使用Spring Boot的一個好處就是它提供了很多的Starter,可以幫助我們實現自動配置,極大的方便我們創建一個Spring應用。如果我們想創建一個自定義的Starter該如何入手呢?一個自動配置包含以下兩個模塊。
一、auto-configuration模塊
一個auto-configuration模塊包含了一個繼承@Configuration的註解,通過@Conditional*條件註解來添加我們的自動配置類的約束條件。可以參考spring-boot-autoconfigure 模塊的META-INF/spring.factories,裏面是官方實現的一些自動配置類。
條件註解
@ConditionalOnClass:表示classpath存在相關的類才生效;
@ConditionalOnMissingClass:表示classpath存在相關的類不生效;
@ConditionalOnBean:表示如果存在相關Bean則該自動配置生效;
@ConditionalOnMissingBean:表示如果存在相關Bean則該自動配置不生效;
@ConditionalOnProperty:根據property條件約束;
@ConditionalOnResource:根據Resource條件約束;
@ConditionalOnExpression:根據SpEL表達式來約束;
二、starter模塊
starter只是一個空的jar包,只有一個pom.xml文件,定義了該自動配置需要的依賴。
三、源碼解析
如spring-cloud-commons模塊裏面的LoadBalancerAutoConfiguration
/*
* Copyright 2013-2017 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.loadbalancer;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import org.springframework.beans.factory.SmartInitializingSingleton;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.client.ClientHttpRequestInterceptor;
import org.springframework.retry.support.RetryTemplate;
import org.springframework.web.client.RestTemplate;
/**
* Auto configuration for Ribbon (client side load balancing).
*
* @author Spencer Gibb
* @author Dave Syer
* @author Will Tran
*/
@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {
@LoadBalanced
@Autowired(required = false)
private List<RestTemplate> restTemplates = Collections.emptyList();
@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
final List<RestTemplateCustomizer> customizers) {
return new SmartInitializingSingleton() {
@Override
public void afterSingletonsInstantiated() {
for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
for (RestTemplateCustomizer customizer : customizers) {
customizer.customize(restTemplate);
}
}
}
};
}
@Autowired(required = false)
private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList();
@Bean
@ConditionalOnMissingBean
public LoadBalancerRequestFactory loadBalancerRequestFactory(
LoadBalancerClient loadBalancerClient) {
return new LoadBalancerRequestFactory(loadBalancerClient, transformers);
}
@Configuration
@ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
static class LoadBalancerInterceptorConfig {
@Bean
public LoadBalancerInterceptor ribbonInterceptor(
LoadBalancerClient loadBalancerClient,
LoadBalancerRequestFactory requestFactory) {
return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
}
@Bean
@ConditionalOnMissingBean
public RestTemplateCustomizer restTemplateCustomizer(
final LoadBalancerInterceptor loadBalancerInterceptor) {
return new RestTemplateCustomizer() {
@Override
public void customize(RestTemplate restTemplate) {
List<ClientHttpRequestInterceptor> list = new ArrayList<>(
restTemplate.getInterceptors());
list.add(loadBalancerInterceptor);
restTemplate.setInterceptors(list);
}
};
}
}
@Configuration
@ConditionalOnClass(RetryTemplate.class)
public static class RetryAutoConfiguration {
@Bean
public RetryTemplate retryTemplate() {
RetryTemplate template = new RetryTemplate();
template.setThrowLastExceptionOnExhausted(true);
return template;
}
@Bean
@ConditionalOnMissingBean
public LoadBalancedRetryPolicyFactory loadBalancedRetryPolicyFactory() {
return new LoadBalancedRetryPolicyFactory.NeverRetryFactory();
}
@Bean
@ConditionalOnMissingBean
public LoadBalancedBackOffPolicyFactory loadBalancedBackOffPolicyFactory() {
return new LoadBalancedBackOffPolicyFactory.NoBackOffPolicyFactory();
}
}
@Configuration
@ConditionalOnClass(RetryTemplate.class)
public static class RetryInterceptorAutoConfiguration {
@Bean
@ConditionalOnMissingBean
public RetryLoadBalancerInterceptor ribbonInterceptor(
LoadBalancerClient loadBalancerClient, LoadBalancerRetryProperties properties,
LoadBalancedRetryPolicyFactory lbRetryPolicyFactory,
LoadBalancerRequestFactory requestFactory,
LoadBalancedBackOffPolicyFactory backOffPolicyFactory) {
return new RetryLoadBalancerInterceptor(loadBalancerClient, properties,
lbRetryPolicyFactory, requestFactory, backOffPolicyFactory);
}
@Bean
@ConditionalOnMissingBean
public RestTemplateCustomizer restTemplateCustomizer(
final RetryLoadBalancerInterceptor loadBalancerInterceptor) {
return new RestTemplateCustomizer() {
@Override
public void customize(RestTemplate restTemplate) {
List<ClientHttpRequestInterceptor> list = new ArrayList<>(
restTemplate.getInterceptors());
list.add(loadBalancerInterceptor);
restTemplate.setInterceptors(list);
}
};
}
}
}
這裏面就用到了我們上面提到那幾個條件註解,通過這個自動配置,我們就可以在spring boot項目裏面引入@LoadBalanced註解來實現客戶端的負載均衡。