How can I append to an existing java.io.ObjectStream? [duplicate]

Question

As for now I will get java.io.StreamCorruptedException when I try to append an Object. I have searched the Internet for a way to overcome that. The answer I found so far is it can't be done. A way around this problem is to write the objects into a list and then write the list to the file.

But I have to overwrite that file everytime when I add new objects. It seems not to be the optimal solution in overtime.

Is there a way to append objects to an existing object stream?

Answer 1

Many thanks to George Hategan for the problem exposing code. I examined it for a while too. Then, it hit me. If you're using a sub-classed ObjectOutputStream with an override of the writeStreamHeader() method to write data, you must use the parallel sub-classed ObjectInputStream with an override of the readStreamHeader() method to read the data. Of course, we can zig-zag between different implementations of writing and reading objects, but as long as we use the corresponding pairs of sub-classes in the write/read process - we'll be (hopefully) fine. Tom.

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.util.HashMap;
import java.util.Map;

public class SerializationDemo {

    public static void main(String[] args) throws Exception {
        File storageFile = new File("test.ser");
        storageFile.delete();
        write(storageFile, getO1());
        write(storageFile, getO2());
        write(storageFile, getO2());
        FileInputStream fis = new FileInputStream(storageFile);
        read(fis, getO1());
        read(fis, getO2());
        read(fis, getO2());
        fis.close();
    }

    private static void write(File storageFile, Map<String, String> o)
                    throws IOException {
        ObjectOutputStream oos = getOOS(storageFile);
        oos.writeObject(o);
        oos.flush();
        oos.close();
    }

    private static void read(FileInputStream fis, Map<String, String> expected)
                    throws ClassNotFoundException, IOException {
        Object actual = getOIS(fis).readObject();
        assertEquals(expected, actual);
        System.out.println("read serialized " + actual);
    }

    private static void assertEquals(Object o1, Object o2) {
        if (!o1.equals(o2)) {
            throw new AssertionError("\n expected: " + o1 + "\n actual:   " + o2);
        }
    }

    private static Map<String, String> getO1() {
        Map<String, String> nvps = new HashMap<String, String>();
        nvps.put("timestamp", "1326382770000");
        nvps.put("length", "246");
        return nvps;
    }

    private static Map<String, String> getO2() {
        Map<String, String> nvps = new HashMap<String, String>();
        nvps.put("timestamp", "0");
        nvps.put("length", "0");
        return nvps;
    }

    private static ObjectOutputStream getOOS(File storageFile)
                    throws IOException {
        if (storageFile.exists()) {
            // this is a workaround so that we can append objects to an existing file
            return new AppendableObjectOutputStream(new FileOutputStream(storageFile, true));
        } else {
            return new ObjectOutputStream(new FileOutputStream(storageFile));
        }
    }

    private static ObjectInputStream getOIS(FileInputStream fis)
                    throws IOException {
        long pos = fis.getChannel().position();
        return pos == 0 ? new ObjectInputStream(fis) : 
            new AppendableObjectInputStream(fis);
    }

    private static class AppendableObjectOutputStream extends
                    ObjectOutputStream {

        public AppendableObjectOutputStream(OutputStream out)
                        throws IOException {
            super(out);
        }

        @Override
        protected void writeStreamHeader() throws IOException {
            // do not write a header
        }
    }

    private static class AppendableObjectInputStream extends ObjectInputStream {

        public AppendableObjectInputStream(InputStream in) throws IOException {
            super(in);
        }

        @Override
        protected void readStreamHeader() throws IOException {
            // do not read a header
        }
    }
}