Problem: "Extracting" JSON using lift-json in an Android (Scala) application

Question

I want to deserialize a JSON-String using the lift-json library. In my Android application I'm using Scala 2.9.0 and lift-json_2.9.0_2.4-M1.

I took a simple example from lift-json readme, but everytime I try to extract values from the JSON-String I get a net.liftweb.json.MappingException when calling the Activity. It seems there are no args delivered to "extract".

Here is my Activity:

import _root_.android.app.Activity
import _root_.android.os.Bundle
import net.liftweb.json._

class JsonTest extends Activity {

  override def onCreate(savedInstanceState: Bundle) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.mainactivity)

    implicit val formats = DefaultFormats
    case class NumList(numbers: List[Int])
    val json = parse(""" { "numbers" : [1, 2, 3, 4] } """)
    json.extract[NumList]
  }
}

and here the exception I get:

06-29 12:09:31.548: ERROR/AndroidRuntime(405): FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{de.fhs.spirit/de.fhs.spirit.JsonTest}: net.liftweb.json.MappingException: Parsed JSON values do not match with class constructor
    args=
    arg types=
    constructor=public de.fhs.spirit.JsonTest$NumList$2(de.fhs.spirit.JsonTest,scala.collection.immutable.List)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
    at android.app.ActivityThread.access$2300(ActivityThread.java:125)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:123)
    at android.app.ActivityThread.main(ActivityThread.java:4627)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:521)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
    at dalvik.system.NativeStart.main(Native Method)
    Caused by: net.liftweb.json.MappingException: Parsed JSON values do not match with class constructor
    args=
    arg types=
    constructor=public de.fhs.spirit.JsonTest$NumList$2(de.fhs.spirit.JsonTest,scala.collection.immutable.List)
    at net.liftweb.json.Meta$.fail(Meta.scala:185)
    at net.liftweb.json.Extraction$.instantiate$1(Extraction.scala:257)
    at net.liftweb.json.Extraction$.newInstance$1(Extraction.scala:280)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:298)
    at net.liftweb.json.Extraction$.extract0(Extraction.scala:345)
    at net.liftweb.json.Extraction$.net$liftweb$json$Extraction$$extract0(Extraction.scala:194)
    at net.liftweb.json.Extraction$.extract(Extraction.scala:42)
    at net.liftweb.json.JsonAST$JValue.extract(JsonAST.scala:290)
    at de.fhs.spirit.JsonTest.onCreate(JsonTest.scala:16)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
    ... 11 more

Would be great if you have an idea how to solve my problem. Thank you!

Greetings, Illaz

score 2 · Accepted Answer · edited May 23 '17 at 12:24

2

I suspect Lift uses reflection in this mode. It might not work at all in Android, or it may require that some stuff that got removed not be removed. I suggest you just try to use some of other alternatives instead.

See also this question -- my suspicion comes from the remark that it doesn't work on REPL. Not that REPL doesn't have reflection, but its package structure might be confusing to the Lift library.

edited May 23 '17 at 12:24

Community

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1

answered Jun 29 '11 at 17:45

Daniel C. Sobral

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Thanks for your answer. I'm now using Google Gson. Unfortunately it's no Scala, but it works. – Illaz Jun 30 '11 at 09:32
Are you able to deserialize Scala objects with GSon, or are you not using Scala at all? – Chopmo Jun 30 '11 at 10:13
1

If you're using Gson and for example you have a List in your Json-String, Gson returns a java.util.List. If you want to use a scala.List instead, you have to convert this list using "import scala.collection.JavaConversions._" and "javaList.toList". Is that what you meant, Chopmo? – Illaz Jul 07 '11 at 12:29

Problem: "Extracting" JSON using lift-json in an Android (Scala) application

1 Answers1