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

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

I took a simple example from read-json readme, but every time I try to extract values ​​from a JSON-String, I get net.liftweb.json.MappingException when I call Activity. It seems that the "extract" is not transmitted anywhere.

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 an 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

It would be great if you have an idea how to solve my problem. Thanks!

Hi Illaz

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1 answer

I suspect that Lift uses reflection in this mode. It may not work at all in Android, or you may need to remove some things that have been deleted. I suggest you just try using some other alternatives.

See also this question - my suspicion comes from the remark that it does not work on REPL. Not that REPL is not reflected, but its package structure can confuse the Lift library.

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