如何阅读 Spring Cloud OpenFein 源码

背景

一直以来，使用 Spring Cloud OpenFeign 都是阅读官方文档，虽然也大概知道其实现原理，但终究是没有"证据"的。关于 Spring 的源码阅读，自认为是一件十分令人头疼的事情。最近，在学习 Feign 的原生 API，乘此机会，也就阅读一下 Spring Cloud OpenFeign 的源码，并将分享出来，希望能帮到有需要的人吧。

概述

关于 Spring Cloud OpenFeign 源码的博客有很多，但是，不知道为什么，照着博客，一边读博客，一边读源码，还一边 debug，总是认为还有很多不清楚的地方。究其原因，我认为，博客都是按照源码的流程讲解，虽然附上了大段代码，可能还是无法清晰的理解。不知道你们是不是，反正我是这样的。

目标

首先，我们明确一下今天探究的问题：

1. 我们知道，当我们使用 @FeignClient，是使用了JDK动态代理，那么是如何实现的，那一步创建的代理类。
2. 当我们知道第一个问题后，我们就基本清楚整个流程了，那么，我们就可以手写一个简易的入门测试了。

源码

启动流程

• org.springframework.cloud.openfeign.EnableFeignClients
• org.springframework.cloud.openfeign.FeignAutoConfiguration
• org.springframework.cloud.openfeign.FeignClientsRegistrar
• org.springframework.cloud.openfeign.FeignClientsRegistrar#registerBeanDefinitions
• org.springframework.cloud.openfeign.FeignClientsRegistrar#registerFeignClient
• org.springframework.cloud.openfeign.FeignClientFactoryBean#getObject
• org.springframework.cloud.openfeign.Targeter
• feign.Feign.Builder#build
• feign.SynchronousMethodHandler.Factory
• feign.ReflectiveFeign.ParseHandlersByName
• feign.ReflectiveFeign#ReflectiveFeign
• feign.Feign#newInstance
• feign.ReflectiveFeign#newInstance
• feign.InvocationHandlerFactory#create
• feign.InvocationHandlerFactory.Default#create
• feign.ReflectiveFeign.FeignInvocationHandler#FeignInvocationHandler

贴上图吧，看看完整版的

注册流程：

自动配置：

调用流程

• feign.ReflectiveFeign.FeignInvocationHandler#invoke
• feign.InvocationHandlerFactory.MethodHandler#invoke
• feign.SynchronousMethodHandler#invoke

MyRpc

经过上面的流程，我们手写一个 RPC。

下面给出主要代码。

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import({MyRpcRegister.class, MyRpcAutoConfig.class})
public @interface EnableMyRpc {
}

getObject()

@Data
public class MyRpcFactoryBean implements FactoryBean<Object> {

    private String url;

    private String contextPath;

    private String name;

    private Class<?> type;

    private BeanFactory beanFactory;

    private MyClient myClient;

    @Override
    public Object getObject() {

        Map<Method, RpcBean> map = new HashMap<>();

        Method[] methods = type.getMethods();

        myClient = beanFactory.getBean(MyClient.class);

        for (Method method : methods) {
            Annotation[] annotations = method.getAnnotations();
            String httpMethod = "";
            String path = "";
            for (Annotation annotation : annotations) {
                if (annotation.annotationType() == PostMapping.class) {
                    httpMethod = "POST";
                    path = ((PostMapping) annotation).value()[0];
                    break;
                } else if (annotation.annotationType() == GetMapping.class) {
                    httpMethod = "GET";
                    path = ((GetMapping) annotation).value()[0];
                    break;
                } else if (annotation.annotationType() == RequestMapping.class) {
                    RequestMapping requestMapping = ((RequestMapping) annotation);
                    httpMethod = requestMapping.method()[0].name();
                    path = requestMapping.value()[0];
                    break;
                }
            }
            RpcBean rpcBean = new RpcBean()
                    .setUrl(url + contextPath)
                    .setPath(path)
                    .setHttpMethod(httpMethod)
                    .setMyClient(myClient)
                    ;
            map.put(method, rpcBean);
        }

        ClassLoader loader = type.getClassLoader();

        return Proxy.newProxyInstance(loader, new Class<?>[] {type}, new MyRpcInvocationHandler(map));
    }

    @Override
    public Class<?> getObjectType() {
        return type;
    }

}

handler

@Slf4j
public class MyRpcInvocationHandler implements InvocationHandler {

    private final Map<Method, RpcBean> map;

    public MyRpcInvocationHandler(Map<Method, RpcBean> map) {
        this.map = map;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

        log.info("proxy handler");

        return request(method, args);
    }

    public Object request(Method method, Object[] args) {
        String result = "";
        RpcBean rpcBean = map.get(method);
        Parameter[] parameters = method.getParameters();
        Class<?> returnType = method.getReturnType();
        String url = rpcBean.getUrl() + rpcBean.getPath();
        String httpMethod = rpcBean.getHttpMethod();
        String param = getParam(httpMethod, parameters, args);
        log.info("url: [{}], param: [{}]", url, param);
        MyClient myClient = rpcBean.getMyClient();
        if ("POST".equals(httpMethod)) {
            result = myClient.post(url, param);
        } else if ("GET".equals(httpMethod)) {
            result = myClient.get(url, param);
        }
        if (StringUtils.hasText(result)) {
            return JsonUtils.convertObject(result, returnType);
        }
        return "";
    }

    public String getParam(String httpMethod, Parameter[] parameters, Object[] args) {
        if ("POST".equals(httpMethod)) {
            return JsonUtils.convertString(args[0]);
        } else if ("GET".equals(httpMethod)) {
            if (Objects.isNull(parameters) || parameters.length == 0
                    || Objects.isNull(args) || args.length == 0) {
                return "";
            }
            String param = "";
            StringBuilder urlBuilder = new StringBuilder(param);
            for (int i = 0; i < parameters.length; i++) {
                if (Objects.nonNull(args[i])) {
                    urlBuilder.append(String.format("%s=%s&", parameters[i].getName(), args[i]));
                }
            }
            param = urlBuilder.toString();
            param = param.substring(0, param.length() - 1);
            return param;
        }
        return "";
    }
}

完整代码