我已经设置了同步．这样的程序为什么还会重复插入!

public class SQLMap {

	private static final String resource = "SqlMap.xml";
	private static SqlMapClient sqlMap = null;
	private static final Logger logger = Logger.getLogger(SQLMap.class);
	private static SQLMap sql = null;

	private SQLMap() {
	}

	public static SqlMapClient getSQLMap() throws SQLMapException {
		return sqlMap;
	}
         //设置同步
	public static synchronized SQLMap getInstance() throws SQLMapException {
		if (sqlMap == null) {
			Reader reader = null;
			try {
				reader = Resources.getResourceAsReader(resource);
				sqlMap = SqlMapClientBuilder.buildSqlMapClient(reader);
			} catch (Exception e) {
				logger.error("Create SQLMap Instance is error:" + e.getMessage());
				throw new SQLMapException(e.getMessage());
			}
		}
		if (sql == null)
			sql = new SQLMap();

		return sql;
	}
	

	public static void start() throws SQLMapException {
		try {
			if (sqlMap == null)
				getInstance();
			sqlMap.startTransaction();
		} catch (Exception e) {
			logger.error("start Transaction is error:" + e.getMessage());
			throw new SQLMapException(e.getMessage());
		}

	}

	public static void end() {
		try {
			if (sqlMap != null)
				sqlMap.endTransaction();
		} catch (Exception e) {
			logger.error("end Transaction is error:" + e.getMessage());
		}
	}


	public static Object insert(String id, Object obj) throws SQLMapException {
		try {
			if (sqlMap != null)
				return sqlMap.insert(id, obj);
		} catch (Exception e) {
			logger.error("insert(String id, Object obj) is error:" + id + ":"
					+ e.getMessage());
			throw new SQLMapException(e.getMessage());
		}
		return null;
	}

}

（１）我做的自定义表存入sequence

public final class SequenceUtil {
	
	
	/**
	 * Get next sequence
	 * @param tableName
	 * @return sequence
	 * @throws SQLException 
	 */
	@SuppressWarnings("unchecked")
	public static synchronized long nextVal(String tableName) throws SQLException{
		if(tableName==null || tableName.equals(""))
			throw new SQLException("The name of table is not null!");
		int val = 0;
		try{
			Map map = new HashMap();
			map.put("name", tableName);
			Object obj = SQLMap.queryForObject("tb_sequences.query", map);
			
			if(obj==null){
				map.put("val", 1);
				SQLMap.insert("tb_sequences.insert", map);
			}else{
				SQLMap.update("tb_sequences.update",map);
				val = (Integer)obj;
			}
			SQLMap.commit();
		}catch(SQLException e){
			throw new SQLException(e.getMessage());
		}
		return (val+1);
	}
}

(3)客户端在多线程调用，会重得插入ID．不能插入唯一SequenceUtil.nextVal("test")

public static void main(String[] args){
		try{
			SQLMap.start();
			int i=0;
			while(true& i<10){
				Map map = new HashMap();
				map.put("name","feng");
				map.put("id",SequenceUtil.nextVal("test"));
				SQLMap.insert("test.insert", map);
				SQLMap.commit();
				i++;
			}

		}catch(SQLException e){
			e.printStackTrace();
		}finally{
			SQLMap.end();
		}

	}