java 回调 spring jdbc 查询模板
spring jdbc 查询模板内部类的使用感觉这种写法让代码显得很清晰，将固定的操作步骤制定成模板方法，通过使用者，自己来实现继承类来决定操作过程。
org.springframework.jdbc.core.RowMapper
public interface RowMapper<T> { T mapRow(ResultSet rs, int rowNum) throws SQLException; }
org.springframework.jdbc.core.RowMapperResultSetExtractor
public class RowMapperResultSetExtractor<T> implements ResultSetExtractor<List<T>> {public List<T> extractData(ResultSet rs) throws SQLException {List<T> results = (this.rowsExpected > 0 ? new ArrayList<T>(this.rowsExpected) : new ArrayList<T>());int rowNum = 0;while (rs.next()) {results.add(this.rowMapper.mapRow(rs, rowNum++));}return results;}}
以下是模板类的操作过程：
org.springframework.jdbc.core.JdbcTemplate
public <T> List<T> query(String sql, RowMapper<T> rowMapper) throws DataAccessException {return query(sql, new RowMapperResultSetExtractor<T>(rowMapper));}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());}//模板方法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());}}
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