If you need to set one font for all TextViews in android application you can use this solution. It will override ALL TextView's typefaces, includes action bar and other standard components, but EditText's password font won't be overriden.

MyApp.java

public class MyApp extends Application {
  
  @Override
  public void onCreate() {
    TypefaceUtil.overrideFont(getApplicationContext(), "SERIF", "fonts/Roboto-Regular.ttf"); // font from assets: "assets/fonts/Roboto-Regular.ttf
  }
}

themes.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <style name="MyAppTheme" parent="@android:style/Theme.Holo.Light">
    <!-- you should set typeface which you want to override with TypefaceUtil -->
    <item name="android:typeface">serif</item>
  </style>
</resources>

TypefaceUtil.java

import android.content.Context;
import android.graphics.Typeface;
import android.util.Log;

import java.lang.reflect.Field;

public class TypefaceUtil {

    /**
     * Using reflection to override default typeface
     * NOTICE: DO NOT FORGET TO SET TYPEFACE FOR APP THEME AS DEFAULT TYPEFACE WHICH WILL BE OVERRIDDEN
     * @param context to work with assets
     * @param defaultFontNameToOverride for example "monospace"
     * @param customFontFileNameInAssets file name of the font from assets
     */
    public static void overrideFont(Context context, String defaultFontNameToOverride, String customFontFileNameInAssets) {
        try {
            final Typeface customFontTypeface = Typeface.createFromAsset(context.getAssets(), customFontFileNameInAssets);

            final Field defaultFontTypefaceField = Typeface.class.getDeclaredField(defaultFontNameToOverride);
            defaultFontTypefaceField.setAccessible(true);
            defaultFontTypefaceField.set(null, customFontTypeface);
        } catch (Exception e) {
            Log.e("Can not set custom font " + customFontFileNameInAssets + " instead of " + defaultFontNameToOverride);
        }
    }
}