PrevaylerJr - "To serve the persistence needs of 99% of information systems in the world using 23 semicolons."

gistfile1.java

import java.io.*;


public class PrevaylerJr {

	public static interface Command extends Serializable {
		Object executeOn(Object system);
	}

	
	private final Object _system;
	private final ObjectOutputStream _journal;


	public PrevaylerJr(Serializable initialState, File storageFile) throws Exception {
		File tempFile = new File(storageFile.getAbsolutePath() + ".tmp");
		_system = restoreState(initialState, storageFile, tempFile);
		_journal = new ObjectOutputStream(new FileOutputStream(tempFile));
		writeToJournal(_system);
		
		if (storageFile.delete() && tempFile.renameTo(storageFile))
			return;
		
		throw new IOException("Unable to rename " + tempFile + " to " + storageFile);
	}


	synchronized
	public Object executeTransaction(Command transaction) throws Exception {
		writeToJournal(transaction);
		return transaction.executeOn(_system);
	}


	synchronized
	public Object executeQuery(Command query) {
		return query.executeOn(_system);
	}


	private Object restoreState(Object initialState, File storageFile, File tempFile) {
		Object state = initialState;
		try {
			File fileToRead = storageFile.exists() ? storageFile : tempFile;
			ObjectInputStream input = new ObjectInputStream(new FileInputStream(fileToRead));
			
			state = restoreImage(input);
			restoreCommands(state, input);
			
		} catch (Exception endOfStreamReached) {}
		
		return state;
	}


	private Serializable restoreImage(ObjectInputStream input) throws IOException, ClassNotFoundException {
		return (Serializable) input.readObject();
	}


	private void restoreCommands(Object state, ObjectInputStream input) throws IOException, ClassNotFoundException {
		while (true)
			((Command) input.readObject()).executeOn(state);
	}


	private void writeToJournal(Object object) throws IOException {
		_journal.writeObject(object);
		_journal.flush();
	}

}

Applying transactions to the system before they are written will cause queries to see unwritten changes. So, unless you sequence all queries with disk writes, you lose the D in ACID.
Recovering the system after a transaction exception, even if done with the RoyalFoodTaster pattern, is a huge hiccup in the system. If there is a silly transaction throwing a frequent but unimportant exception, sysadmins will hate you for this feature.
Throwing an exception is just one particular kind of bug a transaction can cause. All other bugs will not be caught, just as they won't be caught in any DBMS.

Is there a reason to use Object instead of Serializable on lines 67,41 and 11?