Android: change fonts for the whole application

FontsReplacer.java

public class FontsReplacer {

    @SuppressWarnings("unchecked")
    public static void replaceFonts(Context context) {
        try {
            //we need to change all static fields contains fonts:

            //1. static constants
            setStaticFinalField(Typeface.class, "DEFAULT", FontCache.getTypeface(context, Typeface.NORMAL));
            setStaticFinalField(Typeface.class, "DEFAULT_BOLD", FontCache.getTypeface(context, Typeface.BOLD));
            setStaticFinalField(Typeface.class, "SANS_SERIF", FontCache.getTypeface(context, Typeface.NORMAL));
            setStaticFinalField(Typeface.class, "SERIF", FontCache.getTypeface(context, Typeface.NORMAL));
            setStaticFinalField(Typeface.class, "MONOSPACE", FontCache.getTypeface(context, Typeface.NORMAL));

            //2. array with defaults
            setStaticFinalField(Typeface.class, "sDefaults", new Typeface[] {
                    Typeface.DEFAULT,
                    Typeface.DEFAULT_BOLD,
                    FontCache.getTypeface(context, Typeface.ITALIC),
                    FontCache.getTypeface(context, Typeface.BOLD_ITALIC)
            });

            //3. invoke method 'setDefault'
            Method setDefault = Typeface.class.getDeclaredMethod("setDefault", Typeface.class);
            setDefault.setAccessible(true);
            setDefault.invoke(null, Typeface.DEFAULT);

            //4. replace fonts in the map
            Field sSystemFontMapField = Typeface.class.getDeclaredField("sSystemFontMap");
            sSystemFontMapField.setAccessible(true);
            Map<String, Typeface> sSystemFontMap = (Map<String, Typeface>) sSystemFontMapField.get(null);
            for (String key : sSystemFontMap.keySet()) {
                sSystemFontMap.put(key, FontCache.getDefaultTypeface(context));
            }

        } catch (Exception e) {
            Timber.e(e, e.getMessage());
        }
    }

    private static void setStaticFinalField(Class clazz, String fieldName, Object value) throws NoSuchFieldException, IllegalAccessException {
        Field field = clazz.getDeclaredField(fieldName);
        field.setAccessible(true);

        try {
            Field modifiers = Field.class.getDeclaredField("accessFlags");
            modifiers.setAccessible(true);
            modifiers.setInt(field, field.getModifiers() & ~Modifier.FINAL);
        } catch (Exception e) {
            Timber.e(e, e.getMessage());
        }

        field.set(null, value);
    }

    private FontsReplacer() {}
}

If we use reflections to achieve this, Will google play store reject the app during publishing?

If we use reflections to achieve this, Will google play store reject the app during publishing?

Hi. Nope. But this solution pretty old and I wouldn't recommend it anyway. Custom fonts supported now out of the box.

is it possible to replace system font? i am not setting typeface in styles, my requirement is based on some parameter i need to set font, if the parameter is not set i need to set android system font. is that possible?

@jamshadinho this one is obsolete. Custom fonts supported now of of the box.