服务调用 checkSelfPermission 返回 nullPointerException

Question

I have a service checking if an android device has a permission to write to external storage by using this code:

checkSelfPermission(android.Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED

which always return NullPointerException

    public class Repeater extends Service {
    public Repeater() throws Exception {

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {

        if (checkSelfPermission(android.Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED)
        {

 Toast.makeText(getApplicationContext(),"sasa",Toast.LENGTH_SHORT).show();
        } else {

            Log.v(TAG,"Permission is revoked");


        }
    }
    else { //permission is automatically granted on sdk<23 upon installation
        Log.v(TAG,"Permission is granted");
        }
    }

    }

Full stack trace

     java.lang.NullPointerException: Attempt to invoke virtual method 

'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
at android.content.ContextWrapper.getSystemService(ContextWrapper.java:659)
at com.presentiva.repeater.Repeater.isStoragePermissionGranted(Repeater.java:49)
at com.presentiva.repeater.Repeater.<init>(Repeater.java:30)
at java.lang.Class.newInstance(Native Method)
at android.app.ActivityThread.handleCreateService(ActivityThread.java:3173)
at android.app.ActivityThread.-wrap5(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1567)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Answer 1

Problem: You check permissions in constructor of the service, but at that time, the context for the service has not been initialized yet, so that why the app throws the exception.

Solution: Put your code to check permissions in onCreate() method of the service.

public class Repeater extends Service {

    @Override
    public void onCreate() {
        super.onCreate();

        // Context for this service has been initialized.
        try {
            checkPermissions();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void checkPermissions() throws Exception {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            if (checkSelfPermission(android.Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED) {
                Toast.makeText(getApplicationContext(), "sasa", Toast.LENGTH_SHORT).show();
            } else {
                Log.v(TAG, "Permission is revoked");
            }
        } else { //permission is automatically granted on sdk<23 upon installation
            Log.v(TAG, "Permission is granted");
        }
    }

}