如何在Scala中重写Elasticsearch 2.4.x的FilterClient#doExecute()

Hey，我碰到过类似Scala与Java泛型交互的坑，尤其是在Elasticsearch 2.x这种老版本里！让我帮你理清楚问题出在哪，以及怎么解决。

问题回顾 #

你现在卡在Elasticsearch 2.4.x版本（依赖org.elasticsearch % "elasticsearch" % "2.4.x"），没法升级——毕竟后续版本里ActionRequest的泛型参数被移除了，虽然新版本的写法能正常运行，但暂时没法动版本对吧？

你要重写的Java父类方法是这样的：

protected <Request extends ActionRequest, Response extends ActionResponse, RequestBuilder extends ActionRequestBuilder<Request, Response, RequestBuilder>> 
void doExecute(Action<Request, Response, RequestBuilder> action, Request request, ActionListener<Response> listener) { 
    in().execute(action, request, listener); 
}

然后你写了Scala版本的重写代码，结果在Scala 2.11.11、2.11.12、2.12.4里都编译失败：

import org.elasticsearch.action._
import org.elasticsearch.client.{Client, FilterClient}

class DemoFilterClient(underlyingClient: Client) extends FilterClient(underlyingClient) {
 override def doExecute[
 Request <: ActionRequest[_], Response <: ActionResponse, RequestBuilder <: ActionRequestBuilder[Request, Response, RequestBuilder]
 ](
 action: Action[Request, Response, RequestBuilder], request: Request, listener: ActionListener[Response]
 ) = super.doExecute(action, request, listener)
}

编译错误信息看起来挺吓人的：

[info] Compiling 1 Scala source to /home/roberto/development/elasticsearch-scala-client-test/target/scala-2.11/classes ...
[error] /home/roberto/development/elasticsearch-scala-client-test/src/main/scala/com/gu/DemoFilterClient.scala:7:101: type arguments [Request,Response,RequestBuilder] do not conform to class ActionRequestBuilder's type parameter bounds [Request <: org.elasticsearch.action.ActionRequest[_ <: org.elasticsearch.action.ActionRequest[_ <: org.elasticsearch.action.ActionRequest[_ <: AnyRef]]],Response <: org.elasticsearch.action.ActionResponse,RequestBuilder <: org.elasticsearch.action.ActionRequestBuilder[Request,Response,RequestBuilder]]
[error] override def doExecute[Request <: ActionRequest[_], Response <: ActionResponse, RequestBuilder <: ActionRequestBuilder[Request, Response, RequestBuilder]](action: Action[Request, Response, RequestBuilder], request: Request, listener: ActionListener[Response]) = super.doExecute(action, request, listener)
[error] ^
[error] one error found

说白了就是：Scala把Java的泛型边界给过度解析了，尤其是Request的类型被展开成了多层递归的通配符，导致你的泛型约束匹配不上。

解决方法 #

问题的核心是Scala与Java泛型对递归边界的处理方式不同，我们需要调整Scala代码里的泛型约束，让类型检查器能正确匹配Java的定义。

方案一：用通配符替代ActionRequestBuilder的第一个参数 #

修改后的代码如下：

import org.elasticsearch.action._
import org.elasticsearch.client.{Client, FilterClient}

class DemoFilterClient(underlyingClient: Client) extends FilterClient(underlyingClient) {
  override def doExecute[
    Request <: ActionRequest[_], 
    Response <: ActionResponse, 
    RequestBuilder <: ActionRequestBuilder[_, Response, RequestBuilder]
 ](
    action: Action[Request, Response, RequestBuilder], 
    request: Request, 
    listener: ActionListener[Response]
  ): Unit = super.doExecute(action, request, listener)
}

方案二：明确Request的边界为ActionRequest[_ <: AnyRef] #

如果你想要更贴近Java的泛型声明，也可以这样写：

import org.elasticsearch.action._
import org.elasticsearch.client.{Client, FilterClient}

class DemoFilterClient(underlyingClient: Client) extends FilterClient(underlyingClient) {
  override def doExecute[
    Request <: ActionRequest[_ <: AnyRef], 
    Response <: ActionResponse, 
    RequestBuilder <: ActionRequestBuilder[Request, Response, RequestBuilder]
 ](
    action: Action[Request, Response, RequestBuilder], 
    request: Request, 
    listener: ActionListener[Response]
  ): Unit = super.doExecute(action, request, listener)
}

为什么这两种方法能行？ #

• 方案一里，我们把ActionRequestBuilder[Request, Response, RequestBuilder]改成了ActionRequestBuilder[_, Response, RequestBuilder]，用通配符_来兼容Scala过度解析的递归边界，让类型检查器不再纠结Request的多层嵌套约束。
• 方案二则是明确了Request的边界是ActionRequest[_ <: AnyRef]，直接匹配错误信息里最内层的_ <: AnyRef约束，避免Scala展开成多层递归。
• 另外，显式指定返回类型: Unit也是个好习惯，虽然Scala能自动推断，但在重写Java方法时，显式声明能避免一些隐式类型转换的问题。

内容的提问来源于stack exchange，提问作者Roberto Tyley