Appending to a Serialization file (Java)

Question

I'm trying to append objects to a serialized file but when I read it, it only contains the most recent object instead of all of the objects:

import java.io.*;

public class Main implements Serializable {

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        TestClass test = new TestClass();
        test.username = "this one is second";
        test.age = 23;
        test.phone = "+1 (010) 000 0000";
        test.address = "2 Main St.";
        TestClass test2 = new TestClass();
        test2.username = "this one is second      22222";
        test2.age = 23;
        test2.phone = "+1 (010) 000 0000";
        test2.address = "2 Main St.";
        ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("C:\\Java Projects\\ObjectOutputStreamTests\\files\\test.ser"));
        out.writeObject(test);
        out.writeObject(test2);
        out.close();
        ObjectOutputStream out2 = new ObjectOutputStream(new FileOutputStream("C:\\Java Projects\\ObjectOutputStreamTests\\files\\test.ser", true)) {
            @Override
            protected void writeStreamHeader() throws IOException {
                reset();
            }
        };
        out2.writeObject(test);
        out2.writeObject(test2);
        out2.close();

        ObjectInputStream in = new ObjectInputStream(new FileInputStream("C:\\Java Projects\\ObjectOutputStreamTests\\files\\test.ser"));
        try {
            while ((in.readObject()) != null) {
                TestClass testRead = (TestClass) in.readObject();
                System.out.println(testRead.username);
                System.out.println(testRead.age);
                System.out.println(testRead.address);
                System.out.println(testRead.phone);
            }
        } catch (EOFException e) {
            System.out.println("end");
        }
    }
}

TestClass:

import java.io.Serializable;

public class TestClass implements Serializable {

    public String username;
    public int age;
    public String phone;
    public String address;
}

When I run this code, I get the second object (test2) printed twice, instead of the first and second object consecutively. This issue persists with more or less objects. How do I fix this?

Does this help? [Appending to an ObjectOutputStream](https://stackoverflow.com/questions/1194656/appending-to-an-objectoutputstrea) — Dave S, Nov 25 '21 at 23:52
Why not put the objects in a `List` before writing them, so you can get a list back later? — magicmn, Nov 26 '21 at 00:06
The original works, but if I only had one ObjectInputStream (and not anything else, to simulate a full read) I only get the most recent sent. Is there any way of fixing this? — WardenHelmet, Nov 26 '21 at 00:39
Why not start by reading the comments. Try responding to the comments. Show your TestClass — Scary Wombat, Nov 26 '21 at 00:59

Scary Wombat · Accepted Answer · 2021-11-26T02:31:44.167

1

You are reading the same Object twice for each iteration

while ((in.readObject()) != null) {    // once
    TestClass testRead = (TestClass) in.readObject();    // twice

You can change your code to

 Object obj;
 while ((obj = in.readObject()) != null) {
       TestClass testRead = (TestClass) obj;
       System.out.println(testRead.username);
       System.out.println(testRead.age);
       System.out.println(testRead.address);
       System.out.println(testRead.phone);
 }

Note

To get it to work I changed to

  FileOutputStream f = new FileOutputStream("C:\\temp\\test.ser", true);

  ObjectOutputStream out2 = new ObjectOutputStream(f)
  {
            @Override 
            protected void writeStreamHeader() throws IOException {  
                   System.out.println("I am called");  
                   reset();
            }
  }
  ;
        
  out2.writeObject(test);
  out2.writeObject(test2);
  out2.close();

edited Nov 26 '21 at 02:31

answered Nov 26 '21 at 01:59

Scary Wombat

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Thanks, but do I still keep the intial ObjectOutputStream? – WardenHelmet Nov 26 '21 at 02:45
You do not need to if you run the program multiple times. The Objects will be appended. – Scary Wombat Nov 26 '21 at 02:50
Thanks, it works. The only change I made is that when the test.ser file is empty, it must be changed to the original ObjectOutputStream or I will get a StreamCorruptedException when attempting to read. – WardenHelmet Nov 26 '21 at 03:44

Appending to a Serialization file (Java)

1 Answers1