通过反射创建新类示例的两种方式及比较

Posted 2020-11-17 wxdlut

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);
    }
}

 1 package com.tmall.toosimple.algorithmprocess.impl.test;
 2 
 3 import java.lang.reflect.Constructor;
 4 
 5 public class B {
 6 
 7     public static void main(String[] args) {
 8         B b = new B();
 9         b.newInstanceByConstructor();
10         b.newInstanceByClass();
11     }
12 
13 
14     private void newInstanceByClass() {
15         try {
16             A  a = (A) Class.forName("com.tmall.toosimple.algorithmprocess.impl.test.A").newInstance();
17         } catch (InstantiationException e) {
18             e.printStackTrace();
19         } catch (IllegalAccessException e) {
20             e.printStackTrace();
21         } catch (ClassNotFoundException e) {
22             e.printStackTrace();
23         }
24     }
25 
26     private void newInstanceByConstructor() {
27         try {
28             Class clazz = Class.forName("com.tmall.toosimple.algorithmprocess.impl.test.A");
29 
30             Constructor<?>[] constructors = clazz.getDeclaredConstructors();
31             Constructor<?>[] constructors1 = clazz.getConstructors();
32 
33             Constructor constructor = clazz.getDeclaredConstructor();
34             constructor.setAccessible(true);
35             A A0 = (A)constructor.newInstance();
36             System.out.println(constructors1.length);
37 
38 
39             Constructor constructor1 = clazz.getDeclaredConstructor(new Class[]{int.class ,int.class });
40             constructor1.setAccessible(true);
41             A A1 = (A)constructor1.newInstance(new Object[]{5, 6});
42 
43         } catch (Exception e) {
44             e.printStackTrace();
45         }
46     }
47 }

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)