XposedHelpers.findAndHookMethod(ClassLoader.class,
        "loadClass", String.class,
        new XC_MethodHook() {
            @Override
            protected void beforeHookedMethod(MethodHookParam param) throws Throwable {
                if ("com.example.MainActivity".equals(param.args[0])) {
                    param.setResult(getClass().getClassLoader()
                            .loadClass("com.example.TargetActivity"));
                }
            }
        });