Spring Cloud -- 动态创建FeignClient实例

一、Feign使用中存在的问题

我们在介绍Spring Cloud —— OpenFeign 核心原理2.2节时候，举了一个生产者消费者的案例，消费者服务在去调用生产者服务提供的接口时，我们需要定义定义 FeignClient 消费服务接口：

@FeignClient(name= "nacos-produce",contextId="DemoFeignClient")
public interface DemoFeignClient {
    @GetMapping("/hello")
    String sayHello(@RequestParam("name") String name);
}

说到这里，我们就会想到一个问题，如果我们有若干个消费者服务，那我们岂不是需要在每个消费者服务中都定义一次FeignClient 接口。因此为了解决这个问题，生产者服务通常需要提供一套API包，供各个消费者服务相互调用。

消费者服务想调用生产者提供的接口时只需要引入我们提供的API包，并做一些简单配置：

• 引入Feign依赖包（spring-cloud-starter-openfeign）；
• 指定Feign配置类（指定Encoder、Decoder、FeignLoggerFactory以及加入一些请求头信息等）
• 开启@EnableFeignCliens注解

然后向Spring容器中，注入FeignClient实例：

 @Autowired private DemoFeignClient demoFeignClient;

由于API包中提供的feign接口，是依赖于Spring Boot的，因此要求外部系统必须是Spring Boot应用，同时由于feign接口配置中没有指定用户权限服务的url地址，而采用指定服务名的方式，导致外部系统如果想调用feign接口，必须要和用户权限服务注册在同一个注册中心上。

二、动态创建FeignClient实例(SDK)

在上一节中我们曾经介绍过FeignClient的创建流程，其底层是通过FeignClientFactoryBean的getTarget创建了一个FeignClient接口的实例。既然利用FeignClientFactoryBean可以创建这么一个FeignClient接口的实例，那我们是否可以自己去手动创建这样一个实例呢，当然是可以的，spring-cloud-openfeign-core为我们提供了一个这样的类FeignClientBuild，采用FeignClientBuild可以动态创建FeignClient实例。同时为了摆脱对Spring Boot的依赖，我们可以自己创建一个Spring ApplicationContext，用来模拟@FeignClient实例的注入过程。

2.1、DemoResourceApi类

我们首先创建一个DemoResourceApi类，用来封装DemoFeignClient的实例：

package com.jnu.example.feign.sdk.swagger;
import com.jnu.example.feign.sdk.service.DemoFeignClient;
import com.jnu.example.feign.sdk.swagger.client.ApiClient;
import com.jnu.example.feign.sdk.util.FeignClientUtils;

/**
 * @Author： zy
 * @Date：2021/4/15
 * @Description：demo API
 */
public class DemoResourceApi {
    /**
     * demo client feign service
     */
    private DemoFeignClient demoFeignClient;

    /**
     * api client
     */
    private ApiClient apiClient;

    /**
     * constructor
     * @param apiClient: api client
     */
    public DemoResourceApi(ApiClient apiClient){
        if(apiClient == null){
            throw new NullPointerException("Api client NullPointerException");
        }
        this.apiClient = apiClient;
        this.demoFeignClient = FeignClientUtils.build(apiClient,"DemoFeignClient", DemoFeignClient.class);
    }

    /**
     * @Author： zy
     * @Date：2021/4/15 14:51
     * @Description：say hello
     * @param name
     * @return: User
     */
    public String sayHello(String name)  {
        String res = demoFeignClient.sayHello(name);
        return res;
    }

}

DemoFeignClient实例我们是通过FeignClientUtils工具类构建的。

2.2、FeignClientUtils

package com.jnu.example.feign.sdk.util;

import com.jnu.example.feign.sdk.ApplicationContextBuilder;
import com.jnu.example.feign.sdk.feign.FeignContextBuilder;
import com.jnu.example.feign.sdk.swagger.client.ApiClient;
import lombok.extern.slf4j.Slf4j;
import org.springframework.cloud.openfeign.FeignClientBuilder;
import org.springframework.cloud.openfeign.FeignContext;
import org.springframework.context.ApplicationContext;

import java.util.*;
import java.util.concurrent.ConcurrentHashMap;

/**
 * @Author： zy
 * @Date：2021/4/15
 * @Description：动态创建FeignClient实例  https://www.jianshu.com/p/172e002e0eb4
 * 关于动态创建Feign Client的问题 https://blog.csdn.net/qq_37312208/article/details/112476051
 */
@Slf4j
public final class FeignClientUtils {
    /**
     * forbid instantiate
     */
    private FeignClientUtils(){
        throw new AssertionError();
    }

    /**
     * Spring ApplicationContext
     */
    private  static ApplicationContext applicationContext;

    /**
     * save contextId -> feign client mapping
     */
    private static final Map<String, Object> FEIGN_CLIENT_CACHE = new ConcurrentHashMap<>();

    /**
     * build feign client
     */
    public static <T> T build(ApiClient apiClient, String contextId, Class<T> targetClass){
        return buildClient(apiClient,contextId,targetClass);
    }

    /**
     * build feign client without using the {@link FeignClient} annotation
     * @param apiClient: api client
     * @param contextId:  Unique Spring ApplicationContext identification for feign client
     * corresponding to the contextId in @FeignClient.  example  "NamedContextFactory"
     * @param targetClass: target class.  example: NamedContextFactory.class，
     */
    private static <T> T buildClient(ApiClient apiClient,String contextId, Class<T> targetClass) {
        //get
        T t = (T)FEIGN_CLIENT_CACHE.get(contextId);
        if(Objects.isNull(t)){
            synchronized(FeignClientUtils.class) {
                t = (T)FEIGN_CLIENT_CACHE.get(contextId);
                if(Objects.isNull(t)) {
                    //null check
                    if (applicationContext == null) {
                        //create Spring ApplicationContext
                        ApplicationContextBuilder builder = new ApplicationContextBuilder(apiClient.getUseRegistry()
                                ,apiClient.getServerAddr(),apiClient.getNamespace());
                        applicationContext = builder.getApplicationContext();
                    }

                    //get feign context
                    FeignContext feignContext = applicationContext.getBean(FeignContext.class);

                    //create Spring ApplicationContext for feign client
                    new FeignContextBuilder(feignContext, apiClient, contextId);

                    //A builder for creating Feign client without using the {@link FeignClient} annotation
                    FeignClientBuilder.Builder<T> builder = new FeignClientBuilder(applicationContext).forType(targetClass, apiClient.getServerName())
                            .contextId(contextId)
                            .url(apiClient.getUrl());
                    t = builder.build();

                    FEIGN_CLIENT_CACHE.put(contextId, t);
                }
            }
        }
        return t;
    }
}

这里我们通过ApplicationContextBuilder创建了一个Spring ApplicationContext，并且向该容器中手动注入了Feign、nacos自动装配时注入的bean。同时注入了FeignContext。因此我们才可以从容器中获取到FeignContext实例。然后通过FeignContextBuilder为每个FeignClient接口创建了一个子Spring ApplicationContext。并保存到FeignContext中。

 private Map<String, AnnotationConfigApplicationContext> contexts = new ConcurrentHashMap();

最后我们调用FeignClientBuilder创建对应的FeignClient实例（实际上就是通过contextId拿到FeignContext中保存的子容器、然后去构建Feign.Builder的过程），具体创建过程参考上一篇博客。

2.3、FeignClientBuilder

package com.jnu.example.feign.sdk.feign;

import cn.hutool.core.util.ReflectUtil;
import com.jnu.example.feign.sdk.feign.factory.TokenFactory;
import com.jnu.example.feign.sdk.swagger.client.ApiClient;
import feign.Logger;
import feign.Request;
import lombok.Getter;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.ObjectFactory;
import org.springframework.beans.factory.support.BeanDefinitionBuilder;
import org.springframework.boot.autoconfigure.http.HttpMessageConverters;
import org.springframework.cloud.openfeign.FeignContext;
import org.springframework.cloud.openfeign.support.SpringDecoder;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.http.MediaType;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;

import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;

/**
 * @Author： zy
 * @Date： 2021/4/18
 * @Description：A builder for creating Application context for feign client.
 */
@Slf4j
@Getter
public class FeignContextBuilder {

    /**
     * A factory that creates instances of feign classes. It creates a Spring
     * ApplicationContext per feign client, and extracts the beans that it needs from there.
     */
    private FeignContext feignContext;

    /**
     * Unique Spring ApplicationContext identification for feign client
     */
    private String contextId;

    /*
     * Spring ApplicationContext for feign client
     */
    private ApplicationContext feignClientContext;

    /**
     * constructor
     * @param feignContext:
     * @param apiClient
     * @param contextId
     */
    public FeignContextBuilder(FeignContext feignContext, ApiClient apiClient, String contextId){
        //save parameter
        this.feignContext = feignContext;
        this.contextId = contextId;

        //create Spring ApplicationContext for feign client .
        this.feignClientContext = createContext(feignContext,apiClient,contextId);
    }

    /**
     * create Spring ApplicationContext for feign client
     * FeignContext cannot find the Spring ApplicationContext we created based on the contextId, it will create  Spring ApplicationContext for feign client
     * @param apiClient: api client
     * @param contextId : Unique Spring ApplicationContext identification for feign client
     */
    private ApplicationContext createContext(FeignContext feignContext, ApiClient apiClient, String contextId){
        //create Spring ApplicationContext for feign client
        Method getContext =  ReflectUtil.getMethod(feignContext.getClass(),"getContext",String.class);
        getContext.setAccessible(true);

        AnnotationConfigApplicationContext context= null;
        try {
            //Inject Feign request interceptor bean into the Spring ApplicationContext
            context = (AnnotationConfigApplicationContext)getContext.invoke(feignContext,contextId);

            //Inject Feign request interceptor bean into the Spring ApplicationContext
            registerRequestInterceptor(context,apiClient.getLoginName(),apiClient.getPassword(),apiClient.getTokenFactory());

            //Inject Feign request options bean into the Spring ApplicationContext
            registerRequestOptions(context,apiClient.getConnectTimeoutMillis(),apiClient.getReadTimeoutMillis());

            //Inject Feign logger level bean into the Spring ApplicationContext
            registerLoggerLevel(context,apiClient.getLevel());

            //Inject Feign encoder bean into the Spring ApplicationContext
            registerEncoder(context);

            //Inject Feign decoder bean into the Spring ApplicationContext
            registerDecoder(context);

            //Inject Feign error decoder bean into the Spring ApplicationContext
            registerErrorDecoder(context);

            log.info("jnu-feign-server-sdk:" + contextId + " feign client context refresh success");
        } catch (Exception e) {
            log.error(contextId +": create sub context fail");
        }

        return context;
    }


    /**
     * Inject Feign request interceptor bean into the Spring ApplicationContext
     * @param context : Spring ApplicationContext for feign client
     * @param loginName：loginName
     * @param password: password
     */
    private void registerRequestInterceptor(AnnotationConfigApplicationContext context, String loginName, String password
            , TokenFactory tokenFactory){
        BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(FeignRequestInterceptor.class);
        builder.addPropertyValue("loginName",loginName);
        builder.addPropertyValue("password",password);
        builder.addPropertyValue("tokenFactory",tokenFactory);
        context.registerBeanDefinition("feignRequestInterceptor",builder.getBeanDefinition());
    }


    /**
     * Inject Feign request options bean into the Spring ApplicationContext
     * @param context : Spring ApplicationContext for feign client
     * @param connectTimeoutMillis : connect timeout
     * @param readTimeoutMillis: read timeout
     */
    private void registerRequestOptions(AnnotationConfigApplicationContext context,int connectTimeoutMillis,int readTimeoutMillis){
        BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(Request.Options.class);
        builder.addConstructorArgValue(connectTimeoutMillis);
        builder.addConstructorArgValue(readTimeoutMillis);
        builder.addConstructorArgValue(true);
        context.registerBeanDefinition("feignRequestOptions",builder.getBeanDefinition());
    }


    /**
     * Inject Feign encoder bean into the Spring ApplicationContext
     * @param context : Spring ApplicationContext for feign client
     */
    private void registerEncoder(AnnotationConfigApplicationContext context){
        BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(SpringEncoder.class);
        builder.addConstructorArgValue(feignHttpMessageConverter());
        context.registerBeanDefinition("feignEncoder",builder.getBeanDefinition());
    }

    /**
     * Inject Feign decoder bean into the Spring ApplicationContext
     * @param context : Spring ApplicationContext for feign client
     */
    private void registerDecoder(AnnotationConfigApplicationContext context){
        BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(SpringDecoder.class);
        builder.addConstructorArgValue(feignHttpMessageConverter());
        context.registerBeanDefinition("feignDecoder",builder.getBeanDefinition());
    }

    /**
     * HttpMessageConverters factory
     */
    private ObjectFactory<HttpMessageConverters> feignHttpMessageConverter() {
        final HttpMessageConverters httpMessageConverters = new HttpMessageConverters(new GateWayMappingJackson2HttpMessageConverter());
        return () -> httpMessageConverters;
    }

    /**
     * HttpMessageConverter
     */
    private static class GateWayMappingJackson2HttpMessageConverter extends MappingJackson2HttpMessageConverter {
        GateWayMappingJackson2HttpMessageConverter(){
            List<MediaType> mediaTypes = new ArrayList<>();
            mediaTypes.add(MediaType.valueOf(MediaType.TEXT_HTML_VALUE + ";charset=UTF-8"));
            setSupportedMediaTypes(mediaTypes);
        }
    }

    /**
     * Inject Feign logger level bean into the Spring ApplicationContext
     */
    private void registerLoggerLevel(AnnotationConfigApplicationContext context,Logger.Level level){
        //https://blog.csdn.net/u014252478/article/details/84869997
        BeanDefinitionBuilder beanDefinitionBuilder = BeanDefinitionBuilder.genericBeanDefinition(Logger.Level.class, () -> level);
        context.registerBeanDefinition("feignLoggerLevel",beanDefinitionBuilder.getBeanDefinition());
    }


    /**
     * Inject Feign error decoder bean into the Spring ApplicationContext
     */
    private void registerErrorDecoder(AnnotationConfigApplicationContext context){
        BeanDefinitionBuilder beanDefinitionBuilder = BeanDefinitionBuilder.genericBeanDefinition(FeignErrorDecoder.class, () -> new FeignErrorDecoder());
        context.registerBeanDefinition("feignErrorDecoder",beanDefinitionBuilder.getBeanDefinition());
    }
}

三、代码下载

以上只展示了部分代码，更多代码下载：https://github.com/Zhengyang550/jnu-feign-server。