feign 服务调用流程
参考链接：
Feign源码解析
- 首先通过@EnableFeignClients注解开启FeignCleint
- 根据Feign的规则实现接口，并加@FeignCleint注解
- 程序启动后，会进行包扫描，扫描所有的@ FeignCleint的注解的类，并将这些信息注入到ioc容器中。
- 当接口的方法被调用，通过jdk的代理，来生成具体的RequesTemplate
- RequesTemplate在生成Request
- Request交给Client去处理，其中Client可以是HttpUrlConnection、HttpClient也可以是Okhttp
- 最后Client被封装到LoadBalanceClient类，这个类结合类Ribbon做到了负载均衡。
我们用图来标示下流程：
源码解析
首先在启动配置上检查是否有@EnableFeignClients注解，如果有该注解，则开启包扫描，扫描被@FeignClient注解接口。扫描到FeignClient后，将信息取出，以bean的形式注入到ioc容器中，源码如下：
class FeignClientsRegistrar
implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware {
.....
@Override
public void registerBeanDefinitions(AnnotationMetadata metadata,
BeanDefinitionRegistry registry) {
registerDefaultConfiguration(metadata, registry);
registerFeignClients(metadata, registry);
}
private void registerFeignClient(BeanDefinitionRegistry registry,
AnnotationMetadata annotationMetadata, Map attributes) {
String className = annotationMetadata.getClassName();
BeanDefinitionBuilder definition = BeanDefinitionBuilder
.genericBeanDefinition(FeignClientFactoryBean.class);
validate(attributes);
definition.addPropertyValue("url", getUrl(attributes));
definition.addPropertyValue("path", getPath(attributes));
String name = getName(attributes);
definition.addPropertyValue("name", name);
String contextId = getContextId(attributes);
definition.addPropertyValue("contextId", contextId);
definition.addPropertyValue("type", className);
definition.addPropertyValue("decode404", attributes.get("decode404"));
definition.addPropertyValue("fallback", attributes.get("fallback"));
definition.addPropertyValue("fallbackFactory", attributes.get("fallbackFactory"));
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
String alias = contextId + "FeignClient";
AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();
boolean primary = (Boolean) attributes.get("primary"); // has a default, won't be
// null
beanDefinition.setPrimary(primary);
String qualifier = getQualifier(attributes);
if (StringUtils.hasText(qualifier)) {
alias = qualifier;
}
BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,
new String[] { alias });
BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
}
在上面 registerFeignClient 方法中有这样一段代码：
BeanDefinitionBuilder definition = BeanDefinitionBuilder .genericBeanDefinition(FeignClientFactoryBean.class);
将FeignClientFactoryBean 定义进来，而FeignClientFactoryBean 实现了FactoryBean接口，这样把bean 注入容器。
@Override
public Object getObject() throws Exception {
return getTarget();
}
/**
* @param the target type of the Feign client
* @return a {@link Feign} client created with the specified data and the context
* information
*/
T getTarget() {
FeignContext context = this.applicationContext.getBean(FeignContext.class);
Feign.Builder builder = feign(context);
if (!StringUtils.hasText(this.url)) {
if (!this.name.startsWith("http")) {
this.url = "http://" + this.name;
}
else {
this.url = this.name;
}
this.url += cleanPath();
return (T) loadBalance(builder, context,
new HardCodedTarget(this.type, this.name, this.url));
}
if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {
this.url = "http://" + this.url;
}
String url = this.url + cleanPath();
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));
}
上面生成对象 调用了 targeter.target, 跟踪进去调用了 feign.target
/**
* @author Spencer Gibb
*/
class DefaultTargeter implements Targeter {
@Override
public T target(FeignClientFactoryBean factory, Feign.Builder feign,
FeignContext context, Target.HardCodedTarget target) {
return feign.target(target);
}
}
继续跟踪
public T target(Target target) {
return build().newInstance(target);
}
跟踪到 ReflectiveFeign 返回代理对象
/**
* creates an api binding to the {@code target}. As this invokes reflection, care should be taken
* to cache the result.
*/
@SuppressWarnings("unchecked")
@Override
public T newInstance(Target target) {
Map nameToHandler = targetToHandlersByName.apply(target);
Map methodToHandler = new LinkedHashMap();
List defaultMethodHandlers = new LinkedList();
for (Method method : target.type().getMethods()) {
if (method.getDeclaringClass() == Object.class) {
continue;
} else if (Util.isDefault(method)) {
DefaultMethodHandler handler = new DefaultMethodHandler(method);
defaultMethodHandlers.add(handler);
methodToHandler.put(method, handler);
} else {
methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
}
}
InvocationHandler handler = factory.create(target, methodToHandler);
T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),
new Class[] {target.type()}, handler);
for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
defaultMethodHandler.bindTo(proxy);
}
return proxy;
}
注入bean之后，通过jdk的代理，当请求Feign Client的方法时会被拦截，代码就在ReflectiveFeign类。
真正的动态代理是下面的静态类
static class FeignInvocationHandler implements InvocationHandler {
private final Target target;
private final Map dispatch;
FeignInvocationHandler(Target target, Map dispatch) {
this.target = checkNotNull(target, "target");
this.dispatch = checkNotNull(dispatch, "dispatch for %s", target);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if ("equals".equals(method.getName())) {
try {
Object otherHandler =
args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
return equals(otherHandler);
} catch (IllegalArgumentException e) {
return false;
}
} else if ("hashCode".equals(method.getName())) {
return hashCode();
} else if ("toString".equals(method.getName())) {
return toString();
}
return dispatch.get(method).invoke(args);
}
最终调用了 SynchronousMethodHandler类进行拦截处理，当被拦截会根据参数生成RequestTemplate对象，该对象就是http请求的模板，代码如下：
@Override
public Object invoke(Object[] argv) throws Throwable {
RequestTemplate template = buildTemplateFromArgs.create(argv);
Retryer retryer = this.retryer.clone();
while (true) {
try {
return executeAndDecode(template);
} catch (RetryableException e) {
try {
retryer.continueOrPropagate(e);
} catch (RetryableException th) {
Throwable cause = th.getCause();
if (propagationPolicy == UNWRAP && cause != null) {
throw cause;
} else {
throw th;
}
}
if (logLevel != Logger.Level.NONE) {
logger.logRetry(metadata.configKey(), logLevel);
}
continue;
}
}
}
其中有个executeAndDecode()方法，该方法是通RequestTemplate生成Request请求对象，然后根据用client获取response。
Object executeAndDecode(RequestTemplate template) throws Throwable {
Request request = targetRequest(template);
if (logLevel != Logger.Level.NONE) {
logger.logRequest(metadata.configKey(), logLevel, request);
}
Response response;
......
其中Client组件是一个非常重要的组件，Feign最终发送request请求以及接收response响应，都是由Client组件完成的，其中Client的实现类，只要有Client.Default，该类由HttpURLConnnection实现网络请求，另外还支持HttpClient、Okhttp.
首先来看以下在FeignRibbonClient的自动配置类，FeignRibbonClientAutoConfiguration ，主要在工程启动的时候注入一些bean,其代码如下：
@ConditionalOnClass({ ILoadBalancer.class, Feign.class })
@Configuration
@AutoConfigureBefore(FeignAutoConfiguration.class)
public class FeignRibbonClientAutoConfiguration {
@Bean
@ConditionalOnMissingBean
public Client feignClient(CachingSpringLoadBalancerFactory cachingFactory,
SpringClientFactory clientFactory) {
return new LoadBalancerFeignClient(new Client.Default(null, null),
cachingFactory, clientFactory);
}
}
在缺失配置feignClient的情况下，会自动注入new Client.Default(),跟踪Client.Default()源码，它使用的网络请求框架为HttpURLConnection，代码如下：
@Override
public Response execute(Request request, Options options) throws IOException {
HttpURLConnection connection = convertAndSend(request, options);
return convertResponse(connection).toBuilder().request(request).build();
}
所以说，feign是一个伪客户端，即它不做任何的请求处理。Feign通过处理注解生成request，从而实现简化HTTP API开发的目的，即开发人员可以使用注解的方式定制request api模板，在发送http request请求之前，feign通过处理注解的方式替换掉request模板中的参数，这种实现方式显得更为直接、可理解。
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