什么是serialVersionUID？为什么要使用它？

旧竹 2024-07-15 23:57:51

首先我需要解释一下什么是序列化。

序列化允许将对象转换为流，以便通过网络发送该对象或保存到文件或保存到数据库以供以后使用。

序列化有一些规则。

仅当对象的类或其超类实现 Serialized 接口时，对象才是可序列化的
对象是可序列化的（其本身实现了 Serialized 接口），即使其超类不是可序列化的。但是，可序列化类层次结构中的第一个超类（未实现 Serialized 接口）必须具有无参数构造函数。如果违反了这一点，readObject()将在运行时产生java.io.InvalidClassException
所有基本类型都是可序列化的。
瞬态字段（带有瞬态修饰符）不会被序列化（即不保存或恢复）。实现 Serialized 的类必须标记不支持序列化的类（例如文件流）的瞬态字段。
静态字段（带有 static 修饰符）未序列化。

当 Object 被序列化时，Java 运行时会关联序列版本号，即 serialVersionID。

我们需要serialVersionID的地方：

在反序列化期间验证发送方和接收方在序列化方面是否兼容。如果接收方使用不同的 serialVersionID 加载类，则反序列化将以 InvalidClassCastException 结束。
可序列化类可以通过声明名为 serialVersionUID 的字段来显式声明自己的 serialVersionUID，该字段必须是 static、final 且类型为 long。

让我们通过一个例子来尝试一下。

import java.io.Serializable;

public class Employee implements Serializable {
    private static final long serialVersionUID = 1L;
    private String empname;
    private byte empage;

    public String getEmpName() {
        return name;
    }

    public void setEmpName(String empname) {
        this.empname = empname;
    }

    public byte getEmpAge() {
        return empage;
    }

    public void setEmpAge(byte empage) {
        this.empage = empage;
    }

    public String whoIsThis() {
        return getEmpName() + " is " + getEmpAge() + "years old";
    }
}

创建序列化对象

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;

public class Writer {
    public static void main(String[] args) throws IOException {
        Employee employee = new Employee();
        employee.setEmpName("Jagdish");
        employee.setEmpAge((byte) 30);

        FileOutputStream fout = new
                FileOutputStream("/users/Jagdish.vala/employee.obj");
        ObjectOutputStream oos = new ObjectOutputStream(fout);
        oos.writeObject(employee);
        oos.close();
        System.out.println("Process complete");
    }
}

反序列化对象

import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;

public class Reader {
    public static void main(String[] args) throws ClassNotFoundException, IOException {
        Employee employee = new Employee();
        FileInputStream fin = new FileInputStream("/users/Jagdish.vala/employee.obj");
        ObjectInputStream ois = new ObjectInputStream(fin);
        employee = (Employee) ois.readObject();
        ois.close();
        System.out.println(employee.whoIsThis());
    }
}

注意：现在更改 Employee 类的serialVersionUID 并保存：

private static final long serialVersionUID = 4L;

并执行 Reader 类。不执行 Writer 类，您将得到异常。

Exception in thread "main" java.io.InvalidClassException: 
com.jagdish.vala.java.serialVersion.Employee; local class incompatible: 
stream classdesc serialVersionUID = 1, local class serialVersionUID = 4
at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:616)
at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1623)
at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1518)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1774)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1351)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:371)
at com.krishantha.sample.java.serialVersion.Reader.main(Reader.java:14)

First I need to explain what serialization is.

Serialization allows to convert an object to a stream, for sending that object over the network OR Save to file OR save into DB for later usage.

There are some rules for serialization.

An object is serializable only if its class or its superclass implements the Serializable interface
An object is serializable (itself implements the Serializable interface) even if its superclass is not. However, the first superclass in the hierarchy of the serializable class, that does not implements Serializable interface, MUST have a no-arg constructor. If this is violated, readObject() will produce a java.io.InvalidClassException in runtime
All primitive types are serializable.
Transient fields (with transient modifier) are NOT serialized, (i.e., not saved or restored). A class that implements Serializable must mark transient fields of classes that do not support serialization (e.g., a file stream).
Static fields (with static modifier) are not serialized.

When Object is serialized, Java Runtime associates the serial version number aka, the serialVersionID.

Where we need serialVersionID:

During the deserialization to verify that sender and receiver are compatible with respect to serialization. If the receiver loaded the class with a different serialVersionID then deserialization will end with InvalidClassCastException.
A serializable class can declare its own serialVersionUID explicitly by declaring a field named serialVersionUID that must be static, final, and of type long.

Let's try this with an example.

import java.io.Serializable;

public class Employee implements Serializable {
    private static final long serialVersionUID = 1L;
    private String empname;
    private byte empage;

    public String getEmpName() {
        return name;
    }

    public void setEmpName(String empname) {
        this.empname = empname;
    }

    public byte getEmpAge() {
        return empage;
    }

    public void setEmpAge(byte empage) {
        this.empage = empage;
    }

    public String whoIsThis() {
        return getEmpName() + " is " + getEmpAge() + "years old";
    }
}

Create Serialize Object

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;

public class Writer {
    public static void main(String[] args) throws IOException {
        Employee employee = new Employee();
        employee.setEmpName("Jagdish");
        employee.setEmpAge((byte) 30);

        FileOutputStream fout = new
                FileOutputStream("/users/Jagdish.vala/employee.obj");
        ObjectOutputStream oos = new ObjectOutputStream(fout);
        oos.writeObject(employee);
        oos.close();
        System.out.println("Process complete");
    }
}

Deserialize the object

import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;

public class Reader {
    public static void main(String[] args) throws ClassNotFoundException, IOException {
        Employee employee = new Employee();
        FileInputStream fin = new FileInputStream("/users/Jagdish.vala/employee.obj");
        ObjectInputStream ois = new ObjectInputStream(fin);
        employee = (Employee) ois.readObject();
        ois.close();
        System.out.println(employee.whoIsThis());
    }
}

NOTE: Now change the serialVersionUID of the Employee class and save:

private static final long serialVersionUID = 4L;

And execute the Reader class. Not to execute the Writer class and you will get the exception.

Exception in thread "main" java.io.InvalidClassException: 
com.jagdish.vala.java.serialVersion.Employee; local class incompatible: 
stream classdesc serialVersionUID = 1, local class serialVersionUID = 4
at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:616)
at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1623)
at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1518)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1774)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1351)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:371)
at com.krishantha.sample.java.serialVersion.Reader.main(Reader.java:14)

什么是serialVersionUID？为什么要使用它？

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