serialVersionUID作用是什么以及如何生成的？

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正常不设置serialVersionUID 的序列化和反序列化

先定义一个实体Student.class,需要实现Serializable接口，但是不需要实现get()，set()方法

import java.io.Serializable;
public class Student implements Serializable {
    private int age;
    private String name;
    public Student(int age, String name) {
        this.age = age;
        this.name = name;
    }
    @Override
    public String toString() {
        return "Student{" +
                "age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}

测试类，思路是先把Student对象序列化到Student.txt文件，然后再讲Student.txt文件反序列化成对象，输出。

public class SerialTest {
    public static void main(String[] args) {
        serial();
        deserial();
    }
    // 序列化
    private static void serial(){
        Student student = new Student(9, "Mike");
        try {
            FileOutputStream fileOutputStream = new FileOutputStream("Student.txt");
            ObjectOutputStream objectOutputStream= new ObjectOutputStream(fileOutputStream);
            objectOutputStream.writeObject(student);
            objectOutputStream.flush();
        } catch (Exception exception) {
            exception.printStackTrace();
        }
    }
    // 反序列化
    private static void deserial() {
        try {
            FileInputStream fis = new FileInputStream("Student.txt");
            ObjectInputStream ois = new ObjectInputStream(fis);
            Student student = (Student) ois.readObject();
            ois.close();
            System.out.println(student.toString());
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

输出结果，序列化文件我们可以看到Student.txt,反序列化出来，里面的字段都是不变的，说明反序列化成功了。

序列化之后，类文件增加了字段，反序列化会怎么样？

先说结果，会失败！！！

我们在Student.java中增加了一个属性score,重新生成了toString()方法。

package com.aphysia.normal;

import java.io.Serializable;

public class Student implements Serializable {
    private int age;
    private String name;
    private int score;
    public Student(int age, String name) {
        this.age = age;
        this.name = name;
    }
    @Override
    public String toString() {
        return "Student{" +
                "age=" + age +
                ", name='" + name + '\'' +
                ", score=" + score +
                '}';
    }
}

然后重新调用deserial()方法，会报错：

java.io.InvalidClassException: com.aphysia.normal.Student; local class incompatible: stream classdesc serialVersionUID = 7488921480006384819, local class serialVersionUID = 6126416635811747983
    at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:699)
    at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1963)
    at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1829)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2120)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1646)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:482)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:440)
    at com.aphysia.normal.SerialTest.deserial(SerialTest.java:26)
    at com.aphysia.normal.SerialTest.main(SerialTest.java:7)

从上面的报错信息中，我们可以看到，类型不匹配，主要是因为serialVersionUID变化了！！！