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javaandroidclassloaderflyway

Occasionally receiving WarningContextClassLoader instead of PathClassLoader causing flyway to crash

Context

In my android app, I use flyway to update my Db schema. Up to now I have one and only one migration, that creates 3 tables. Simple and usual business for flyway. It does work very well... most of the time! Sometimes it crashes with this error (full stack hereunder):

java.lang.ClassCastException: android.app.LoadedApk$WarningContextClassLoader
cannot be cast to dalvik.system.PathClassLoader

I have several questions related to that error :

  • What is LoadedApk.WarningContextClassLoader ?
  • Why is it sometimes used instead of dalvik.system.PathClassLoader ?
  • Is it a bug in my app, in Android system or in Flyway ?

I also noticed that I can inject explicitely the classloader inside flyway (Flyway.setClassLoader() )

  • How can I get for sure the right classloader ?

Note that I inject the Application Context in Flyway, not a particular activity's context.

Versions

I use org.flywaydb:flyway-core:4.0.2

I noticed this problem in Android 4.2.2 (JellyBean, api level 17) and 4.4 (KitKat, api level 19). AFAIK, it never occured on higher versions.

Stack trace

Exception java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mycompany.myapp.free.debug/com.mycompany.MainActivity}: java.lang.ClassCastException: android.app.LoadedApk$WarningContextClassLoader cannot be cast to dalvik.system.PathClassLoader
    android.app.ActivityThread.performLaunchActivity (ActivityThread.java:2306)
    android.app.ActivityThread.handleLaunchActivity (ActivityThread.java:2358)
    android.app.ActivityThread.access$600 (ActivityThread.java:156)
    android.app.ActivityThread$H.handleMessage (ActivityThread.java:1340)
    android.os.Handler.dispatchMessage (Handler.java:99)
    android.os.Looper.loop (Looper.java:153)
    android.app.ActivityThread.main (ActivityThread.java:5299)
    java.lang.reflect.Method.invokeNative (Method.java)
    java.lang.reflect.Method.invoke (Method.java:511)
    com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run (ZygoteInit.java:833)
    com.android.internal.os.ZygoteInit.main (ZygoteInit.java:600)
    dalvik.system.NativeStart.main (NativeStart.java)

Caused by java.lang.ClassCastException: android.app.LoadedApk$WarningContextClassLoader cannot be cast to dalvik.system.PathClassLoader
    org.flywaydb.core.internal.util.scanner.classpath.android.AndroidScanner.<init> (AndroidScanner.java:46)
    org.flywaydb.core.internal.util.scanner.Scanner.<init> (Scanner.java:38)
    org.flywaydb.core.Flyway.execute (Flyway.java:1353)
    org.flywaydb.core.Flyway.info (Flyway.java:1040)
    com.mycompany.db.FlywayHelper.info (FlywayHelper.java:54)
    com.mycompany.db.FlywayHelper.migrate (FlywayHelper.java:45)
    com.mycompany.db.GW2DatabaseHelper.migrate (GW2DatabaseHelper.java:56)
    com.mycompany.db.DbInit.initKeyGuild (DbInit.java:62)
    com.mycompany.db.DbInit.init (DbInit.java:42)
    com.mycompany.business.BootApp.boot (BootApp.java:79)
    com.mycompany.MainActivity.onCreate (MainActivity.java:51)
    android.app.Activity.performCreate (Activity.java:5122)
    android.app.Instrumentation.callActivityOnCreate (Instrumentation.java:1081)
    android.app.ActivityThread.performLaunchActivity (ActivityThread.java:2270)
    android.app.ActivityThread.handleLaunchActivity (ActivityThread.java:2358)
    android.app.ActivityThread.access$600 (ActivityThread.java:156)
    android.app.ActivityThread$H.handleMessage (ActivityThread.java:1340)
    android.os.Handler.dispatchMessage (Handler.java:99)
    android.os.Looper.loop (Looper.java:153)
    android.app.ActivityThread.main (ActivityThread.java:5299)
    java.lang.reflect.Method.invokeNative (Method.java)
    java.lang.reflect.Method.invoke (Method.java:511)
    com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run (ZygoteInit.java:833)
    com.android.internal.os.ZygoteInit.main (ZygoteInit.java:600)
    dalvik.system.NativeStart.main (NativeStart.java)

Research update

Although its purpose is not clear to me, LoadedApk.WarningContextClassLoader appears to be a valid class loader, and should then be supported by Flyway. A pull request has been submitted.

In between, a workaround needs to be implemented, to inject the expected class loader:

private void injectClassLoader(Flyway flyway) {
    ClassLoader classLoader = flyway.getClassLoader();
    if (classLoader != null && !(classLoader instanceof PathClassLoader)){
        flyway.setClassLoader(classLoader.getParent());
    }
}
Solution

  • What is LoadedApk.WarningContextClassLoader ?

    Why is it sometimes used instead of dalvik.system.PathClassLoader ?

    Not really sure, but the purpose of this specific class loader seems to warn the user that its package could be sharing its virtual machine with other packages. More info here, thanks to @orip.

    Is it a bug in my app, in Android system or in Flyway ?

    Anyway the WarningContextClassLoader is still a valid class loader, and must be accepted by flyway. The cast made in AndroidScanner was just useless.

    Solution