Untitled

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();
        }

}