SpringCloud - Feign负载均衡实战与Ribbon关系

【1】Feign是什么

Feign是一个声明式Web服务客户端。使用Feign能让编写Web服务客户端更加简单，它的使用方法是定义一个接口，然后在上面添加注解，同时也支持JAX-RS标准的注解。

Feign也支持可插拔式的编码器和解码器。SpringCloud对Feign进行了封装，使其支持了SpringMVC标准注解和HttpMessageConverters。Feign可以与Eureka和Ribbon组合使用以支持负载均衡。

官网文档地址：http://projects.spring.io/spring-cloud/spring-cloud.html#spring-cloud-feign

源码地址：https://github.com/OpenFeign/feign

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【2】Feign能干什么

Feign旨在使编写Java Http客户端变得更加容易。

前面在使用Ribbon+RestTemplate时，利用了RestTemplate对http请求的封装处理，形成了一套模板化的调用方法。但是在实际开发中，由于对服务依赖的调用可能不止一处，往往一个接口会被多处调用，所以通常都会针对每个微服务自行封装一些客户端类来包装这些依赖服务的调用。

Feign在此基础上做了进一步封装，由他来帮助我们定义和实现依赖服务接口的定义。在Feign的实现下，我们只需创建一个接口并使用注解的方式来配置(比如以前是一个Dao接口上面标注@Mapper注解，现在是一个微服务接口上面标注一个Feign注解即可)，即可完成对服务提供方的接口绑定，简化了使用Spring Cloud Ribbon时自动封装服务调用客户端的开发量。

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Feign集成了Ribbon

利用Ribbon维护了MicroServiceCloud-Dept的服务列表信息，并且通过轮询实现了客户端的负载均衡。而与Ribbon不同的是，通过Feign只需要定义服务绑定接口且以声明式的方法，优雅而简单的实现了服务调用。

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【3】Feign项目实战

① 参考microservicecloud-consumer-dept-80项目创建microservicecloud-consumer-dept-feign

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② pom中添加对Feign的支持

     <dependencies>
        <dependency><!-- 自己定义的api -->
            <groupId>com.web.springcloud</groupId>
            <artifactId>microservicecloud-api</artifactId>
            <version>${project.version}</version>
        </dependency>
        <!-- Cloud-Feign相关 -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-feign</artifactId>
        </dependency>
        <!-- Cloud-Ribbon相关 -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-eureka</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-ribbon</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-config</artifactId>
        </dependency>
            <!-- Spring Boot相关 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <!-- 修改后立即生效，热部署相关 -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>springloaded</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
        </dependency>
    </dependencies>

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③ 修改microservicecloud-api Module

• pom中添加feign依赖

        <!-- Cloud-Feign相关 -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-feign</artifactId>
        </dependency>

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• com.web.springcloud.service包下添加DeptClientService

package com.web.springcloud.service;

import java.util.List;

import org.springframework.cloud.netflix.feign.FeignClient;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

import com.web.springcloud.entities.Dept;


@FeignClient(value = "MICROSERVICECLOUD-DEPT")//注意这里
public interface DeptClientService
{
    @RequestMapping(value = "/dept/get/{id}", method = RequestMethod.GET)
    public Dept get(@PathVariable("id") long id);

    @RequestMapping(value = "/dept/list", method = RequestMethod.GET)
    public List<Dept> list();

    @RequestMapping(value = "/dept/add", method = RequestMethod.POST)
    public boolean add(Dept dept);
}

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④ 修改microservicecloud-consumer-dept-feign 的Controller

import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import com.web.springcloud.entities.Dept;
import com.web.springcloud.service.DeptClientService;

@RestController
public class DeptController_Consumer
{
    @Autowired
    private DeptClientService service;
    //引入公共Module(api)中的DeptClientService ，直接调用

    @RequestMapping(value = "/consumer/dept/get/{id}")
    public Dept get(@PathVariable("id") Long id)
    {
        return this.service.get(id);
    }

    @RequestMapping(value = "/consumer/dept/list")
    public List<Dept> list()
    {
        return this.service.list();
    }

    @RequestMapping(value = "/consumer/dept/add")
    public Object add(Dept dept)
    {
        return this.service.add(dept);
    }
}

由此可见Feign只是又进一步进行了封装，使得调用服务更加方便。

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⑤ 修改microservicecloud-consumer-dept-feign 的主启动类

@SpringBootApplication
@EnableEurekaClient 
@EnableFeignClients(basePackages={"com.web.springcloud"})//启用Feign
@ComponentScan("com.web.springcloud")
public class DeptConsumer80_Feign_App
{
    public static void main(String[] args)
    {
        SpringApplication.run(DeptConsumer80_Feign_App.class, args);
    }
}

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⑥ 测试

依次启动服务中心 7001 7002 7003和服务提供者 8001 8002 8003，最后启动microservicecloud-consumer-dept-feign。

浏览器访问http://localhost/consumer/dept/list如下：

默认使用轮询算法，当然此时使用RestTemplate+Ribbon也是可以正常访问。

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如下图所示，注册自定义IRule，则可以更改默认轮询算法：

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【4】@FeignClient注解

仔细阅读源码，至少最上面几句话要看完！！！

/**
 * Annotation for interfaces declaring that a REST client with that interface should be
 * created (e.g. for autowiring into another component). If ribbon is available it will be
 * used to load balance the backend requests, and the load balancer can be configured
 * using a <code>@RibbonClient</code> with the same name (i.e. value) as the feign client.
 *
 * @author Spencer Gibb
 * @author Venil Noronha
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FeignClient {

    /**
     * The name of the service with optional protocol prefix. Synonym for {@link #name()
     * name}. A name must be specified for all clients, whether or not a url is provided.
     * Can be specified as property key, eg: ${propertyKey}.
     */
    @AliasFor("name")
    String value() default "";

    /**
     * The service id with optional protocol prefix. Synonym for {@link #value() value}.
     *// 已经过时，使用name替代
     * @deprecated use {@link #name() name} instead
     */
    @Deprecated
    String serviceId() default "";

    /**
     * The service id with optional protocol prefix. Synonym for {@link #value() value}.
     */
    @AliasFor("value")
    String name() default "";
    // 即，ServiceId===name===value
    /**
     * Sets the <code>@Qualifier</code> value for the feign client.
     */
    String qualifier() default "";

    /**
     * An absolute URL or resolvable hostname (the protocol is optional).
     */
    String url() default "";

    /**
     * Whether 404s should be decoded instead of throwing FeignExceptions
     */
    boolean decode404() default false;

    /**
     * A custom <code>@Configuration</code> for the feign client. Can contain override
     * <code>@Bean</code> definition for the pieces that make up the client, for instance
     * {@link feign.codec.Decoder}, {@link feign.codec.Encoder}, {@link feign.Contract}.
     *
     * @see FeignClientsConfiguration for the defaults
     */
    Class<?>[] configuration() default {};

    /**
     * Fallback class for the specified Feign client interface. The fallback class must
     * implement the interface annotated by this annotation and be a valid spring bean.
     */
    Class<?> fallback() default void.class;

    /**
     * Define a fallback factory for the specified Feign client interface. The fallback
     * factory must produce instances of fallback classes that implement the interface
     * annotated by {@link FeignClient}. The fallback factory must be a valid spring
     * bean.
     *
     * @see feign.hystrix.FallbackFactory for details.
     */
    Class<?> fallbackFactory() default void.class;

    /**
     * Path prefix to be used by all method-level mappings. Can be used with or without
     * <code>@RibbonClient</code>.
     */
    String path() default "";

    /**
     * Whether to mark the feign proxy as a primary bean. Defaults to true.
     */
    boolean primary() default true;

}

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【5】FeignClientsConfiguration

Feign Client的配置类：

@Configuration
public class FeignClientsConfiguration {

    // 请求/响应信息转换器
    @Autowired
    private ObjectFactory<HttpMessageConverters> messageConverters;

    @Autowired(required = false)
    private List<AnnotatedParameterProcessor> parameterProcessors = new ArrayList<>();

    @Autowired(required = false)
    private List<FeignFormatterRegistrar> feignFormatterRegistrars = new ArrayList<>();

    @Autowired(required = false)
    private Logger logger;

    @Bean
    @ConditionalOnMissingBean
    public Decoder feignDecoder() {
        return new ResponseEntityDecoder(new SpringDecoder(this.messageConverters));
    }

    @Bean
    @ConditionalOnMissingBean
    public Encoder feignEncoder() {
        return new SpringEncoder(this.messageConverters);
    }

    @Bean
    @ConditionalOnMissingBean
    public Contract feignContract(ConversionService feignConversionService) {
        return new SpringMvcContract(this.parameterProcessors, feignConversionService);
    }

    // 注册FormattingConversionService--数据类型转换
    @Bean
    public FormattingConversionService feignConversionService() {
        FormattingConversionService conversionService = new DefaultFormattingConversionService();
        for (FeignFormatterRegistrar feignFormatterRegistrar : feignFormatterRegistrars) {
            feignFormatterRegistrar.registerFormatters(conversionService);
        }
        return conversionService;
    }

    // 与Hystrix整合
    @Configuration
    @ConditionalOnClass({ HystrixCommand.class, HystrixFeign.class })
    protected static class HystrixFeignConfiguration {
        @Bean
        @Scope("prototype")
        @ConditionalOnMissingBean
        @ConditionalOnProperty(name = "feign.hystrix.enabled", matchIfMissing = false)
        public Feign.Builder feignHystrixBuilder() {
            return HystrixFeign.builder();
        }
    }

    @Bean
    @ConditionalOnMissingBean
    public Retryer feignRetryer() {
        return Retryer.NEVER_RETRY;
    }

    @Bean
    @Scope("prototype")
    @ConditionalOnMissingBean
    public Feign.Builder feignBuilder(Retryer retryer) {
        return Feign.builder().retryer(retryer);
    }
    //如果缺少FeignLoggerFactory就注册FeignLoggerFactory
    @Bean
    @ConditionalOnMissingBean(FeignLoggerFactory.class)
    public FeignLoggerFactory feignLoggerFactory() {
        return new DefaultFeignLoggerFactory(logger);
    }

}

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【6】@EnableFeignClients 注解

源码如下：

/**
 * Scans for interfaces that declare they are feign clients (via {@link FeignClient
 * <code>@FeignClient</code>}). 
 * // 扫描那些声明为feign client 的接口
 * Configures component scanning directives for use with
 * {@link org.springframework.context.annotation.Configuration
 * <code>@Configuration</code>} classes.
 *
 * @author Spencer Gibb
 * @author Dave Syer
 * @since 1.0
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {

    /**
     * Alias for the {@link #basePackages()} attribute. Allows for more concise annotation
     * declarations e.g.: {@code @ComponentScan("org.my.pkg")} instead of
     * {@code @ComponentScan(basePackages="org.my.pkg")}.
     * @return the array of 'basePackages'.
     */
    String[] value() default {};

    /**
     * Base packages to scan for annotated components.
     * <p>
     * {@link #value()} is an alias for (and mutually exclusive with) this attribute.
     * <p>//  value与该属性是互斥，不能共同使用。
     * Use {@link #basePackageClasses()} for a type-safe alternative to String-based
     * package names.
     *
     * @return the array of 'basePackages'.
     */
    String[] basePackages() default {};

    /**
     * Type-safe alternative to {@link #basePackages()} for specifying the packages to
     * scan for annotated components. The package of each class specified will be scanned.
     * <p>// 指定扫描的基础类型，可以与basePackages共同使用
     * Consider creating a special no-op marker class or interface in each package that
     * serves no purpose other than being referenced by this attribute.
     *
     * @return the array of 'basePackageClasses'.
     */
    Class<?>[] basePackageClasses() default {};

    /**
     * A custom <code>@Configuration</code> for all feign clients. Can contain override
     * <code>@Bean</code> definition for the pieces that make up the client, for instance
     * {@link feign.codec.Decoder}, {@link feign.codec.Encoder}, {@link feign.Contract}.
     *
     * @see FeignClientsConfiguration for the defaults
     */
    Class<?>[] defaultConfiguration() default {};

    /**
     * List of classes annotated with @FeignClient. If not empty, disables classpath scanning.
     * @return
     */
    Class<?>[] clients() default {};
}

本篇博文项目地址：https://github.com/JanusJ/SpringCloud/