IllegalArgumentException：Java Constructor.newInstance() 中的参数数量错误

Question

考虑以下代码，

public class StartUp {

    public StartUp(String[] test){}

    public static void main(String[] args) throws Exception{
        Constructor cd = StartUp.class.getConstructor(String[].class);
        System.out.println(cd.newInstance(new String[]{}).toString());
    }
}

它出什么问题了？ 我收到以下异常：

线程“main”中的异常 java.lang.IllegalArgumentException：sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance0(Native Method) 处的参数数量错误newInstance(DelegatingConstructorAccessorImpl.java:27) at java.lang.reflect.Constructor.newInstance(Constructor.java:513) at com.test.StartUp.main(StartUp.java:10)

Answer 1

您的String[]被隐式转换为Object[]并作为一个空的参数数组，而不是作为一个空数组的单个参数。 尝试这个：

Object arg = new String[0];
System.out.println(cd.newInstance(arg).toString());

或者

System.out.println(cd.newInstance(((Object)new String[0]).toString());

甚至完全避免编译器必须为您创建数组：

System.out.println(cd.newInstance(new Object[] { new String[0] }).toString());

基本上这是可变参数处理和数组协方差的混合:(

Answer 2

您可以使用 dp4j 详细选项来回答您的问题，并获得您需要的正确反射代码：

$ vim ReflectedAcces.java
 class StartUp {

    private StartUp(String[] test){}

}

public class ReflectedAcces{

        @com.dp4j.InjectReflection
    public static void main(String[] args) throws Exception{
                StartUp su = new StartUp(new String[]{});
                System.out.println(su.toString());
    }

}
 

$ javac -cp dp4j-1.0-jar-with-dependencies.jar -Averbose=true ReflectedAcces.java 
...
ReflectedAcces.java:10: Note: 
class StartUp {
    
    private StartUp(String[] test) {
    }
}
public class ReflectedAcces {
    
    public ReflectedAcces() {
        super();
    }
    
    @com.dp4j.InjectReflection()
    public static void main(String[] args) java.lang.ClassNotFoundException, java.lang.NoSuchFieldException, java.lang.IllegalAccessException, java.lang.NoSuchMethodException, java.lang.reflect.InvocationTargetException, java.lang.InstantiationException {
        final java.lang.reflect.Constructor startUpConstructor = Class.forName("StartUp").getDeclaredConstructor(.java.lang.String[].class);
        startUpConstructor.setAccessible(true);
        StartUp su = (.StartUp)startUpConstructor.newInstance(new .java.lang.Object[1][]{new String[]{}});
        System.out.println(su.toString());
    }
}