Feign源码解析系列-那些注解们

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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">
    * &#064;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。

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