializer(
+			LoadBalancerClientFactory clientFactory, LoadbalancerEagerLoadProperties properties) {
+		return new LoadBalancerEagerContextInitializer(clientFactory, properties.getClients());
+	}
 }

spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/support/LoadBalancerEagerContextInitializer.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2012-2021 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
+ *
+ *      https://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.loadbalancer.support;
+
+import org.springframework.boot.context.event.ApplicationReadyEvent;
+import org.springframework.context.ApplicationListener;
+
+import java.util.List;
+
+/**
+ * @author Andrii Bohutskyi
+ */
+public class LoadBalancerEagerContextInitializer implements ApplicationListener<ApplicationReadyEvent> {
+
+	private final LoadBalancerClientFactory factory;
+
+	private final List<String> serviceNames;
+
+	public LoadBalancerEagerContextInitializer(LoadBalancerClientFactory factory, List<String> serviceNames) {
+		this.factory = factory;
+		this.serviceNames = serviceNames;
+	}
+
+	@Override
+	public void onApplicationEvent(ApplicationReadyEvent applicationReadyEvent) {
+		warnUpFunction();
+	}
+
+	private void warnUpFunction() {
+		this.serviceNames.forEach(factory::getInstance);
+	}
+
+}

spring-cloud-loadbalancer/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -33,6 +33,16 @@
 			"name": "spring.cloud.loadbalancer.enabled",
 			"description": "Enables Spring Cloud LoadBalancer.",
 			"type": "java.lang.Boolean"
+		},
+		{
+			"name": "spring.cloud.loadbalancer.eager-load.enabled",
+			"description": "Enables eager load of clients context.",
+			"type": "java.lang.Boolean"
+		},
+		{
+			"name": "spring.cloud.loadbalancer.eager-load.clients",
+			"description": "Names of the clients.",
+			"type": "java.util.List"
 		}
 	]
 }

spring-cloud-loadbalancer/src/test/java/org/springframework/cloud/loadbalancer/support/LoadBalancerEagerContextInitializerTest.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2012-2021 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
+ *
+ *      https://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.loadbalancer.support;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.cloud.loadbalancer.annotation.LoadBalancerClient;
+import org.springframework.cloud.loadbalancer.config.LoadBalancerAutoConfiguration;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.test.context.junit.jupiter.SpringExtension;
+
+import java.util.Collections;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Andrii Bohutskyi
+ */
+@ExtendWith(SpringExtension.class)
+@SpringBootTest(classes = {
+		LoadBalancerAutoConfiguration.class,
+		LoadBalancerEagerContextInitializerTest.SomeTestConfig.class
+})
+class LoadBalancerEagerContextInitializerTest {
+
+	private static final String LOAD_BALANCER = "testLoadBalancer";
+
+	@Autowired
+	private LoadBalancerClientFactory factory;
+
+	@Test
+	void testLoadBalancerInstanceShouldBeInitialized() {
+		assertThat(LoadBalancerCounter.getCount()).isEqualTo(1);
+		factory.getInstance(LOAD_BALANCER);
+		assertThat(LoadBalancerCounter.getCount()).isEqualTo(1);
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	@LoadBalancerClient(name = LOAD_BALANCER, configuration = TestLoadBalancerConfiguration.class)
+	static class SomeTestConfig {
+
+		@Bean
+		public LoadBalancerEagerContextInitializer loadBalancerEagerContextInitializer(
+				LoadBalancerClientFactory factory) {
+			return new LoadBalancerEagerContextInitializer(factory, Collections.singletonList(LOAD_BALANCER));
+		}
+
+	}
+
+	static class TestLoadBalancerConfiguration {
+
+		@Bean
+		public LoadBalancerCounter loadBalancerCounter() {
+			return new LoadBalancerCounter();
+		}
+
+	}
+
+	static class LoadBalancerCounter {
+
+		private static final AtomicInteger COUNT = new AtomicInteger();
+
+		public LoadBalancerCounter() {
+			COUNT.incrementAndGet();
+		}
+
+		public static int getCount() {
+			return COUNT.get();
+		}
+	}
+}