Android Work Manager: “Could not instantiate Worker”

Question

I've followed the Android Developer's tutorial on using the Worker Manager structure to run my code in background but anytime I try to enqueue my worker it doesn't run and I get the following error:

2018-10-04 22:25:47.004 28669-28885/app.package.com.debug E/DefaultWorkerFactory: Could not instantiate app.package.com.MyWorker
    java.lang.NoSuchMethodException: <init> []
        at java.lang.Class.getConstructor0(Class.java:2320)
        at java.lang.Class.getDeclaredConstructor(Class.java:2166)
        at androidx.work.DefaultWorkerFactory.createWorker(DefaultWorkerFactory.java:58)
        at androidx.work.impl.WorkerWrapper.runWorker(WorkerWrapper.java:180)
        at androidx.work.impl.WorkerWrapper.run(WorkerWrapper.java:117)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1162)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:636)
        at java.lang.Thread.run(Thread.java:764)
2018-10-04 22:25:47.005 28669-28885/app.package.com.debug E/WorkerWrapper: Could for create Worker app.package.com.MyWorker

I've seen that this problem could be related to the default constructor on the worker but I already use the correct one instead of the deprecated default function.

My worker is declared as:

public class MyWorker extends Worker {

    public MyWorker(@NonNull Context context, @NonNull WorkerParameters workerParams) {
        super(context, workerParams);
    }

    @NonNull
    @Override
    public Result doWork() {
        // Doesn't even get called
    }
}

And it gets queued like so:

WorkManager workManager = WorkManager.getInstance();
if (myWorkerRequest == null) {
    myWorkerRequest = new OneTimeWorkRequest.Builder(MyWorker.class)
            .setConstraints(new Constraints.Builder()
                    .setRequiredNetworkType(NetworkType.CONNECTED)
                    .build())
            .build();
}
WorkStatus workStatus = workManager.getStatusById(myWorkerRequest.getId()).getValue();
if (workStatus == null || !workStatus.getState().isFinished()) {
    workManager.enqueue(myWorkerRequest);
}

I'm not seeing anything different from the examples so I'd like to understand what else could affect my code to cause this crash. Could it be something ProGuard related?

My version is 1.0.0-alpha09

Thanks!

Answer 1

This is a known issue with WorkManager 1.0.0-alpha09 that is already marked as fixed for alpha10.

As a workaround, you can add the following lines to your proguard configuration:

-keepclassmembers class * extends androidx.work.Worker {
    public <init>(android.content.Context,androidx.work.WorkerParameters);
}

Answer 2

I had the same issue. The cause of the issue for me was that my Worker class was a nested class. The moment I made it an independent class, it worked.

Answer 3

I had this problem in stable version 1.0.0 and I fixed it by making my worker class public.

class MyWorker extends Worker {...} > public class MyWorker extends Worker {...}

Answer 4

The problem for me was the constructor of my kotlin class:

WRONG

class MyWorker(val app: Application, workerParams: WorkerParameters): Worker(app, workerParams)

GOOD

class MyWorker(val context: Context, workerParams: WorkerParameters): Worker(context, workerParams)