Fragment的空指针问题_android could not find fragment constructor-优快云博客

本文链接：https://blog.youkuaiyun.com/MarketAndTechnology/article/details/104936872

本文深入探讨了Android中Fragment的初始化过程，揭示了其内部通过反射调用无参构造函数的机制，并介绍了如何正确地传递参数。

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很多人平时已经写过许多Fragment的代码，觉得自己对Fragment已经很熟悉。但是其实对于很多人来说，Fragment并不像自己想象的那么熟悉。

很多时候使用Fragment总是会莫名其妙出现空指针问题。下面就来说说很多人不太熟悉的地方。

我们初始化Fragment的时候经常会习惯性的 new Fragment(参数...); 但是，当我们扒开android的源码时，却发现不是那么回事。

public static Fragment instantiate(Context context, String fname, @Nullable Bundle args) {
    try {
        Class<?> clazz = sClassMap.get(fname);
        if (clazz == null) {
            // Class not found in the cache, see if it's real, and try to add it
            clazz = context.getClassLoader().loadClass(fname);
            sClassMap.put(fname, clazz);
        }
        Fragment f = (Fragment) clazz.getConstructor().newInstance();
        if (args != null) {
            args.setClassLoader(f.getClass().getClassLoader());
            f.setArguments(args);
        }
        return f;
    } catch (ClassNotFoundException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (java.lang.InstantiationException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (IllegalAccessException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (NoSuchMethodException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": could not find Fragment constructor", e);
    } catch (InvocationTargetException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": calling Fragment constructor caused an exception", e);
    }
}

你会发现Fagment初始化调用的是Fragment里面的静态方法，而且是通过反射的方式调用的。反射则意味着他只会调用无参构造函数。

你会发现传参数用的方法是setArguments(Bundle)