微服务负载均衡组件Ribbon——源码解读篇

从这几篇源码解读可以发现一个规律，那就是引进某个服务，会经常看到@Configuration和@Import（核心服务注册类）这两个注解，伴随着@ConditonalOnClass(XXX.class)、@ConditionalOnBean(XXX.class)、@ConditionOnProperty(***)和@ConditionalOnMissingBean(XXX.class)。作为Spring Boot神奇的特性-简化配置，@Configuration起了很大的作用，此处做个伏笔，感兴趣的可以先阅读JavaDoc，关于@Configuration的详细解释会在之后的Spring Boot系列文章中阐明。按照惯例，此处从核心服务类开始展开对Ribbon的源码解读

@Configuration
@Import(RibbonClientConfigurationRegistrar.class)
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface RibbonClient {

   String value() default "";

   // 配置Ribbon客户端名字
   String name() default "";

    // Ribbon客户端的自定义配置，可以配置生成客户端的各个组件，比如说ILoadBalancer、ServerListFilter和IRule。默认的配置为RibbonClientConfiguration.java
   Class<?>[] configuration() default {};

}

RibbonClientConfigurationRegistrar.class实现了接口ImportBeanDefinitionRegistrar，这个接口是Spring动态注册BeanDefinition的接口，用来注册Ribbon所需要的BeanDefinition

我们看下接口ImportBeanDefinitionRegistrar的描述，处理被注解@Configuration标注的类时用来标记bean definition，说白了就是这个接口用来实现动态注入，可以通过下面的小Demo进行Debug，一步一步看下代码Spring Boot做了什么

Interface to be implemented by types that register additional bean definitions when processing @{@link Configuration} classes. Useful when operating at the bean definition level (as opposed to {@code @Bean} method/instance level) is desired or necessary.  

小Demo，案例来自Spring - Using ImportBeanDefinitionRegistrar

package thinking.in.spring.boot;

/**
* @Author: Bruce
* @Date: 2019/7/10 0:18
* @Version 1.0
*/
public class AppBean {

    private String str;

    public void setStr(String str) {
        this.str = str;
    }

    public void process() {
        System.out.println(str);
    }
}

package thinking.in.spring.boot.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import thinking.in.spring.boot.MyBeanRegistrar;
import thinking.in.spring.boot.bean.ClientBean;

/**
* @Author: Bruce
* @Date: 2019/7/10 0:19
* @Version 1.0
*/
@Configuration
@Import(MyBeanRegistrar.class)
public class MyConfig {

    @Bean
    ClientBean clientBean() {
        return new ClientBean();
    }
}

package thinking.in.spring.boot.bean;

import org.springframework.beans.factory.annotation.Autowired;
import thinking.in.spring.boot.AppBean;

/**
* @Author: Bruce
* @Date: 2019/7/10 0:20
* @Version 1.0
*/
public class ClientBean {

    @Autowired
    private AppBean appBean;

    public void doSomething() {
        appBean.process();
    }
}

package thinking.in.spring.boot;

import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.GenericBeanDefinition;
import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
import org.springframework.core.type.AnnotationMetadata;

/**
* @Author: Bruce
* @Date: 2019/7/10 0:14
* @Version 1.0
*/
public class MyBeanRegistrar implements ImportBeanDefinitionRegistrar {

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        GenericBeanDefinition gbd = new GenericBeanDefinition();
        gbd.setBeanClass(AppBean.class);
        gbd.getPropertyValues().addPropertyValue("str", "value set from registrar");
        registry.registerBeanDefinition("AppBean", gbd);
    }
}

package thinking.in.spring.boot;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.A