探究清楚 feign 的原理，自定义 feign 功能
spring-cloud-openfeign-core-2.1.1.RELEASE.jar 中 HystrixFeign 的详细构建过程：
@EnableFeignClients -> FeignClientsRegistrar 扫描 @Feign注解的类 -> FeignClientFactoryBean通过Targeter生产FeignClient -> Targeter通过Feign.Builder构建Feign -> Feign.Builder
1. 准备工作（配置）- FeignAutoConfiguration自动配置类
@Configuration
@ConditionalOnClass(name = "feign.hystrix.HystrixFeign")
protected static class HystrixFeignTargeterConfiguration {
@Bean
@ConditionalOnMissingBean
public Targeter feignTargeter() {
return new HystrixTargeter();
}
}
@Configuration
@ConditionalOnMissingClass("feign.hystrix.HystrixFeign")
protected static class DefaultFeignTargeterConfiguration {
@Bean
@ConditionalOnMissingBean
public Targeter feignTargeter() {
return new DefaultTargeter();
}
}
-
feign.hystrix.HystrixFeign类存在时，将 HystrixTargeter 注册为 Targeter 类型的 bean
-
feign.hystrix.HystrixFeign类不存在时，使用 DefaultTargeter 。
-
看起来似乎可以使用自定义的Targeter代替Hystrix或默认的，这样就可以自定义各种功能了。实际上不行，因为 Targeter 是 package 访问级别的。
-
FeignClientsConfiguration
@Configuration
public class FeignClientsConfiguration {
@Bean
@ConditionalOnMissingBean
public Retryer feignRetryer() {
return Retryer.NEVER_RETRY;
}
@Bean
@Scope("prototype")
@ConditionalOnMissingBean
public Feign.Builder feignBuilder(Retryer retryer) {
return Feign.builder().retryer(retryer);
}
@Configuration
@ConditionalOnClass({ HystrixCommand.class, HystrixFeign.class })
protected static class HystrixFeignConfiguration {
@Bean
@Scope("prototype")
@ConditionalOnMissingBean
@ConditionalOnProperty(name = "feign.hystrix.enabled")
public Feign.Builder feignHystrixBuilder() {
return HystrixFeign.builder();
}
}
}
重要：Feign 以及内部类 Feign.Builder 都是 public 访问级别，可以注入自定义的bean。
2.EnableFeignClients与FeignClientsRegistrar类将使用@FeignClient注解的类注册成spring bean，并使用注解中的配置
- 在@EnableFeignClients注解中导入FeignClientsRegistrar类
- FeignClientsRegistrar类实现了importBeanDefinitionRegistrar类，会由spring框架执行实现方法 registerBeanDefinitions(AnnotationmetaData, BeanDefinitionRegistry)
- FeignClientsRegistrar中的 registerBeanDefinitions方法调用两个方法
- registerDefaultConfiguration：注册默认的配置
- registerFeignClients：注册Feign客户端（重点）
- registerFeignClients：获取 @EnableFeignClients注解中定义的配置扫描feign客户端
- registerFeignClients：通过registerFeignClient(BeanDefinitionRegistry, Annotationmetadata, Map)方法注册每一个feignClient，过程：先获取 @FeignClient注解中定义的配置，将配置应用在spring bean 工厂 FeignClientFactoryBean， 通过工厂类 FeignClientFactoryBean 为每一个使用@FeignClient注解的类生产 FeignClient，详细过程见下一节
FeignClient工厂bean。
class FeignClientFactoryBean
implements FactoryBean通过实现方法 FactoryBean#getObject()来由spring框架生产FeignClient。
@Override
public Object getObject() throws Exception {
return getTarget();
}
T getTarget() {
FeignContext context = this.applicationContext.getBean(FeignContext.class);
Feign.Builder builder = feign(context);
//省略部分代码
// ......
Client client = getOptional(context, Client.class);
if (client != null) {
if (client instanceof LoadBalancerFeignClient) {
// not load balancing because we have a url,
// but ribbon is on the classpath, so unwrap
client = ((LoadBalancerFeignClient) client).getDelegate();
}
builder.client(client);
}
Targeter targeter = get(context, Targeter.class);
return (T) targeter.target(this, builder, context,
new HardCodedTarget<>(this.type, this.name, url));
}
protected Feign.Builder feign(FeignContext context) {
FeignLoggerFactory loggerFactory = get(context, FeignLoggerFactory.class);
Logger logger = loggerFactory.create(this.type);
// @formatter:off
Feign.Builder builder = get(context, Feign.Builder.class)
// required values
.logger(logger)
.encoder(get(context, Encoder.class))
.decoder(get(context, Decoder.class))
.contract(get(context, Contract.class));
// @formatter:on
configureFeign(context, builder);
return builder;
}
工厂获得对象（目标）：
1. 获得FeignContext（feign上下文） 2. 从FeignContext中获得Feign构建器Feign.Builder（public，可以在此使用自定义构建器） 3. 从FeignContext中获得Client，判断是否进行负载均衡 4. 从FeignContext中获得Target，并执行Target的默认方法target(FeignClientFactoryBean, Feign.Builder, FeignContext, Target.HardCodedTarget4.Targeter 4.1.HystrixTargeter); 5. 由于一开始注入的 *Targeter* 是 *HystrixTargeter* ,则此处是调用 HystrixTargeter 里的对应方法（从第一节的配置来看，只要 *feign.hystrix.HystrixFeign* 类存在，就是注入的 *HystrixTargeter *， 否则是 *DefaultTargeter*，对于需要**自定义构建feign的，这里不太重要**）
class HystrixTargeter implements Targeter {
@Override
public T target(FeignClientFactoryBean factory, Feign.Builder feign,
FeignContext context, Target.HardCodedTarget target) {
// 若不是 HystrixFeign，则执行其对应的默认target方法。
// 此处只处理HystrixFeign。
if (!(feign instanceof feign.hystrix.HystrixFeign.Builder)) {
return feign.target(target);
}
feign.hystrix.HystrixFeign.Builder builder = (feign.hystrix.HystrixFeign.Builder) feign;
SetterFactory setterFactory = getOptional(factory.getName(), context,
SetterFactory.class);
if (setterFactory != null) {
builder.setterFactory(setterFactory);
}
Class fallback = factory.getFallback();
if (fallback != void.class) {
return targetWithFallback(factory.getName(), context, target, builder,
fallback);
}
Class fallbackFactory = factory.getFallbackFactory();
if (fallbackFactory != void.class) {
return targetWithFallbackFactory(factory.getName(), context, target, builder,
fallbackFactory);
}
// 调用从Feign.Builder继承的方法。
return feign.target(target);
}
private T targetWithFallbackFactory(String feignClientName, FeignContext context,
Target.HardCodedTarget target, HystrixFeign.Builder builder,
Class fallbackFactoryClass) {
FallbackFactory fallbackFactory = (FallbackFactory) getFromContext(
"fallbackFactory", feignClientName, context, fallbackFactoryClass,
FallbackFactory.class);
return builder.target(target, fallbackFactory);
}
private T targetWithFallback(String feignClientName, FeignContext context,
Target.HardCodedTarget target, HystrixFeign.Builder builder,
Class fallback) {
T fallbackInstance = getFromContext("fallback", feignClientName, context,
fallback, target.type());
return builder.target(target, fallbackInstance);
}
//...
}
- HystrixTarget只处理 Feign.Builder 类型为 feign.hystrix.HystrixFeign.Builder 的
- 若feign构建器不是 feign.hystrix.HystrixFeign.Builder 类型，则执行注入的 feign 构建器的默认target方法
- 因此，即使注入的 Targeter 是 HystrixTargeter,此处也可以执行自定义 Feign.Builder。
- 理解：Feign.Builder#target(Target) 方法通常不会被 override（后续会讲解为什么不重写此方法）
class DefaultTargeter implements Targeter {
@Override
public T target(FeignClientFactoryBean factory, Feign.Builder feign,
FeignContext context, Target.HardCodedTarget target) {
return feign.target(target);
}
}
- 执行 Feign.Builder （子）类型对应的 默认 target方法。
- 理解：Feign.Builder#target(Target) 方法通常不会被 override（后续会讲解为什么不重写此方法）
feign构建器：构建feign对象。
Feign的目的是将 http api 包装成 restful 风格以便开发。
在实现中，Feign 是一个为目标http apis 生成 feign对象（Feign#newInstance）的工厂。
上述步骤目前需要的都是通过对应的 Builder 构建对应的Feign。
public abstract class Feign {
public static Builder builder() {
return new Builder();
}
public abstract T newInstance(Target target);
public static class Builder {
public T target(Target target) {
return build().newInstance(target);
}
public Feign build() {
SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =
new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger, logLevel, decode404, closeAfterDecode, propagationPolicy);
ParseHandlersByName handlersByName =
new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder,
errorDecoder, synchronousMethodHandlerFactory);
return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);
}
}
}
- Feign.Builder#target(Target) 方法里面实际上调用的是 build() 方法来构建对象，因此重写 build() 方法即可，没有必要还重写 target(Target) 方法
- Feign 以及内部类 Feign.Builder 都是 public ，可以重写并注入自定义的bean。
public final class HystrixFeign {
public static final class Builder extends Feign.Builder {
@Override
public Feign build() {
return build(null);
}
// 提供一系列的target方法，支持各种配置：fallback、FallBackFactory等
public T target(Target target, T fallback) {
return build(fallback != null ? new FallbackFactory.Default(fallback) : null)
.newInstance(target);
}
public T target(Target target, FallbackFactory fallbackFactory) {
return build(fallbackFactory).newInstance(target);
}
public T target(Class apiType, String url, T fallback) {
return target(new Target.HardCodedTarget(apiType, url), fallback);
}
public T target(Class apiType,
String url,
FallbackFactory fallbackFactory) {
return target(new Target.HardCodedTarget(apiType, url), fallbackFactory);
}
Feign build(final FallbackFactory nullableFallbackFactory) {
super.invocationHandlerFactory(new InvocationHandlerFactory() {
@Override
public InvocationHandler create(Target target,
Map dispatch) {
return new HystrixInvocationHandler(target, dispatch, setterFactory,
nullableFallbackFactory);
}
});
super.contract(new HystrixDelegatingContract(contract));
return super.build();
}
基本到了这一步，需要设置的东西，都可以配置了。
- 虽然 build 方法中涉及到 InvocationHandler,但基本不需要改什么，而 InvocationHandler 竟然也是 package 访问级别，所以只好复制一个，使用自己的。
- HystrixDelegatingContract 是 public 级别，不需要修改的话，仍然用这个。
以下示例参考 SentinelFeign
其中的 YiFeiXiInvocationHandler 和 YiFeiXiFeignFallbackFactory是自定义的。
@Override
public Feign build() {
super.invocationHandlerFactory(new InvocationHandlerFactory() {
@Override
public InvocationHandler create(Target target,
Map dispatch) {
// using reflect get fallback and fallbackFactory properties from
// FeignClientFactoryBean because FeignClientFactoryBean is a package
// level class, we can not use it in our package
Object feignClientFactoryBean = Builder.this.applicationContext
.getBean("&" + target.type().getName());
Class fallback = (Class) getFieldValue(feignClientFactoryBean,
"fallback");
Class fallbackFactory = (Class) getFieldValue(feignClientFactoryBean,
"fallbackFactory");
String name = (String) getFieldValue(feignClientFactoryBean, "name");
Object fallbackInstance;
FallbackFactory fallbackFactoryInstance;
// check fallback and fallbackFactory properties
// 以下逻辑在HystrixTargeter中有，但执行自定义的builder，不会执行到那段逻辑，因此此处加上。
if (void.class != fallback) {
fallbackInstance = getFromContext(name, "fallback", fallback,
target.type());
return new YiFeiXiInvocationHandler(target, dispatch, setterFactory,
new FallbackFactory.Default(fallbackInstance));
}
if (void.class != fallbackFactory) {
fallbackFactoryInstance = (FallbackFactory) getFromContext(name,
"fallbackFactory", fallbackFactory,
FallbackFactory.class);
return new YiFeiXiInvocationHandler(target, dispatch, setterFactory,
fallbackFactoryInstance);
}
// 若注解中没有使用fallback或fallbackFactory，则使用一个默认的FallbackFactory。
return new YiFeiXiInvocationHandler(target, dispatch, setterFactory, new YiFeiXiFeignFallbackFactory<>(target));
}
private Object getFromContext(String name, String type,
Class fallbackType, Class targetType) {
Object fallbackInstance = feignContext.getInstance(name,
fallbackType);
if (fallbackInstance == null) {
throw new IllegalStateException(String.format(
"No %s instance of type %s found for feign client %s",
type, fallbackType, name));
}
if (!targetType.isAssignableFrom(fallbackType)) {
throw new IllegalStateException(String.format(
"Incompatible %s instance. Fallback/fallbackFactory of type %s is not assignable to %s for feign client %s",
type, fallbackType, targetType, name));
}
return fallbackInstance;
}
});
super.contract(new HystrixDelegatingContract(contract));
return super.build();
}
需要自定义fallbackFactory，则实现 feign.hystrix.FallbackFactory类，需要自定义fallback，则实现 org.springframework.cglib.proxy.MethodInterceptor即可
6.总结- 由于Feign构建过程所用到的 Targeter 是 package 访问级别的，不能使用自定义的
- Feign以及Feign.Builder是 publilc，给了我们扩展的空间。
