How Can I Append Objects to an Existing ObjectOutputStream?

Appending Objects to ObjectOutputStream

While the ObjectOutputStream provides a convenient way to serialize objects, it does not support appending to an existing stream. A common scenario where this limitation arises is when trying to maintain a list of objects in a file, with new objects being added as they are created.

To overcome this challenge, a custom solution is required. One approach is to subclass ObjectOutputStream and override the writeStreamHeader method. This method is responsible for writing the stream header, which contains information about the object types being serialized. By overriding this method, we can create a stream that does not write a header, allowing us to append objects to an existing file.

Here's an example of the AppendingObjectOutputStream class:

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, check if the history file exists. If it does, instantiate the AppendingObjectOutputStream to append objects. Otherwise, instantiate the standard ObjectOutputStream to create a new file with a header.

By overriding the writeStreamHeader method, the AppendingObjectOutputStream allows us to append objects to an existing file, providing a way to maintain a persistent list of objects without overwriting existing data.

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