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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 <code>null</code> * @throws DataAccessException in case of SQL Maps errors */ public <T> T execute(SqlMapClientCallback<T> 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> T execute(SqlMapClientCallback<T> 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<Object>() { public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException { return executor.queryForObject(statementName, parameterObject); } }); } public List queryForList(final String statementName, final Object parameterObject) throws DataAccessException { return execute(new SqlMapClientCallback<List>() { public List doInSqlMapClient(SqlMapExecutor executor) throws SQLException { return executor.queryForList(statementName, parameterObject); } }); } public Map queryForMap( final String statementName, final Object parameterObject, final String keyProperty) throws DataAccessException { return execute(new SqlMapClientCallback<Map>() { public Map doInSqlMapClient(SqlMapExecutor executor) throws SQLException { return executor.queryForMap(statementName, parameterObject, keyProperty); } }); }
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> T execute(ConnectionCallback<T> 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()); } }
Spring中Template模式与callback的结合使用浅析
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原文地址：http://www.cnblogs.com/digdeep/p/4515008.html
