基于Feign实现异步调用

一、背景

希望将http的调用由同步等待改为异步，仍使用feign的便捷。

二、使用feign理由

本质上其实feign就是将httpclient常用的操作进行简单封装，且屏蔽底层的httpclient，无感知具体的client实现，轻松完成具体client的替换

三、解决方案

feign在10.8版本后提供了Async接口,如下：

四、demo代码实现

4.1 接口编写

public interface OriginFeignClient {

    @RequestLine("POST /async/server/api")

    CompletableFuture<String> api(@Param("param") String param);

}

4.2 接口发布

@Configuration

public class OriginFeignClientConfig {

    @Bean

    public SpringEncoder springEncoder(ObjectFactory<HttpMessageConverters> messageConverters) {

        return new SpringEncoder(messageConverters);

    }

    @Bean

    public SpringDecoder springDecoder(ObjectFactory<HttpMessageConverters> messageConverters) {

        return new SpringDecoder(messageConverters);

    }

    @Bean

    public OriginFeignClient originFeignClient(SpringEncoder springEncoder, SpringDecoder springDecoder) {

        return AsyncFeign.asyncBuilder()

                .encoder(springEncoder)

                .decoder(springDecoder)

                .target(OriginFeignClient.class, "http://localhost.charlesproxy.com:8090");

    }

}

4.3 调用

@GetMapping("testApi")

    public String testAsyncClient() throws ExecutionException, InterruptedException {

        List<CompletableFuture<String>> results = new ArrayList<>();

        for(int i = 0; i < 10; i++) {

            results.add(originFeignClient.api(i+""));

        }

        Thread.sleep(3000);

        int index = 0;

        for (CompletableFuture<String> result : results) {

            String str = result.get();

            log.info(String.format("%d, result:%s, ", index, str));

            index++;

        }

        return "success";

    }

4.4 结果（很明显，是异步调用）

2021-05-11 14:31:29.989 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 0, result:receive: {"param":"0"} at 2021-05-11 02:31:27.243,

2021-05-11 14:31:29.993 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 1, result:receive: {"param":"1"} at 2021-05-11 02:31:27.243,

2021-05-11 14:31:29.995 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 2, result:receive: {"param":"2"} at 2021-05-11 02:31:27.244,

2021-05-11 14:31:29.995 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 3, result:receive: {"param":"3"} at 2021-05-11 02:31:27.245,

2021-05-11 14:31:29.995 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 4, result:receive: {"param":"4"} at 2021-05-11 02:31:27.243,

2021-05-11 14:31:29.996 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 5, result:receive: {"param":"5"} at 2021-05-11 02:31:27.243,

2021-05-11 14:31:29.996 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 6, result:receive: {"param":"6"} at 2021-05-11 02:31:27.244,

2021-05-11 14:31:29.997 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 7, result:receive: {"param":"7"} at 2021-05-11 02:31:27.243,

2021-05-11 14:31:29.997 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 8, result:receive: {"param":"8"} at 2021-05-11 02:31:27.244,

2021-05-11 14:31:29.998 - INFO [TraceId: , SpanId : ] 24745 --- [nio-8091-exec-5] c.m.a.c.controller.ClientController : 9, result:receive: {"param":"9"} at 2021-05-11 02:31:27.245,

五、问题

feign增加的async实现是10.8版本，我们找到Spring boot 2.2.13和相应cloud最新版Hoxton.SR11（当然不是最新，是该小版本下最新，因为目前项目中使用的就是该版本），排查思路就是以@EnableFeignClients为入口

1.EnableFeignClients内容如下

@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {
}

引入了FeignClientsRegistrar

2.FeignClientsRegistrar功能如下：

(1)扫描@FeignClient，并将其下入到BeanDefinition中

(2)调用registerFeignClient，构造FeignClientFactoryBean（创建feign的工厂类，关键在此）

(3)进入FeignClientFactoryBean内部看下，构建feign的方法：getTarget()：

从context中去找feign.Feign.Builder的实现（即@Bean），看下当前项目中有哪几种实现：

1.org.springframework.cloud.openfeign.FeignCircuitBreaker.Builder Spring Cloud定义的通用断路器

2.feign.hystrix.HystrixFeign.Builder 断路器的原生实现

先不管哪种builder，最终执行：loadBalance，方法内容如下：

protected <T> T loadBalance(Feign.Builder builder, FeignContext context,
		HardCodedTarget<T> target) {
	Client client = getOptional(context, Client.class);
	if (client != null) {
		builder.client(client);
		Targeter targeter = get(context, Targeter.class);
		return targeter.target(this, builder, context, target);
	}
 
	throw new IllegalStateException(
			"No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-netflix-ribbon?");
}

好了！！看到我们想要的了，Client的创建，不对，等等！！Client，我们先看下他的实现类是不是包含我们的AsyncClient，

一切幻想破灭

好了，所以目前的Spring cloud openfeign中仍未支持该特性，先这样用吧

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