春节回来之后瞄了眼 bugly，发现了一个发生频次高影响用户广的 bug:

java.lang.RuntimeException:Unable to start activity ComponentInfo{.ui.activity.HomeActivity}: androidx.fragment.app.Fragment$InstantiationException: Unable to instantiate fragment .ui.fragment.q: could not find Fragment constructor

搜索了问题之后发现原因很简单，同事在创建的Fragment的时候用了带参数的构造方法，而没有添加无参构造，从而导致了这个 runtime exception，来看一下官方文档的解释：

All subclasses of Fragment must include a public no-argument constructor. The framework will often re-instantiate a fragment class when needed, in particular during state restore, and needs to be able to find this constructor to instantiate it. If the no-argument constructor is not available, a runtime exception will occur in some cases during state restore

从这段说明中可以看出所有的Fragment子类都必须包含无参的构造方法，系统在根据需要重新实例化Fragment子类时会默认去调用无参构造方法，如果没有找到这个无参方法，则会导致运行时错误。