Spring不论是与ibatis，还是与Hibernate的结合中，都使用到了Template模式与callback技术，来达到简化代码实现的目的。Template模式也即模板模式，用于对一些不太变化的流程进行模板化，与callback结合，可以将变化的部分出离出来，使用callback实现。然后根据不同的情况，向template注入不同的callback。那些模板代码就没有必要重复写了。我们看下spring和ibatis的结合中，Template和callback的使用：

public class SqlMapClientTemplate extends JdbcAccessor implements SqlMapClientOperations {    // ... ...    /**     * Execute the given data access action on a SqlMapExecutor.     * @param action callback object that specifies the data access action     * @return a result object returned by the action, or null     * @throws DataAccessException in case of SQL Maps errors     */    public 
     
       T execute(SqlMapClientCallback
      
        action) throws DataAccessException {        Assert.notNull(action, "Callback object must not be null");        Assert.notNull(this.sqlMapClient, "No SqlMapClient specified");        // We always need to use a SqlMapSession, as we need to pass a Spring-managed        // Connection (potentially transactional) in. This shouldn't be necessary if        // we run against a TransactionAwareDataSourceProxy underneath, but unfortunately        // we still need it to make iBATIS batch execution work properly: If iBATIS        // doesn't recognize an existing transaction, it automatically executes the        // batch for every single statement...        SqlMapSession session = this.sqlMapClient.openSession();        if (logger.isDebugEnabled()) {            logger.debug("Opened SqlMapSession [" + session + "] for iBATIS operation");        }        Connection ibatisCon = null;        try {            Connection springCon = null;            DataSource dataSource = getDataSource();            boolean transactionAware = (dataSource instanceof TransactionAwareDataSourceProxy);            // Obtain JDBC Connection to operate on...            try {                ibatisCon = session.getCurrentConnection();                if (ibatisCon == null) {                    springCon = (transactionAware ?                            dataSource.getConnection() : DataSourceUtils.doGetConnection(dataSource));                    session.setUserConnection(springCon);                    if (logger.isDebugEnabled()) {                        logger.debug("Obtained JDBC Connection [" + springCon + "] for iBATIS operation");                    }                }                else {                    if (logger.isDebugEnabled()) {                        logger.debug("Reusing JDBC Connection [" + ibatisCon + "] for iBATIS operation");                    }                }            }            catch (SQLException ex) {                throw new CannotGetJdbcConnectionException("Could not get JDBC Connection", ex);            }            // Execute given callback...            try {                return action.doInSqlMapClient(session);            }            catch (SQLException ex) {                throw getExceptionTranslator().translate("SqlMapClient operation", null, ex);            }            finally {                try {                    if (springCon != null) {                        if (transactionAware) {                            springCon.close();                        }                        else {                            DataSourceUtils.doReleaseConnection(springCon, dataSource);                        }                    }                }                catch (Throwable ex) {                    logger.debug("Could not close JDBC Connection", ex);                }            }            // Processing finished - potentially session still to be closed.        }        finally {            // Only close SqlMapSession if we know we've actually opened it            // at the present level.            if (ibatisCon == null) {                session.close();            }        }    }
      
     

public 
    
      T execute(SqlMapClientCallback
     
       action) throws DataAccessException 该方法就是一个模板方法，方法的参数是一个回调对象。在模板方法中，将一些相同的处理过程模板化，比如获得数据库连接，处理事务，处理异常，关闭资源等等， 这些都是每一个sql执行时都要面临的相同的过程，所以我们将他们房子模板方法中。然后将不同的部分通过 callback 对象作为参数传入进去，这样将模板代码和非 模板代码进行了隔离。没有必要将模板代码每次都写一遍。
     
    

 

public Object queryForObject(final String statementName, final Object parameterObject)            throws DataAccessException {        return execute(new SqlMapClientCallback

 

我们看一下上面这些方法，都是借助模板方法来处理那些每次都相同的流程，然后传入一个自己实现的 callback 对象。模板会自动回调我们在 callback 对象中定义的方法。

我们看下JdbcTemplate中的模板方法也是相似的：

public 
     
       T execute(ConnectionCallback
      
        action) throws DataAccessException {        Assert.notNull(action, "Callback object must not be null");        Connection con = DataSourceUtils.getConnection(getDataSource());        try {            Connection conToUse = con;            if (this.nativeJdbcExtractor != null) {                // Extract native JDBC Connection, castable to OracleConnection or the like.                conToUse = this.nativeJdbcExtractor.getNativeConnection(con);            }            else {                // Create close-suppressing Connection proxy, also preparing returned Statements.                conToUse = createConnectionProxy(con);            }            return action.doInConnection(conToUse);        }        catch (SQLException ex) {            // Release Connection early, to avoid potential connection pool deadlock            // in the case when the exception translator hasn't been initialized yet.            DataSourceUtils.releaseConnection(con, getDataSource());            con = null;            throw getExceptionTranslator().translate("ConnectionCallback", getSql(action), ex);        }        finally {            DataSourceUtils.releaseConnection(con, getDataSource());        }    }