LarryDpk
发布于 2020-05-09 / 1079 阅读 / 0 评论 / 0 点赞

Java创建对象的六种方法-权当记录一下

1 简介

Java是面向对象的编程语言,只要使用它,就需要创建对象。Java创建对象有六种方法,实际常用的不会这么多,这里权当是记录一下。

2 六种方法

(1)使用new关键字

Pumpkin p1 = new Pumpkin();

(2)反射之Class类newInstance()

Pumpkin p2 = Pumpkin.class.newInstance();

(3)反射之Constructor类的newInstance()

Pumpkin p3 = Pumpkin.class.getDeclaredConstructor().newInstance();

(4)Object对象的clone方法

Pumpkin p4 = (Pumpkin) p1.clone();

注意Object类的clone方法是protected的,在Override的时候,可以改成public,这样让其它所有类都可以调用。

注意浅拷贝和深拷贝。

(5)反序列化

ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("object.bin"));
oos.writeObject(p1);
oos.close();

ObjectInputStream ois = new ObjectInputStream(new FileInputStream("object.bin"));
Pumpkin p5 = (Pumpkin) ois.readObject();
ois.close();

必须要实现Serializable接口;

需要注意哪些字段可序列化,哪些字段不会被序列化,如何控制;

注意serialVersionUID的作用;

了解Externalizable的不同之处。

(6)使用Unsafe类

Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
Unsafe unsafe = (Unsafe) f.get(null);
Pumpkin p6 = (Pumpkin) unsafe.allocateInstance(Pumpkin.class);

很少用的方法,一般不用了解这个方法。

3 示例代码

示例代码如下:

package com.pkslow.basic;


import sun.misc.Unsafe;

import java.io.*;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;

public class CreateObject {

    public static class Pumpkin implements Cloneable, Serializable {
        public Pumpkin(){
            System.out.println("Constructor called");
        }
        @Override
        public Object clone() throws CloneNotSupportedException {
            return super.clone();
        }
    }

    public static void main(String[] args) throws IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, CloneNotSupportedException, IOException, ClassNotFoundException, NoSuchFieldException {

        System.out.println("---start---");
        System.out.println("(1) new");
        Pumpkin p1 = new Pumpkin();

        System.out.println("(2) Class newInstance");
        Pumpkin p2 = Pumpkin.class.newInstance();

        System.out.println("(3) Constructor newInstance");
        Pumpkin p3 = Pumpkin.class.getDeclaredConstructor().newInstance();

        System.out.println("(4) clone");
        Pumpkin p4 = (Pumpkin) p1.clone();

        System.out.println("(5)Serialization");
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("object.bin"));
        oos.writeObject(p1);
        oos.close();

        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("object.bin"));
        Pumpkin p5 = (Pumpkin) ois.readObject();
        ois.close();

        System.out.println("(6) Unsafe");
        Field f = Unsafe.class.getDeclaredField("theUnsafe");
        f.setAccessible(true);
        Unsafe unsafe = (Unsafe) f.get(null);
        Pumpkin p6 = (Pumpkin) unsafe.allocateInstance(Pumpkin.class);

        System.out.println("---end---");
    }
}

输出结果如下:

---start---
(1) new
Constructor called
(2) Class newInstance
Constructor called
(3) Constructor newInstance
Constructor called
(4) clone
(5)Serialization
(6) Unsafe
---end---

所以会执行构造函数的有:new关键字、两种反射;

不会执行构造函数的有:clone、序列化、Unsafe类。

4 总结

要学会生产对象,也要学会管理对象、回收对象。