通过反射创建新类示例的两种方式及比较

https://blog.csdn.net/fenglibing/article/details/4531033

通过反射创建新的类示例，有两种方式：

Class.newInstance()

Constructor.newInstance()

以下对两种调用方式给以比较说明：

l             Class.newInstance() 只能够调用无参的构造函数，即默认的构造函数；而Constructor.newInstance() 可以根据传入的参数，调用任意构造构造函数。

l             Class.newInstance() 抛出所有由被调用构造函数抛出的异常。

l             Class.newInstance() 要求被调用的构造函数是可见的，也即必须是public类型的; Constructor.newInstance() 在特定的情况下，可以调用私有的构造函数。

package com.tmall.toosimple.algorithmprocess.impl.test;

public class A {
    private A() {
        System.out.println("A' default constructor is call");
    }

    private A(int a, int b) {
        System.out.println("a=" + a + ", b=" + b);
    }
}

package com.tmall.toosimple.algorithmprocess.impl.test;

import java.lang.reflect.Constructor;

public class B {

    public static void main(String[] args) {
        B b = new B();
        b.newInstanceByConstructor();
        b.newInstanceByClass();
    }


    private void newInstanceByClass() {
        try {
            A  a = (A) Class.forName("com.tmall.toosimple.algorithmprocess.impl.test.A").newInstance();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    private void newInstanceByConstructor() {
        try {
            Class clazz = Class.forName("com.tmall.toosimple.algorithmprocess.impl.test.A");

            Constructor<?>[] constructors = clazz.getDeclaredConstructors();
            Constructor<?>[] constructors1 = clazz.getConstructors();

            Constructor constructor = clazz.getDeclaredConstructor();
            constructor.setAccessible(true);
            A A0 = (A)constructor.newInstance();
            System.out.println(constructors1.length);


            Constructor constructor1 = clazz.getDeclaredConstructor(new Class[]{int.class ,int.class });
            constructor1.setAccessible(true);
            A A1 = (A)constructor1.newInstance(new Object[]{5, 6});

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

结果输出：

A' default constructor is call
0
a=5, b=6
java.lang.IllegalAccessException: Class com.tmall.toosimple.algorithmprocess.impl.test.B can not access a member of class com.tmall.toosimple.algorithmprocess.impl.test.A with modifiers "private"
at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:102)
at java.lang.Class.newInstance(Class.java:436)
at com.tmall.toosimple.algorithmprocess.impl.test.B.newInstanceByClass(B.java:16)
at com.tmall.toosimple.algorithmprocess.impl.test.B.main(B.java:10)

//invoke示例

package com.tmall.toosimple.algorithmprocess.impl.test;

import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import org.junit.Test;

public class Msym {
    public void test(String[] arg){
        for (String string : arg) {
            System.out.println(string);
        }
    }

    public void test1(String... args) {
        for (String arg : args) {
            System.out.println(arg);
        }
    }


    @Test
    public void demo1() throws Exception {

        Msym msym = new Msym();
        msym.test1("a", "b");


        //获取字节码对象
        Class<Msym> clazz = (Class<Msym>) Class.forName("com.tmall.toosimple.algorithmprocess.impl.test.Msym");
        //获取一个对象
        Constructor con =  clazz.getConstructor();
        Msym m = (Msym) con.newInstance();
        String[] s = new String[]{"aa","bb"};
        //获取Method对象
        Method method = clazz.getMethod("test", String[].class);
        //调用invoke方法来调用
        method.invoke(m, new Object[]{s});
        method.invoke(m, (Object)s);
        String[] s1 = new String[]{"cc", "dd"};
        method.invoke(m, (Object)(new String[]{"cc", "dd"}));

        Object object = (Object)s1;
        System.out.println("object=" + object.toString());
    }
}