jdbcTemplate里的StatementCallback等实现类里为什么要实现SqlProvider接口？

代码片断一

public <T> T query(final String sql, final ResultSetExtractor<T> rse) throws DataAccessException {
        Assert.notNull(sql, "SQL must not be null");
        Assert.notNull(rse, "ResultSetExtractor must not be null");
        if (logger.isDebugEnabled()) {
            logger.debug("Executing SQL query [" + sql + "]");
        }
        class QueryStatementCallback implements StatementCallback<T>, SqlProvider {
            public T doInStatement(Statement stmt) throws SQLException {
                ResultSet rs = null;
                try {
                    rs = stmt.executeQuery(sql);
                    ResultSet rsToUse = rs;
                    if (nativeJdbcExtractor != null) {
                        rsToUse = nativeJdbcExtractor.getNativeResultSet(rs);
                    }
                    return rse.extractData(rsToUse);
                }
                finally {
                    JdbcUtils.closeResultSet(rs);
                }
            }
            public String getSql() {
                return sql;
            }
        }
        return execute(new QueryStatementCallback());
    }
代码片断二

public <T> T execute(StatementCallback<T> action) throws DataAccessException {
        Assert.notNull(action, "Callback object must not be null");
 
        Connection con = DataSourceUtils.getConnection(getDataSource());
        Statement stmt = null;
        try {
            Connection conToUse = con;
            if (this.nativeJdbcExtractor != null &&
                    this.nativeJdbcExtractor.isNativeConnectionNecessaryForNativeStatements()) {
                conToUse = this.nativeJdbcExtractor.getNativeConnection(con);
            }
            stmt = conToUse.createStatement();
            applyStatementSettings(stmt);
            Statement stmtToUse = stmt;
            if (this.nativeJdbcExtractor != null) {
                stmtToUse = this.nativeJdbcExtractor.getNativeStatement(stmt);
            }
            T result = action.doInStatement(stmtToUse);
            handleWarnings(stmt);
            return result;
        }
        catch (SQLException ex) {
            // Release Connection early, to avoid potential connection pool deadlock
            // in the case when the exception translator hasn't been initialized yet.
            JdbcUtils.closeStatement(stmt);
            stmt = null;
            DataSourceUtils.releaseConnection(con, getDataSource());
            con = null;
            throw getExceptionTranslator().translate("StatementCallback", getSql(action), ex);
        }
        finally {
            JdbcUtils.closeStatement(stmt);
            DataSourceUtils.releaseConnection(con, getDataSource());
        }
    }
 

代码片断三

/**
     * Determine SQL from potential provider object.
     * @param sqlProvider object that's potentially a SqlProvider
     * @return the SQL string, or <code>null</code>
     * @see SqlProvider
     */
    private static String getSql(Object sqlProvider) {
        if (sqlProvider instanceof SqlProvider) {
            return ((SqlProvider) sqlProvider).getSql();
        }
        else {
            return null;
        }
    }
 

代码片断四

package org.springframework.jdbc.core;
 
/**
 * Interface to be implemented by objects that can provide SQL strings.
 *
 * <p>Typically implemented by PreparedStatementCreators, CallableStatementCreators
 * and StatementCallbacks that want to expose the SQL they use to create their
 * statements, to allow for better contextual information in case of exceptions.
 *
 * @author Juergen Hoeller
 * @since 16.03.2004
 * @see PreparedStatementCreator
 * @see CallableStatementCreator
 * @see StatementCallback
 */
public interface SqlProvider {
 
    /**
     * Return the SQL string for this object, i.e.
     * typically the SQL used for creating statements.
     * @return the SQL string, or <code>null</code>
     */
    String getSql();
 
}
 

执行代码片断一的query方法execute(newQueryStatementCallback())会调用代码片断二的方法。execute方法的异常处理中有throw getExceptionTranslator().translate("StatementCallback", getSql(action), ex);此处getSql(action)就需要使用SqlProvider里的getSql()。getSql(action)方法见代码片断三。
接口SqlProvider定义见代码片断四。SqlProvider接口注释中写道：
Typically implemented by PreparedStatementCreators, CallableStatementCreators and StatementCallbacks that want to expose the SQL they use to create their statements, to allow for better contextual information in case of exceptions.
此处QueryStatementCallback实现SqlProvider接口的作用是，如果出现异常，将告知当前执行的sql。这样就很巧妙地通过回调接口来获取相关数据，供调用者使用。这也是模板方法(Template method)模式的应用。
由此可见，spring的代码实现非常精妙。我们通过多阅读，多学习，多研究，并在软件实践中加以运用，一定会提高自身的编程能力。