static 不可变对象（字符串）和 static 可变对象（单例 class 对象）在 Z913F725F48742 中的序列化

Question

i am a bit confuse about serialization of static variables as they cannot be serialization. Although singleton class objects can be serialization. i have a sample code:

1. in TestSerilization class: i first compiled an run the code without comments i got the result, but as i comment the part and recompile and run i didnot got the desired result for static String values while de-serialization

2. in TestSerilization2 class: i first compiled an run the code without comments i got the result, but as i comment the part and recompile and run i did got the same result for private static Student instance values while de-serialization.

   i want to ask that why the private static Student instance gave values but private static String name didn't, although both are static .

    import java.io.*;
    class Student implements Serializable{
        private static String  name;
        private int rollNo;

        public void setName(String name){
            this.name=name;
        }
        public String getName(){
            return this.name;
        }

        public void setRollNo(int rollNo){
            this.rollNo=rollNo;
        }
        public int getRollNo(){
            return this.rollNo;
        }
    }
    public class TestSerilization{
        public static void main(String args[]) throws FileNotFoundException, IOException, ClassNotFoundException{
            /*
            Student s1=new Student();
            s1.setName("Apoorv");
            s1.setRollNo(13);

            ObjectOutputStream oos=new ObjectOutputStream(new FileOutputStream("D:\\JAVA Practice\\output.ser"));
            oos.writeObject(s1);
            System.out.println("s1: name-"+s1.getName()+" rollno-"+s1.getRollNo()+" hascode: "+s1.hashCode());
            */
            ObjectInputStream ins=new ObjectInputStream(new FileInputStream("D:\\JAVA Practice\\output.ser"));
            Student s2=(Student)ins.readObject();
            System.out.println("s2: name-"+s2.getName()+" rollno-"+s2.getRollNo()+" hascode: "+s2.hashCode());
        }
    }




    import java.io.*;
    final class Student implements Serializable{
        private static Student instance;
        private String  name;
        private int rollNo;

        private Student(){} 
        public static Student getInstance(){
            if(instance==null){
                instance=new Student();
            }
            return instance;
        }

        public void setName(String name){
            this.name=name;
        }
        public String getName(){
            return this.name;
        }

        public void setRollNo(int rollNo){
            this.rollNo=rollNo;
        }
        public int getRollNo(){
            return this.rollNo;
        }
    }
    public class TestSerilization2{
        public static void main(String args[]) throws FileNotFoundException, IOException, ClassNotFoundException{
            /*
            Student s1=Student.getInstance();
            s1.setName("Apoorv");
            s1.setRollNo(13);
            System.out.println("s1: name-"+s1.getName()+" rollno-"+s1.getRollNo()+" hascode: "+s1.hashCode());
            Student s2=Student.getInstance();
            System.out.println("s2: name-"+s2.getName()+" rollno-"+s2.getRollNo()+" hascode: "+s2.hashCode());
            */

            /*
            Student s1=Student.getInstance();
            s1.setName("Apoorv");
            s1.setRollNo(13);

            ObjectOutputStream oos=new ObjectOutputStream(new FileOutputStream("D:\\JAVA Practice\\output2.ser"));
            oos.writeObject(s1);
            System.out.println("s1: name-"+s1.getName()+" rollno-"+s1.getRollNo()+" hascode: "+s1.hashCode());
            */
            ObjectInputStream ins=new ObjectInputStream(new FileInputStream("D:\\JAVA Practice\\output2.ser"));
            Student s2=(Student)ins.readObject();
            System.out.println("s2: name-"+s2.getName()+" rollno-"+s2.getRollNo()+" hascode: "+s2.hashCode());

        }
    }

Answer 1

You say that the " private static Student instance gave values" when serialized, and that the " private static String name didn't", but what you are comparing is different from what you think your are comparing. You seem to think that in the first example you saw values when serializing private static Student instance but didn't when serializing private static String name , but you never serialized private static String name . Actually, you saw values when serializing a Student instance with String name and didn't when serializing a Student instance with static String name . To clarify,

• In the 1st example, the name field IS static , and it IS NOT serialized.
• In the 2nd example, the name field IS NOT static , and it IS serialized.

A field marked as static indicates that it is owned by the class, which is why they are referenced via the class ( MyClass.field ) rather than via an instance ( ((MyClass) instance).field ). Since you are dealing with Java object serialization, the static fields are not serialized as part of the object.

Whether or not a reference to an object is static is irrelevant when serializing that object. Ie Student student (non-static reference) will serialize just the same as static Student student (static reference).