Feign详细构建过程及自定义扩展

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）

1. FeignAutoConfiguration自动配置类

```java
@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();
}

}
```

1. feign.hystrix.HystrixFeign类存在时，将 `HystrixTargeter` 注册为 `Targeter` 类型的 bean
2. feign.hystrix.HystrixFeign类不存在时，使用 `DefaultTargeter` 。
3. 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。

2. FeignClientsConfiguration

```java
@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

1. 在@EnableFeignClients注解中导入FeignClientsRegistrar类
2. FeignClientsRegistrar类实现了ImportBeanDefinitionRegistrar类，会由spring框架执行实现方法 ` registerBeanDefinitions(AnnotationMetaData, BeanDefinitionRegistry)`
3. FeignClientsRegistrar中的 `registerBeanDefinitions`方法调用两个方法
1. `registerDefaultConfiguration`：注册默认的配置
2. `registerFeignClients`：注册Feign客户端（**重点**）
4. `registerFeignClients`：获取 `@EnableFeignClients`注解中定义的配置扫描feign客户端
5. `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

# 3.FeignClientFactoryBean

FeignClient工厂bean。

```java
class FeignClientFactoryBean
implements FactoryBean, InitializingBean, ApplicationContextAware{
//...
}
```

通过实现方法 `FactoryBean#getObject()`来由spring框架生产FeignClient。

```java
@Override
public Object getObject() throws Exception {
return getTarget();
}

/**
* 获得目标
* 1. 获得FeignContext
* 2. 从FeignContext中获得Feign构建器Feign.Builder
* 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.HardCodedTarget);
* 5.由于一开始注入的Feign.Builder是HystrixFeign.Builder,则此处是调用HystrixFeign.Builder里的对应方法
*/
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.HardCodedTarget);
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**）

# 4.Targeter

## 4.1.HystrixTargeter

```java
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);
}

//...
}
```

1. HystrixTarget只处理 *Feign.Builder* 类型为 *feign.hystrix.HystrixFeign.Builder* 的
2. 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
3. 因此，即使注入的 Targeter 是 *HystrixTargeter*,此处也可以执行自定义 *Feign.Builder*。
4. 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）

## 4.2.DefaultTargeter

```java
class DefaultTargeter implements Targeter {
@Override
public T target(FeignClientFactoryBean factory, Feign.Builder feign,
FeignContext context, Target.HardCodedTarget target) {
return feign.target(target);
}
}
```

1. 执行 Feign.Builder （子）类型对应的 默认 target方法。
2. 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）

# 5.FeignBuilder

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。

```java
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);
}
}
}

```

1. *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
2. **Feign** 以及内部类 **Feign.Builder** 都是 *public* ，**可以重写并注入自定义的bean**。

## 5.1.HystrixFeign

```java
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);
}

/** Configures components needed for hystrix integration. */
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。

1. 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。
2. HystrixDelegatingContract 是 public 级别，不需要修改的话，仍然用这个。

## 5.2示例

以下示例参考 *SentinelFeign*
其中的 `YiFeiXiInvocationHandler` 和 `YiFeiXiFeignFallbackFactory`是自定义的。
```java
@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

1. 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
2. **Feign**以及**Feign.Builder**是 *publilc*，给了我们扩展的空间。

# 7.参考资料

1. **feign-hystrix-10.1.0.jar**和**spring-cloud-openfeign-core-2.1.1.RELEASE.jar**
2. **spring-cloud-alibaba-sentinel-0.9.0.RELEASE.jar**中的 *sentinelFeign* 实现
3. [Spring Cloud Alibaba Sentinel 整合 Feign 的设计实现](https://www.cnblogs.com/javastack/p/10129409.html)