How Can I Append Objects to an Existing Object File in Java?

Appending to an Existing Object File

In Java, ObjectOutputStream allows you to serialize and write objects to a file. However, by default, this stream doesn't support appending new objects to an existing file. This topic explores a solution to append data to an existing ObjectOutputStream.

To append to an existing file, the writeStreamHeader() method of ObjectOutputStream needs to be overridden to avoid writing a header. Here's a custom AppendingObjectOutputStream class that does this:

public class AppendingObjectOutputStream extends ObjectOutputStream {

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

  @Override
  protected void writeStreamHeader() throws IOException {
    // do not write a header, but reset:
    reset();
  }

}

To use this class for appending objects to an existing file, check if the file exists before creating the stream:

FileOutputStream fos = null;
ObjectOutputStream out = null;

File file = new File(preferences.getAppDataLocation() + "history");

if (file.exists()) {
  // Append to existing file
  fos = new FileOutputStream(file, true);
  out = new AppendingObjectOutputStream(fos);
} else {
  // Create new file with header
  fos = new FileOutputStream(file);
  out = new ObjectOutputStream(fos);
}

// Append objects to the file
out.writeObject(new Stuff(stuff));
out.close();

For reading the appended objects, use an ObjectInputStream as usual. The StreamCorruptedException exception would no longer occur when reading objects from the file.

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