07 导出服务（接口级别）

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@Override
public <T> Exporter<T> export(final Invoker<T> originInvoker) throws RpcException {
    ...
    // 获取 registry，比如 ZookeeperRegistry (接口级注册)
    final Registry registry = getRegistry(registryUrl);
    final URL registeredProviderUrl = getUrlToRegistry(providerUrl, registryUrl);

    // decide if we need to delay publish (provider itself and registry should both need to register)
    // 如果是接口级别，register 为 true
    boolean register = providerUrl.getParameter(REGISTER_KEY, true) && registryUrl.getParameter(REGISTER_KEY, true);
    if (register) {
        // 注册 providerUrl，最终调用 ZookeeperRegistry#registry 方法
        register(registry, registeredProviderUrl);
    }
    ...
    return new DestroyableExporter<>(exporter);
}

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// 注册 url, 这个类是 ZookeeperRegistry 的 父类
@Override
public void register(URL url) {
    ...
    super.register(url);
    // 移除 url
    removeFailedRegistered(url);
    removeFailedUnregistered(url);
    try {
        // Sending a registration request to the server side
        // 调用子类的注册方法, 这里以 ZookeeperRegistry 为例
        doRegister(url);
    } catch (Exception e) {
        Throwable t = e;

        // If the startup detection is opened, the Exception is thrown directly.
        boolean check = getUrl().getParameter(Constants.CHECK_KEY, true)
            && url.getParameter(Constants.CHECK_KEY, true)
            && (url.getPort() != 0);
        boolean skipFailback = t instanceof SkipFailbackWrapperException;
        // 检查 check 参数，如果是 true，表示第一次一定要注册成功
        if (check || skipFailback) {
            if (skipFailback) {
                t = t.getCause();
            }
            throw new IllegalStateException("Failed to register " + url + " to registry " + getUrl().getAddress() + ", cause: " + t.getMessage(), t);
        } else {
            logger.error(INTERNAL_ERROR, "unknown error in registry module", "", "Failed to register " + url + ", waiting for retry, cause: " + t.getMessage(), t);
        }

        // Record a failed registration request to a failed list, retry regularly
        // 添加失败的 url，稍后定时任务会重新注册
        addFailedRegistered(url);
    }
}

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@Override
public void doRegister(URL url) {
    try {
        checkDestroyed();
        // 创建 zookeeper 临时节点， 路径为 /dubbo/${interfaceName}/providers/
        zkClient.create(toUrlPath(url), url.getParameter(DYNAMIC_KEY, true), true);
    } catch (Throwable e) {
        throw new RpcException("Failed to register " + url + " to zookeeper " + getUrl() + ", cause: " + e.getMessage(), e);
    }
}