How Can I Append to an Existing ObjectOutputStream File Without Overwriting Data?

ObjectOutputStream Appendation: A Solution with Subclassing

Appending to an ObjectOutputStream can be a problematic task. As you discovered, the default behavior of ObjectOutputStream when writing to an existing file is to overwrite it, leading to data loss.

To resolve this issue, the key is to override the behavior of ObjectOutputStream. Specifically, by overriding the writeStreamHeader method, we can prevent the header from being written when appending to an existing file. Consider the following code example:

public class AppendingObjectOutputStream extends ObjectOutputStream {

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

  @Override
  protected void writeStreamHeader() throws IOException {
    // Do not write a header
    reset(); // Added to fix potential issues with previous implementation
  }
}

The AppendingObjectOutputStream subclass inherits from the original ObjectOutputStream class and overrides the writeStreamHeader method. Within this overridden method, we do not write a header but instead reset the stream. This effectively allows us to append data to an existing file without overwriting it.

To use this technique, you can check whether the history file exists or not. If the file exists, instantiate the AppendingObjectOutputStream; otherwise, use the regular ObjectOutputStream. Here's an updated version of your code using this approach:

OutputStream out;
if (historyFile.exists()) {
  out = new AppendingObjectOutputStream(new FileOutputStream(historyFile, true));
} else {
  out = new ObjectOutputStream(new FileOutputStream(historyFile));
}
out.writeObject(new Stuff(stuff));
out.close();

By using the AppendingObjectOutputStream class when appending to an existing file, you can preserve the existing data while adding new objects. This technique allows you to create a persistent data store that can grow as needed without the need for overwriting or data loss.

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