在微服务架构中总需要各个服务相互调用，各个服务又是以接口方式暴露，所以需要Http远程调用，于是我们就用到了Feign。

Feign是一种声明式、模板化的HTTP客户端。在Spring Cloud中使用Feign, 我们可以做到使用HTTP请求远程服务时能与调用本地方法一样的编码体验，开发者完全感知不到这是远程方法，更感知不到这是个HTTP请求。

首先加入依赖

<!-- spring cloud  feign组件 -->
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-openfeign</artifactId>
</dependency>

在启动类上使用注解@EnableFeignClients启用feign客户端

@EnableFeignClients

创建Feign客户端

@FeignClient("common-wx")
//@FeignClient(value = "common-wx", url = "www.baidu.com/common-wx")
public interface InfoCommonWXService {
    /**
     * 获取ticket
     */
    @PostMapping(value = "/wx/auth/jsapi/ticket/v1")
    ApiResponse getWxTicket(@RequestBody Map<String, Object> map);
}

value()和name()一样，是被调用的服务的名称（微服务名），url()直接填写调用的地址（同http的url地址），注意一点，如果存在url的话，会直接调到url上。

在其他service中直接依赖注入feign的接口名便可以使用，或者可以把feign看成一个服务调用就好。

@SuppressWarnings("SpringJavaInjectionPointsAutowiringInspection")
@Autowired
private InfoCommonWXService infoCommonWXService;

以上就是一般使用过程，对于开发中使用已经足够了！

下面贴出FeignClient源码

/*
 * Copyright 2013-2016 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.openfeign;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.core.annotation.AliasFor;

/**
 * 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}.
	 *
	 * @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 "";
	
	/**
	 * 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;

}

FeignClient注解被@Target(ElementType.TYPE)修饰，表示FeignClient的作用目标在接口上。

@Retention(RetentionPolicy.RUNTIME)注解表明该注解会在Class字节码中存在，在运行时可以通过反射获取到。

@Documented表明该注解被包含在javadoc中。

@FeignClient用于创建声明是API接口，该接口是RESTful风格的。Feign被设计成插拔式的，可注入其他组件和Feign一起使用。最典型的是如果Ribbon可用，Feign会和Ribbon相结合进行负载均衡。

name：指定FeignClient的名称，如果项目使用了Ribbon，name属性会作为微服务的名称，用于服务发现

url: url一般用于调试，可以手动指定@FeignClient调用的地址

decode404:当发生http 404错误时，如果该字段位true，会调用decoder进行解码，否则抛出FeignException

configuration: Feign配置类，可以自定义Feign的Encoder、Decoder、LogLevel、Contract

fallback: 定义容错的处理类，当调用远程接口失败或超时时，会调用对应接口的容错逻辑，fallback指定的类必须实现@FeignClient标记的接口

fallbackFactory: 工厂类，用于生成fallback类示例，通过这个属性我们可以实现每个接口通用的容错逻辑，减少重复的代码

path: 定义当前FeignClient的统一前缀