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());
}
}