java 实例化一个内部类
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Instantiating an inner class
提问by Jon Skeet
I have a utility method and when irrelevant logic is removed from it, the simplified method would look like this:
我有一个实用方法，当从中删除不相关的逻辑时，简化的方法将如下所示：
public static <A extends Foo> List<A> getFooList(Class<A> clazz) {
List<A> returnValue = new ArrayList<A>();
for(int i=0; i < 5; i++) {
A object = clazz.newInstance();
returnValue.add(object);
}
return returnValue;
}
The problem is, that if clazzis an inner class such as Foo.Bar.class, then the newInstance()method will not work even if Barwould be public, as it will throw a java.lang.InstantiationException.
问题是，如果clazz是一个内部类，例如Foo.Bar.class，那么newInstance()即使Bar是公开的方法也不会工作，因为它会抛出一个java.lang.InstantiationException.
Is there a way to dynamically instantiate inner classes?
有没有办法动态实例化内部类？
回答by Jon Skeet
If it's genuinely an innerclass instead of a nested(static) class, there's an implicit constructor parameter, which is the reference to the instance of the outer class. You can't use Class.newInstanceat that stage - you have to get the appropriate constructor. Here's an example:
如果它真的是一个内部类而不是嵌套（静态）类，则有一个隐式构造函数参数，它是对外部类实例的引用。您不能Class.newInstance在那个阶段使用- 您必须获得适当的构造函数。下面是一个例子：
import java.lang.reflect.*;
class Test
{
public static void main(String[] args) throws Exception
{
Class<Outer.Inner> clazz = Outer.Inner.class;
Constructor<Outer.Inner> ctor = clazz.getConstructor(Outer.class);
Outer outer = new Outer();
Outer.Inner instance = ctor.newInstance(outer);
}
}
class Outer
{
class Inner
{
// getConstructor only returns a public constructor. If you need
// non-public ones, use getDeclaredConstructors
public Inner() {}
}
}
回答by pierrefevrier
Something more generic:
更通用的东西：
public static <T> T createInstance(final Class<T> clazz) throws SecurityException, NoSuchMethodException,
IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException {
T instanceToReturn = null;
Class< ? > enclosingClass = clazz.getEnclosingClass();
if (enclosingClass != null) {
Object instanceOfEnclosingClass = createInstance(enclosingClass);
Constructor<T> ctor = clazz.getConstructor(enclosingClass);
if (ctor != null) {
instanceToReturn = ctor.newInstance(instanceOfEnclosingClass);
}
} else {
instanceToReturn = clazz.newInstance();
}
return instanceToReturn;
}
回答by TheJeff
In Jmockit 1.41, use this:
在 Jmockit 1.41 中，使用这个：
ConstructorReflection.newInstance
构造函数反射.newInstance
回答by Sandman
This exception will be thrown only if clazz represents either an abstract class or an interface. Are you sure you're passing a Class object that represents a concrete class?
仅当 clazz 表示抽象类或接口时才会抛出此异常。您确定要传递代表具体类的 Class 对象吗？
