Feign源码解析系列-那些注解们

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，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：

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。