logrecorditerator.java

用java语言简单实现数据库的初步功能

package simpledb.tx.recovery;

import static simpledb.tx.recovery.LogRecord.*;
import java.util.Iterator;
import simpledb.log.BasicLogRecord;
import simpledb.server.SimpleDB;

/**
 * A class that provides the ability to read records
 * from the log in reverse order.
 * Unlike the similar class simpledb.buffer.LogIterator, this class 
 * understands the meaning of the log records.
 * @author Edward Sciore
 */
class LogRecordIterator implements Iterator<LogRecord> {
	private Iterator<BasicLogRecord> iter = SimpleDB.logMgr().iterator();

	public boolean hasNext() {
		return iter.hasNext();
	}

	/**
	 * Constructs a log record from the values in the current log record.
	 * The method first reads an integer, which denotes
	 * the type of the log record.  Based on that type,
	 * the method calls the appropriate LogRecord constructor
	 * to read the remaining values.
	 * @return the next log record, or null if no more records
	 */
	public LogRecord next() {
		BasicLogRecord rec = iter.next();
		int op = rec.nextInt();
		switch (op) {
			case CHECKPOINT:
				return new CheckpointRecord(rec);
			case START:
				return new StartRecord(rec);
			case COMMIT:
				return new CommitRecord(rec);
			case ROLLBACK:
				return new RollbackRecord(rec);
			case SETINT:
				return new SetIntRecord(rec);
			case SETSTRING:
				return new SetStringRecord(rec);
			default:
				return null;
		}
	}	
	
	public void remove() {
		throw new UnsupportedOperationException();
	}
}