Question

我正在构建一个非常简单的服务，它应该返回一个通过递归case类定义的树状结构：

case class Node(id: Int, name: String, children: Seq[Node] = Seq())

但由于某种原因，我不断收到以下编译错误：

错误：（24,70）无法找到参数编码器的隐含值：   io.circe.Encoder [序号[com.ansarada.ds.docviewer.server.Main.Node]]
  隐式val nodesEncoder：EntityEncoder [Seq [Node]] =   jsonEncoderOf [Seq [Node]]

错误：（24,70）没有足够的参数   方法jsonEncoderOf :(隐式编码器：   io.circe.Encoder [序号[com.ansarada.ds.docviewer.server.Main.Node]]）org.http4s.EntityEncoder [序号[com.ansarada.ds.docviewer.server.Main.Node]。   未指定的值参数编码器。隐式val nodesEncoder：   EntityEncoder [Seq [Node]] = jsonEncoderOf [Seq [Node]]

一旦删除子元素定义并将Node转换为扁平对象，代码就会被编译：

case class Node(id: Int, name: String)

任何人都可以帮我为嵌套子项的案例定义正确的Json编码器吗？

完整代码：

import org.http4s.circe._
import org.http4s.dsl._
import org.http4s.server.blaze.BlazeBuilder
import org.http4s.server.{Server, ServerApp}
import org.http4s.{EntityEncoder, HttpService}

import scalaz.concurrent.Task

object Main extends ServerApp {
  import io.circe.generic.auto._
  import io.circe.syntax._

  case class Node(id: Int, name: String, children: Seq[Node] = Seq())

  def getNodes: Seq[Node] = Seq(
    Node(0, "#One"),
    Node(1, "#Two"),
    Node(2, "#Three")
  )

  implicit val nodeEncoder: EntityEncoder[Node] = jsonEncoderOf[Node]
  implicit val nodesEncoder: EntityEncoder[Seq[Node]] = jsonEncoderOf[Seq[Node]]

  override def server(args: List[String]): Task[Server] = {
    val nodesService = HttpService {
      case _ @ GET -> Root / "nodes" =>
        Ok(getNodes.asJson)
    }

    BlazeBuilder
      .bindHttp(8080, "localhost")
      .mountService(nodesService, "/api")
      .start
  }
}

Answer 1

确定。使用circe json注释工作 - @JsonCodec。解决上述问题的分步指南：

先决条件 - 导入circe lib：

"io.circe" %% "circe-core" % "0.8.0",
"io.circe" %% "circe-generic" % "0.8.0",
"io.circe" %% "circe-literal" % "0.8.0",
"io.circe" %% "circe-parser" % "0.8.0",

在build.sbt中打开自动编译器插件：
```
autoCompilerPlugins := true
```

添加scalamacros paradise编译器插件：

lazy val root = Project("root", file("."))
    .settings(
        addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.patch)
)

使用@JsonCodec注释案例类：

@JsonCodec case class Node(id: Int, children: Seq[Node] = Seq())

对json投射回复：
```
Ok(getNodes.asJson)
```

完整的代码段：

import org.http4s.circe._
import org.http4s.dsl._
import org.http4s.server.blaze.BlazeBuilder
import org.http4s.server.{Server, ServerApp}
import org.http4s.{EntityEncoder, HttpService}

import scalaz.concurrent.Task
object Main extends ServerApp {
  import io.circe.generic.auto._
  import io.circe.syntax._

  @JsonCodec case class Node(id: Int, name: String, children: Seq[Node] = Seq())

  def getNodes: Seq[Node] = Seq(
    Node(0, "#One"),
    Node(1, "#Two"),
    Node(2, "#Three")
  )

  override def server(args: List[String]): Task[Server] = {
    val nodesService = HttpService {
      case _ @ GET -> Root / "nodes" =>
        Ok(getNodes.asJson)
    }

    BlazeBuilder
      .bindHttp(8080, "localhost")
      .mountService(nodesService, "/api")
      .start
  }
}

在http4s上运行时，将带有Circe的递归数据结构编码到Json中

1 个答案: