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ProGuard and Gson on Android (ClassCastException)

Posted by: admin April 23, 2020 Leave a comment


I am having a hell of a time with Gson and ProGuard. I have a simple object and when I parse tojson, save to sqllite and read back from the database in order to load the json back to my object, I get a java.lang.classcastexception. If I dont use ProGuard, everthing works fine.

I have verified that the json string being sent to and gotten from the database is the same. The exception is not thrown when it converts from json, but rather when I try to access the object.

Here is my simple object:

public class ScanLog extends ArrayList<SingleFrame>
     private static final long serialVersionUID = 1L;

     public ScanLog()


public final class SingleFrame 
    public int Position;
    public int Time;
    public Map<Integer,String> MainDataMap;
    public Map<Integer,String> DataMap;

    public SingleFrame(int position, int time, 
                    Map<Integer,String> mainDataMap, Map<Integer,String> dataMap)
        this.Position = position;
        this.Time = time;
        this.MainDataMap = mainDataMap;
        this.DataMap = dataMap;


All other aspects of my app are fine, but something with proguard is causing this to happen….Ive tried all kinds of -keep commands in the proguard.cfg but I am not sure what Im doing is right.



-keep public class * extends android.app.Activity
-keep public class * extends android.app.Application
-keep public class * extends android.app.Service
-keep public class * extends android.content.BroadcastReceiver
-keep public class * extends android.content.ContentProvider
-keep public class * extends android.app.backup.BackupAgentHelper
-keep public class * extends android.preference.Preference
-keep public class com.android.vending.licensing.ILicensingService

#keep all classes that might be used in XML layouts
-keep public class * extends android.view.View
-keep public class * extends android.app.Fragment
-keep public class * extends android.support.v4.Fragment

#keep all classes
-keep public class *{
public protected *;

#keep all public and protected methods that could be used by java reflection
-keepclassmembernames class * {
    public protected <methods>;

-keepclasseswithmembernames class * {
    native <methods>;

-keep public class org.scanner.scanlog.SingleFrame

-keepclassmembers class org.scanner.scanlog.ScanLog { 
        private <fields>; 
        public <fields>; 

-keepclassmembers class org.scanner.scanlog.SingleFrame { 
        private <fields>; 
        public <fields>; 

-keepclasseswithmembernames class * {
    public <init>(android.content.Context, android.util.AttributeSet);

-keepclasseswithmembernames class * {
    public <init>(android.content.Context, android.util.AttributeSet, int);

-keepclassmembers enum * {
    public static **[] values();
    public static ** valueOf(java.lang.String);

-keep class * implements android.os.Parcelable {
  public static final android.os.Parcelable$Creator *;

-dontwarn **CompatHoneycomb
-dontwarn org.htmlcleaner.*
#-keep class android.support.v4.** { *; }

EDIT – Okay I got ACRA set up successfully in my app, pretty awesome feature!
Here is the stack trace:

java.lang.ClassCastException: java.lang.Object
    at org.scanner.activity.ReaderMainActivity.AdvanceScanLog(SourceFile:1499)
    at org.scanner.activity.r.onProgressChanged(SourceFile:271)
    at android.widget.SeekBar.onProgressRefresh(SeekBar.java:89)
    at android.widget.ProgressBar.doRefreshProgress(ProgressBar.java:507)
    at android.widget.ProgressBar.refreshProgress(ProgressBar.java:516)
    at android.widget.ProgressBar.setProgress(ProgressBar.java:565)
    at android.widget.AbsSeekBar.trackTouchEvent(AbsSeekBar.java:337)
    at android.widget.AbsSeekBar.onTouchEvent(AbsSeekBar.java:292)
    at android.view.View.dispatchTouchEvent(View.java:3932)
    at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:906)
    at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:906)
    at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:906)
    at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:906)
    at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:906)
    at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:906)
    at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:906)
    at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:906)
    at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:906)
    at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1784)
    at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1157)
    at android.app.Activity.dispatchTouchEvent(Activity.java:2181)
    at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1759)
    at android.view.ViewRoot.deliverPointerEvent(ViewRoot.java:2336)
    at android.view.ViewRoot.handleMessage(ViewRoot.java:1976)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:143)
    at android.app.ActivityThread.main(ActivityThread.java:4263)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:507)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
    at dalvik.system.NativeStart.main(Native Method)
How to&Answers:

For the latest version of the recommended proguard configuraiton file, please see the gson supplied android proguard example at:


These settings in the config worked for me in one of my apps:

# Add the gson class
-keep public class com.google.gson

# Add any classes the interact with gson
-keep class com.someapp.android.models.ChatModel { *; }
-keep class com.someapp.android.models.FeedModel { *; }

# Add the path to the jar
-libraryjars /Users/someuser/Documents/workspace/someapp/lib/gson-1.7.1.jar

Hopefully this helps you out.


Applying the changes found in the Android example in the Gson project worked for me

The lines needed were:

-keepattributes Signature
-keep class sun.misc.Unsafe { *; }
# and keeping the classes that will be serialized/deserialized


I know the original question was resolved by taking a different approach, but I was having a very similar issue using flexjson and Proguard on Android, and I’ve solved it, in case anyone runs into it themselves.

When converting back from JSON to my value object which included some ArrayLists, I would get the same ClassCastException. I got it to work by basically having obfuscation enabled but turning all parts of obfuscation off (-keep everything, -keepclassmembers everything and -keepattributes everything) and then working backwards by enabling things a bit at a time.

The result; keeping the entire flexjson library:

-keep class flexjson**
--keepclassmembers class flexjson** {

and keeping the Signature and Annotation attribute:

-keepattributes Signature, *Annotation*

I was able to use the flexjson library without incident after that in a proguarded, release version of my app.


I was getting errors for Model classes with proguard If you look at

GSON Proguard you will find a line

# Application classes that will be serialized/deserialized over Gson
-keep class com.google.gson.examples.android.model.** { *; }

replace com.google.gson.examples.android.model. with your model package likewise in my case i replaced it with -keep class com.consumer.myProject.model.** { *; }

rest I copied as such


It does look like your are keeping everything from your class (fields, methods and the class itself). But to make sure you can add -printseeds outputfile.txt to the proguard.cfg file to verify that proguard really keeps everything you need once obfuscation is done.

BTW, you might think about adding something like ACRA or Android Remote stacktrace that allows you to inspect stacktraces on a built app.


So, I ended up ditching the Gson library and instead of converting my object to json using gson, I created a custom class in my app to serialize and deserialize the object and store the data that way.

I am overall more happy, even though this has cost me over 12 hours of trying to figure it out. Apparently, PROGUARD and gson must not like each other too much?

As A HUGELY added benefit to not having to use GSON, I noticed that by taking out the GSON library, my app size is cut in half. My app was 577kb and is now only 260kb after removing the gson lib.