Java中的序列化时对static字段的处理

November 29, 2017

起因
static 域会不会被序列化
如何实现 static 域的序列化

起因

今天工作时遇到一个 Java 序列化对象中带有 static 字段的问题, 一时无法确定 Java 默认的序列化机制会怎么处理静态域, Google 的时候发现以下文章此文将 static 和 transient 两个关键字做对比,初看时觉得不错, 但是仔细看时发现文章得出的结论和我的印象中的 Java 序列化有大出入, 于是自己动手写代码验证一下.

static 域会不会被序列化

改文章中的第二部分通过一段代码得出以下结论:

Static variables value can be stored while serializing if the same is provided while initialization.

If variable is defined as Static and Transient both, than static modifier will govern the behavior of variable and not Transient.

该文作者认为:

如果序列化时静态域的值等于初始化时候的值，那么静态域就会被序列化
如果一个域同时拥有 static 和 transient 关键字,那么这个域也会被序列化其实以上这个两个结论都是不对的

实例代码

import java.io.*;

class SuperEmployee implements Serializable {
    public String superField;
    public static String superStaticField;
}

class Employee extends SuperEmployee {
    public String simpleField;
    public static String staticField = "defaultStatic01";
    public transient String transientField = "defaultTransientField01";
    public static transient String staticTransientField;
}

public class TestSerialization {

    private static void serialization() {
        System.out.println("Serialization start...");
        Employee employee = new Employee();
        employee.superField = "superFieldValue";
        employee.superStaticField = "superStaticFieldValue";
        employee.simpleField = "simpleFieldValue";
        // employee.staticField = "staticFieldValue";
        employee.transientField = "transientFieldValue";
        employee.staticTransientField = "staticTransientFieldValue";
        try (
                FileOutputStream fileOutputStream = new FileOutputStream("./employee.dat");
                ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);
        ) {
            objectOutputStream.writeObject(employee);
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("Serialization finished");
        showEmployee(employee);
    }

    private static void deserialization() {
        System.out.println("Deserialization start...");
        Employee employee = null;
        try (
                FileInputStream fileInputStream = new FileInputStream("./employee.dat");
                ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);
        ) {
            employee = (Employee) objectInputStream.readObject();
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
        System.out.println("Deserialization finished");
        showEmployee(employee);
    }

    private static void showEmployee(Employee employee) {
        System.out.println("Employee:");
        System.out.println("employee.superField = " + employee.superField);
        System.out.println("employee.superStaticField = " + employee.superStaticField);
        System.out.println("employee.simpleField = " + employee.simpleField);
        System.out.println("employee.staticField = " + employee.staticField);
        System.out.println("employee.staticTransientField = " + employee.staticTransientField);
    }


    public static void main(String[] args) {
        //serialization();
        deserialization();
    }
}

代码运行方法:

调用 serialization 方法,生成出 employee.dat 文件
修改staticField和staticTransientField的初始值
调用 deserialization 方法

可以看出所有 static 字段其实并没有被序列化,都会等于代码中的值, 其实类变量(静态域)的赋值是在类的初始化过程中发生的, 序列化的是目标是对象而不是类,所以静态变量不会被序列化.

如何实现 static 域的序列化

Java 默认的 serialization 机制不会序列化 static 的域,但是 Java 允许程序员自定义序列化的对象,以下两种方法都可以实现序列化 static 域(或者 transient 域)

重写 writeObject()方法和 readObject()方法代码实例
实现 Externalizable 接口

import java.io.*;
class CustomObject implements Serializable {

    public static String staticField = "defaultStaticValue";
    public transient String transientField = "defaultTransientValue";
    public static transient String staticTransientField = "defaultStaticTransientValue";

    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        out.writeObject(staticField);
        out.writeObject(transientField);
        out.writeObject(staticTransientField);
    }

    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        staticField = (String) in.readObject();
        transientField = (String) in.readObject();
        staticTransientField = (String) in.readObject();
    }
}
public class TestCustomSerialization {

    private static void serialization() {
        System.out.println("Serialization start...");
        CustomObject customObject = new CustomObject();
        customObject.staticField = "staticFieldValue";
        customObject.transientField = "transientFieldValue";
        customObject.staticTransientField = "staticTransientFieldValue";
        try (
                FileOutputStream fileOutputStream = new FileOutputStream("./customObject.dat");
                ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);
        ) {
            objectOutputStream.writeObject(customObject);
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("Serialization finished");
        showEmployee(customObject);
    }

    private static void deserialization() {
        System.out.println("Deserialization start...");
        CustomObject customObject = null;
        try (
                FileInputStream fileInputStream = new FileInputStream("./customObject.dat");
                ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);
        ) {
            customObject = (CustomObject) objectInputStream.readObject();
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
        System.out.println("Deserialization finished");
        showEmployee(customObject);
    }

    private static void showEmployee(CustomObject customObject) {
        System.out.println("CustomObject:");
        System.out.println("CustomObject.staticField = " + customObject.staticField);
        System.out.println("CustomObject.transientField = " + customObject.transientField);
        System.out.println("CustomObject.staticTransientField = " + customObject.staticTransientField);
    }

    public static void main(String[] args) {
        // serialization();
        deserialization();
    }
}