开始
Feign在Spring Cloud体系中被整合进来作为web service客户端，使用HTTP请求远程服务时能就像调用本地方法，可见在未来一段时间内，大多数Spring Cloud架构的微服务之间调用都会使用Feign来完成。
所以准备完整解读一遍Feign的源码，读源码，我个人觉得一方面，可以在使用的基础上对内部实现的细节的了解，提高使用时对组件功能的信心，另一方面，开源组件的代码质量一般都比较高，对代码结构组织一般比较优秀，还有，内部实现的一些细节可能优秀开发的思考所得，值得仔细揣摩。我对后两个好处比较感兴趣，虽然现如今写的代码好与坏，其实不会太多的影响平时的工作，不过如果内心是真的爱代码，也会不断追求细节的极致。
因为是Spring Cloud体系下使用Feign，必然会涉及到：服务注册（Euraka），负载均衡（Rinbon），熔断器（Hystrix）等方面的整合知识。
另外，能思考的高度和广度必然有限，但是源码阅读学习又难以共同参与，所以刚好你也在这个位置，有自己的思路或想法，不吝留言。
内容
1，EnableFeignClients注解
大流程上，就是扫描FeignClient注解的接口，将接口方法动态代理成http客户端的接口请求操作就完成了Feign的目的。所以一个FeignClient注解对应一个客户端。
- EnableFeignClients这个注解可以配置扫描FeignClient注解的路径。可以通过value属性或basePackages属性来制定扫描的包路径。
- basePackageClasses属性并不是精准扫描哪几个Class，而是指定这些指定的class在的package会被扫描。所以注释中推荐写一个空接口来标记这个package要被扫描的方式来关联。
- defaultConfiguration属性是可以定义全局Feign配置的类，默认使用FeignClientsConfiguration类。想要自定义需要好好确认下FeignClientsConfiguration定义了那一些bean。当然如果只是想覆盖部分bean，完全不用这个，直接在Configuration定义对应bean即可。
- clients属性才是精准指定Class扫描，与package扫描互斥。
@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>
* 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>
* 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 {};
}从EnableFeignClients注解的属性看，我们可以了解到，在解析这个注解属性的时候，需要利用配置的扫描的package或Class，扫描FeignClient注解，进而解析那些FeignClient注解的配置属性。并且我们还可以配置全局的Feign相关的配置。
回头我们再看一下EnableFeignClients定义的元数据，@Import注解的使用值得学习一下。
关于这个注解，我们可以理解成导入
@Import注解导入的类 FeignClientsRegistrar 是继承 ImportBeanDefinitionRegistrar 的，ImportBeanDefinitionRegistrar的方法一般实现动态注册bean使用，在由@Import注解导入后，Spring容器启动时会执行registerBeanDefinitions方法。
所以一般@Import注解和ImportBeanDefinitionRegistrar实现动态注册bean而配合使用。
前面提到大流程，这篇文章的思路基本描述了：扫描+动态代理接口+http请求，其中也对@Import和ImportBeanDefinitionRegistrar使用场景进行了解释，可以做参考学习。
2，FeignClient注解
每个FeignClient代表一个http客户端，定义的每一个方法对应这个一个接口。
- value和name用于定义http客户端服务的名称，在spring cloud为服务之间调用服务总要有负载均衡的，比如Rinbon。所以这里定义的会是服务提供方的应用名（serviceId）。
- qualifier属性在spring容器中定义FeignClient的bean时，配置名称，在装配bean的时候可以用这个名称装配。使用spring的注解：Qualifier。
- url属性用来定义请求的绝对URL。
- decode404属性，在客户端返回404时是进行decode操作还是抛出异常的标记。
- configuration属性，自定义配置类，可以定义Decoder, Encoder，Contract来覆盖默认的配置，可以参考默认的配置类：FeignAutoConfiguration
- fallback属性 使用fallback机制时可以配置的类属性，继承客户端接口，实现fallback逻辑。如果要使用fallback机制需要配合Hystrix一起，所以需要开启Hystrix。
- fallbackFactory属性 生产fallback实例，生产的自然是继承客户端接口的实例。
- path属性 每个接口url的统一前缀
- primary属性 标记在spring容器中为primary bean
/**
* 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;
}通过FeignClient注解的属性，可以看到针对单个Feign客户端可以做自定义的配置。
3，定义客户端接口的注解
在Feign中需要定义http接口的办法，注解是个好解决方案。这里就看到Contract的接口，解析这些注解用的，下面是抽象类BaseContract，它有默认实现，即Contract.Default，解析了自定义注解：feign.Headers，feign.RequestLine，feign.Body，feign.Param，feign.QueryMap，feign.HeaderMap，这些注解都是用来定义描述http客户端提供的接口信息的。
但是因为这里默认将Feign和Spring Cloud体系中使用，而提供了SpringMvcContract类来解析使用的注解，而这个注解就是RequestMapping。这个注解使用过spring mvc的同学必然非常熟悉，这里就是利用了这个注解的定义进行解析，只是功能上并不是和spring保持完全一致，毕竟它这里只需要考虑将接口信息定义出来即可。
在SpringMvcContract的代码里，可以看到解析RequestMapping注解属性的逻辑代码，如此在使用中可以直接使用RequestMapping来定义接口。
- value属性和path属性定义接口路径
- method属性配置HTTP请求方法
- params属性在feign中不支持
- headers属性配置http头信息
- consumes属性配置http头信息，只解析使用配置了 Content-Type 属性的值
- produces属性配置http头信息，只解析使用配置了 Accept 属性的值
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Mapping
public @interface RequestMapping {
/**
* Assign a name to this mapping.
* <p><b>Supported at the type level as well as at the method level!</b>
* When used on both levels, a combined name is derived by concatenation
* with "#" as separator.
* @see org.springframework.web.servlet.mvc.method.annotation.MvcUriComponentsBuilder
* @see org.springframework.web.servlet.handler.HandlerMethodMappingNamingStrategy
*/
String name() default "";
/**
* The primary mapping expressed by this annotation.
* <p>In a Servlet environment this is an alias for {@link #path}.
* For example {@code @RequestMapping("/foo")} is equivalent to
* {@code @RequestMapping(path="/foo")}.
* <p>In a Portlet environment this is the mapped portlet modes
* (i.e. "EDIT", "VIEW", "HELP" or any custom modes).
* <p><b>Supported at the type level as well as at the method level!</b>
* When used at the type level, all method-level mappings inherit
* this primary mapping, narrowing it for a specific handler method.
*/
@AliasFor("path")
String[] value() default {};
/**
* In a Servlet environment only: the path mapping URIs (e.g. "/myPath.do").
* Ant-style path patterns are also supported (e.g. "/myPath/*.do").
* At the method level, relative paths (e.g. "edit.do") are supported within
* the primary mapping expressed at the type level. Path mapping URIs may
* contain placeholders (e.g. "/${connect}")
* <p><b>Supported at the type level as well as at the method level!</b>
* When used at the type level, all method-level mappings inherit
* this primary mapping, narrowing it for a specific handler method.
* @see org.springframework.web.bind.annotation.ValueConstants#DEFAULT_NONE
* @since 4.2
*/
@AliasFor("value")
String[] path() default {};
/**
* The HTTP request methods to map to, narrowing the primary mapping:
* GET, POST, HEAD, OPTIONS, PUT, PATCH, DELETE, TRACE.
* <p><b>Supported at the type level as well as at the method level!</b>
* When used at the type level, all method-level mappings inherit
* this HTTP method restriction (i.e. the type-level restriction
* gets checked before the handler method is even resolved).
* <p>Supported for Servlet environments as well as Portlet 2.0 environments.
*/
RequestMethod[] method() default {};
/**
* The parameters of the mapped request, narrowing the primary mapping.
* <p>Same format for any environment: a sequence of "myParam=myValue" style
* expressions, with a request only mapped if each such parameter is found
* to have the given value. Expressions can be negated by using the "!=" operator,
* as in "myParam!=myValue". "myParam" style expressions are also supported,
* with such parameters having to be present in the request (allowed to have
* any value). Finally, "!myParam" style expressions indicate that the
* specified parameter is <i>not</i> supposed to be present in the request.
* <p><b>Supported at the type level as well as at the method level!</b>
* When used at the type level, all method-level mappings inherit
* this parameter restriction (i.e. the type-level restriction
* gets checked before the handler method is even resolved).
* <p>In a Servlet environment, parameter mappings are considered as restrictions
* that are enforced at the type level. The primary path mapping (i.e. the
* specified URI value) still has to uniquely identify the target handler, with
* parameter mappings simply expressing preconditions for invoking the handler.
* <p>In a Portlet environment, parameters are taken into account as mapping
* differentiators, i.e. the primary portlet mode mapping plus the parameter
* conditions uniquely identify the target handler. Different handlers may be
* mapped onto the same portlet mode, as long as their parameter mappings differ.
*/
String[] params() default {};
/**
* The headers of the mapped request, narrowing the primary mapping.
* <p>Same format for any environment: a sequence of "My-Header=myValue" style
* expressions, with a request only mapped if each such header is found
* to have the given value. Expressions can be negated by using the "!=" operator,
* as in "My-Header!=myValue". "My-Header" style expressions are also supported,
* with such headers having to be present in the request (allowed to have
* any value). Finally, "!My-Header" style expressions indicate that the
* specified header is <i>not</i> supposed to be present in the request.
* <p>Also supports media type wildcards (*), for headers such as Accept
* and Content-Type. For instance,
* <pre class="code">
* @RequestMapping(value = "/something", headers = "content-type=text/*")
* </pre>
* will match requests with a Content-Type of "text/html", "text/plain", etc.
* <p><b>Supported at the type level as well as at the method level!</b>
* When used at the type level, all method-level mappings inherit
* this header restriction (i.e. the type-level restriction
* gets checked before the handler method is even resolved).
* <p>Maps against HttpServletRequest headers in a Servlet environment,
* and against PortletRequest properties in a Portlet 2.0 environment.
* @see org.springframework.http.MediaType
*/
String[] headers() default {};
/**
* The consumable media types of the mapped request, narrowing the primary mapping.
* <p>The format is a single media type or a sequence of media types,
* with a request only mapped if the {@code Content-Type} matches one of these media types.
* Examples:
* <pre class="code">
* consumes = "text/plain"
* consumes = {"text/plain", "application/*"}
* </pre>
* Expressions can be negated by using the "!" operator, as in "!text/plain", which matches
* all requests with a {@code Content-Type} other than "text/plain".
* <p><b>Supported at the type level as well as at the method level!</b>
* When used at the type level, all method-level mappings override
* this consumes restriction.
* @see org.springframework.http.MediaType
* @see javax.servlet.http.HttpServletRequest#getContentType()
*/
String[] consumes() default {};
/**
* The producible media types of the mapped request, narrowing the primary mapping.
* <p>The format is a single media type or a sequence of media types,
* with a request only mapped if the {@code Accept} matches one of these media types.
* Examples:
* <pre class="code">
* produces = "text/plain"
* produces = {"text/plain", "application/*"}
* produces = "application/json; charset=UTF-8"
* </pre>
* <p>It affects the actual content type written, for example to produce a JSON response
* with UTF-8 encoding, {@code "application/json; charset=UTF-8"} should be used.
* <p>Expressions can be negated by using the "!" operator, as in "!text/plain", which matches
* all requests with a {@code Accept} other than "text/plain".
* <p><b>Supported at the type level as well as at the method level!</b>
* When used at the type level, all method-level mappings override
* this produces restriction.
* @see org.springframework.http.MediaType
*/
String[] produces() default {};
}和注解RequestMapping组合使用在传参的注解目前包含：PathVariable，RequestHeader，RequestParam。
PathVariable：url占位符参数绑定
RequestHeader：可以设置业务header
RequestParam：将传参映射到http请求的参数，get/post请求都支持
关于RequestParam，前面有文章涉及到细节：链接
结束
先看一眼将涉及到的注解，通过这些注解，我们可以大致了解到Feign能提供的能力范围和实现机制，而对应这些注解的源码在后续文章中也将一一学习到。