java 反射 java.lang.IllegalArgumentException

package cn.com.reflect;

public class Person {

private String name;
private int age;

public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
public int getAge() {
    return age;
}
public void setAge(int age) {
    this.age = age;
}
public Person(int age) {
    super();
    this.age = age;
}
public Person(String name) {
    super();
    this.name = name;
}
public Person() {
    super();
}
public Person(String name, int age) {
    super();
    this.name = name;
    this.age = age;
}

}

package cn.com.reflect;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class ReflectDemo4 {

public static void main(String[] args) {

    Class<?> clazz = null;
    try {
        clazz = Class.forName("cn.com.reflect.Person");
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
    Person demo1 = null;
    Constructor<?>[] con = clazz.getConstructors();
    try {
        demo1 = (Person) con[0].newInstance(12);
    } catch (InstantiationException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (IllegalArgumentException e) {
        e.printStackTrace();
    } catch (InvocationTargetException e) {
        e.printStackTrace();
    }

    System.out.println(demo1.getAge());
}

}
java.lang.IllegalArgumentException: wrong number of arguments
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at cn.com.reflect.ReflectDemo4.main(ReflectDemo4.java:20)
Exception in thread "main" java.lang.NullPointerException
at cn.com.reflect.ReflectDemo4.main(ReflectDemo4.java:31)

通过clazz.getConstructors()所获得的构造器的顺序，不一定是书写顺序。很可能是按参数从少到多排列的（未查证）。
你的con[0]获得的应该是个默认构造器，所以报错。你可以通过con[0]的getParameterTypes()来获得参数的个数，类型等信息进行判断，
进而确定要使用的构造器。
或者，干脆通过带参数的clazz.getConstructors(xxx，yyy。。。)函数，来明确获得一个你指定的参数列表的构造器。

我按你的例子打了个断点，发现那些构造器的顺序是Person(),Person(java.lang.String), Person(java.lang.String,int), Person(int)。可见con[0]是默认构造器，所以报了个参数非法

yimiguan 童鞋给的是正解，顶一下。