java - 使用反射实例化内部类时出现 InstantiationException。为什么？

Question

我无法创建 B 对象，但为什么？

public class AFactory {

    public int currentRange;

    private abstract class A {
        protected final Object range = currentRange;

        public int congreteRange = 28;
    }

    public class B extends A {
        public int congreteRange = 42;
    }

    synchronized A createNew(Class<? extends A> clazz) throws Exception {
        // EDIT: there is accessible default constructor
        currentRange = clazz.newInstance().congreteRange;
        return clazz.newInstance();
    }

    public static void main(String[] args) throws Exception {
        AFactory factory = new AFactory();
        System.out.println(factory.createNew(B.class).range);
    }
}

异常(exception)是:

Exception in thread "main" java.lang.InstantiationException: AFactory$B
at java.lang.Class.newInstance0(Class.java:357)
at java.lang.Class.newInstance(Class.java:325)
at AFactory.createNew(AFactory.java:15)
at AFactory.main(AFactory.java:21)

Answer 1

问题是您正在尝试实例化一个内部类，您只能在外部类的实例上访问它。内部类的构造函数采用封闭类的隐式隐藏实例。分析这个简单类的字节码就可以看出:

public class Demo {
    class Test {
    }
}

现在，编译代码:

javac Demo.java

这将创建两个类文件:

Demo.class
Demo$Test.class

运行以下命令查看Demo$Test.class的字节码:

javap -c . Demo$Test

你会得到如下结果:

class Demo$Test {
  final Demo this$0;   

  Demo$Test(Demo);
    Code:
       0: aload_0
       1: aload_1
       2: putfield      #1                  // Field this$0:LDemo;
       5: aload_0
       6: invokespecial #2                  // Method java/lang/Object."<init>":
()V
       9: return
}

那么，你看到类的构造函数了吗？它以 Demo 作为参数。因此，没有 0-arg 构造函数。

但是，如果您将内部类设置为static，它就会起作用，因为这样您就不需要任何封闭类的实例来调用内部类构造函数。

使用 static 内部类 - 备选方案:

public class AFactory {

    public static int currentRange;

    private static abstract class A {
        protected final Object range = AFactory.currentRange;
    }

    public static class B extends A {
        public int congreteRange = 42;
    }

    synchronized A createNew(Class<? extends B> clazz) throws Exception {
        currentRange = clazz.newInstance().congreteRange;
        return clazz.newInstance();
    }

    public static void main(String[] args) throws Exception {
        AFactory factory = new AFactory();
        System.out.println(factory.createNew(B.class).range);
    }
}

---

使用非static 内部类 - 最终代码:

如果您不想让它们成为static，那么您必须首先创建一个封闭类的实例。并将其传递给内部类的构造函数。要获取内部类的构造函数，可以使用Class#getDeclaredConstructor方法。

现在，您必须修改工厂方法以将 Constructor 作为参数。像这样修改您的代码:

public class AFactory {

    public int currentRange;

    private abstract class A {
        protected final Object range = currentRange;
    }

    public class B extends A {
        public int congreteRange = 42;
    }

    synchronized A createNew(Constructor<? extends A> ctor) throws Exception {
        // Pass `this` as argument to constructor. 
        // `this` is reference to current enclosing instance
        return ctor.newInstance(this); 
    }

    public static void main(String[] args) throws Exception {
        AFactory factory = new AFactory();

        // Get constructor of the class with `AFactory` as parameter
        Class<B> bClazz = B.class;
        Constructor<B> ctor = bClazz.getDeclaredConstructor(AFactory.class);

        System.out.println(factory.createNew(ctor));
    }
}