android - 无法通过 `cls.newInstance()` 创建异步任务的实例？

Question

android代码很简单:

public void createNewTask(View v) {
    try {
        MyTask task = MyTask.class.newInstance();
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

private class MyTask extends AsyncTask<Void, Void, Void> {
    @Override
    protected Void doInBackground(Void... params) {
        return null;  
    }
}

但是在调用createNewTask方法时，会抛出异常:

Caused by: java.lang.RuntimeException: 
    java.lang.InstantiationException: com.example.MyActivity$MyTask
    at com.example.MyActivity.createNewTask(MyActivity.java:214)
    ... 14 more
    Caused by: java.lang.InstantiationException: com.example.MyActivity$MyTask
    at java.lang.Class.newInstanceImpl(Native Method)
    at java.lang.Class.newInstance(Class.java:1440)
    at com.example.MyActivity.createNewTask(MyActivity.java:211)

但是如果我选择另一个类，例如

MyActivity task = MyActivity.class.newInstance();

一切都会好起来的。

我的代码哪里错了？我在装有 android 2.3.6 的 android 手机上对其进行了测试。

Answer 1

找到了解决这个问题的两种方法:

public static class MyTask {
}

或

private static class MyTask {
    public MyTask(){}
}

根据 documentation :

InstantiationException - if this Class represents an abstract class, an interface, an array class, a primitive type, or void; or if the class has no nullary constructor; or if the instantiation fails for some other reason.

尝试向您的类中添加 nullary 构造函数:

public MyTask() {}

编辑

根据 this documentation :

Class.newInstance() requires that the constructor be visible; Constructor.newInstance() may invoke private constructors under certain circumstances.