Appending Objects to an existing file

Question

I'm trying to create a system where users can save a series of Message objects to a .txt file, which can then be de-serialized and read by the recipient of the Message. However each time I try and write to the file I simply overwrite the object that was there previously.

My current code is as follows:

public class Comms{
    Message msg;
    public void sendMessage(Message myMessage){
        msg = myMessage;

        //Generate Message ID       
        String msgID = "MSG" + genMsgID();
        myMessage.setMessageID(msgID);

        //********************************************************
        try
        {
            FileOutputStream logFile = new FileOutputStream("logFile.txt");
            NoHeaderObjectOutputStream out = new NoHeaderObjectOutputStream(logFile);
            out.writeObject(myMessage);
            out.close();
            logFile.close();
        }catch(IOException i)
        {
        }
    }
}

class NoHeaderObjectOutputStream extends ObjectOutputStream {
    public NoHeaderObjectOutputStream(OutputStream os) throws IOException {
        super(os);
    }
    protected void writeStreamHeader() {}
}

I appreciate that there are similar questions out there but with my limited knowledge I'm struggling to make sense of the answers provided.

If anyone has a solution to this or some pointers that'd be greatly appreciated.

Don't think on object serialization like an easier way to store things like a database. The whole idea of serialization is to just persist everything before shutdown and restore everything on startup. It's not like a table where you can add and remove things. That said, please take a look on http://stackoverflow.com/questions/2094637/how-can-i-append-to-an-existing-java-io-objectstream — Leo, May 16 '14 at 01:41
Thanks for your response. So I've implemented the suggested class, but I'm still overwriting the entire file each time I write to the file, updated code above. — user3352349, May 16 '14 at 02:03

Mark W · Accepted Answer · 2014-05-16T02:37:32.253

You can store your objects in a collection type which is serializable. Like one of the many List implementations such as ArrayList. Then you serialize the collection. Deserializing the collection back into a List, and adding and removing as you see fit.

Or alternatively, you could store each object in its own file.

Here is a quick example:

package arg;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.ArrayList;

public class Serialization {
    private static final String FILE_NAME = "test.dat";
    private static ArrayList<Name> names;

    public static void main(String[] args) {
        Serialization s = new Serialization();
        s.start();
    }

    private void start() {
        loadFile();
        promptForName();
    }


    private void promptForName() {
        String input = "";
        try (BufferedReader br = new BufferedReader(new InputStreamReader(System.in))) {
            while (!input.equals("Exit")) {
                System.out.println("Press 1 to enter new name.\r\nPress 2 to save.\r\nPress 3 to display.\r\nType Exit to exit.");
                input = br.readLine();
                if (input.equals("1")) {
                    System.out.println("Enter a name");
                    input = br.readLine();
                    Name n = new Name();                    
                    n.name = input;
                    names.add(n);
                }
                else if (input.equals("2")){
                    saveFile();
                }
                else if (input.equals("3")){
                    for (Name n : names) {
                        System.out.println(n.name);
                    }
                }               
            }
        } catch (IOException e) {
            e.printStackTrace();
        }       
    }

    private void saveFile() {
        File f = new File(FILE_NAME);
        f.delete();
        try {
            f.createNewFile();
        } catch (IOException e1) {
            e1.printStackTrace();
        }
        try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(f))){
            oos.writeObject(names);
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    @SuppressWarnings("unchecked")
    private void loadFile() {
        File f = new File(FILE_NAME);
        if (f.exists()) {
            try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream(f)))
            {
                names = (ArrayList<Name>) ois.readObject();
            } catch (IOException e) {
                f.delete();
                e.printStackTrace();
            } catch (ClassNotFoundException e) {                
                e.printStackTrace();
                f.delete();
            }
        }
        else
        {
            names = new ArrayList<Name>();
        }
    }

    private static class Name implements Serializable {
        public String name;
    }

}

Edit - Your code sorta came after the answer. If this is for logging purposes you should use log4j or some similar ready made logging API.

I went with your list suggestion, seems so obvious now, much appreciated. — user3352349, May 16 '14 at 02:24

score 2 · Answer 2 · answered May 16 '14 at 02:22

2

Look at http://docs.oracle.com/javase/7/docs/api/java/io/FileOutputStream.html#FileOutputStream(java.lang.String, boolean)

and you will see that the FileOutputStream constructor has an append option.

answered May 16 '14 at 02:22

Scary Wombat

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Appending Objects to an existing file

2 Answers2