// 是否允許開啟定時任務(wù)檢查更新元數(shù)據(jù)
if (ENABLE_TABLE_META_CHECKER_ENABLE) {
    // 開啟定時任務(wù)，默認(rèn)一分鐘更新檢查一下
    tableMetaExecutor.scheduleAtFixedRate(() -> {
        // 獲取數(shù)據(jù)庫鏈接
        try (Connection connection = dataSource.getConnection()) {
             // 更新緩存中的數(shù)據(jù)表元數(shù)據(jù)
             TableMetaCacheFactory.getTableMetaCache(DataSourceProxy.this.getDbType())
                        .refresh(connection, DataSourceProxy.this.getResourceId());
        } catch (Exception ignore) {}
   }, 0, TABLE_META_CHECKER_INTERVAL, TimeUnit.MILLISECONDS);
}

public abstract class AbstractTableMetaCache implements TableMetaCache {
    @Override
    public TableMeta getTableMeta(final Connection connection, final String tableName, String resourceId) {
        // 如果緩存中有對應(yīng)數(shù)據(jù)就返回，否則就去查詢元數(shù)據(jù)并放在緩存中。
    }
    @Override
    public void refresh(final Connection connection, String resourceId) {
        // 更新緩存
    }
}

@Override
    protected TableMeta fetchSchema(Connection connection, String tableName) throws SQLException {
        String sql = "SELECT * FROM " + ColumnUtils.addEscape(tableName, JdbcConstants.MYSQL) + " LIMIT 1";
        try (Statement stmt = connection.createStatement();
            // 執(zhí)行SQL語句：SELECT * FROM [tableName] LIMIT 1；
            ResultSet rs = stmt.executeQuery(sql)) {
            // 根據(jù)執(zhí)行結(jié)果獲取元數(shù)據(jù)
            return resultSetMetaToSchema(rs.getMetaData(), connection.getMetaData());
        } catch (SQLException sqlEx) {
            throw sqlEx;
        } catch (Exception e) {
            throw new SQLException(String.format("Failed to fetch schema of %s", tableName), e);
        }
    }

private TableMeta resultSetMetaToSchema(ResultSetMetaData rsmd, DatabaseMetaData dbmd)
        throws SQLException {
        //always "" for mysql
        String schemaName = rsmd.getSchemaName(1);
        String catalogName = rsmd.getCatalogName(1);
        /*
         * 通過ResultSetMetaData獲取tableName可以避免以下情況
         *
         * select * from account_tbl
         * select * from account_TBL
         * select * from `account_tbl`
         * select * from account.account_tbl
         */
        String tableName = rsmd.getTableName(1);
        TableMeta tm = new TableMeta();
        tm.setTableName(tableName);
        /*
         * here has two different type to get the data
         * make sure the table name was right
         * 1. show full columns from xxx from xxx(normal)
         * 2. select xxx from xxx where catalog_name like ? and table_name like ?(informationSchema=true)
         */
        // 通過dbmd發(fā)送查詢語句獲取指定表中的所有列信息
        try (ResultSet rsColumns = dbmd.getColumns(catalogName, schemaName, tableName, "%");
             // 發(fā)送查詢語句獲取表中索引信息
             ResultSet rsIndex = dbmd.getIndexInfo(catalogName, schemaName, tableName, false, true);
             // 查詢更新行中的任何值時自動更新的列的信息
             ResultSet onUpdateColumns = dbmd.getVersionColumns(catalogName, schemaName, tableName)) {
            // 收集列信息
            while (rsColumns.next()) {
                ColumnMeta col = new ColumnMeta();
                col.setTableCat(rsColumns.getString("TABLE_CAT"));
                col.setTableSchemaName(rsColumns.getString("TABLE_SCHEM"));
                col.setTableName(rsColumns.getString("TABLE_NAME"));
                col.setColumnName(rsColumns.getString("COLUMN_NAME"));
                col.setDataType(rsColumns.getInt("DATA_TYPE"));
                col.setDataTypeName(rsColumns.getString("TYPE_NAME"));
                col.setColumnSize(rsColumns.getInt("COLUMN_SIZE"));
                col.setDecimalDigits(rsColumns.getInt("DECIMAL_DIGITS"));
                col.setNumPrecRadix(rsColumns.getInt("NUM_PREC_RADIX"));
                col.setNullAble(rsColumns.getInt("NULLABLE"));
                col.setRemarks(rsColumns.getString("REMARKS"));
                col.setColumnDef(rsColumns.getString("COLUMN_DEF"));
                col.setSqlDataType(rsColumns.getInt("SQL_DATA_TYPE"));
                col.setSqlDatetimeSub(rsColumns.getInt("SQL_DATETIME_SUB"));
                col.setCharOctetLength(rsColumns.getInt("CHAR_OCTET_LENGTH"));
                col.setOrdinalPosition(rsColumns.getInt("ORDINAL_POSITION"));
                col.setIsNullAble(rsColumns.getString("IS_NULLABLE"));
                col.setIsAutoincrement(rsColumns.getString("IS_AUTOINCREMENT"));
                if (tm.getAllColumns().containsKey(col.getColumnName())) {
                    throw new NotSupportYetException("Not support the table has the same column name with different case yet");
                }
                tm.getAllColumns().put(col.getColumnName(), col);
            }
            while (onUpdateColumns.next()) {
                tm.getAllColumns().get(onUpdateColumns.getString("COLUMN_NAME")).setOnUpdate(true);
            }
            // 收集索引信息
            while (rsIndex.next()) {
                String indexName = rsIndex.getString("INDEX_NAME");
                String colName = rsIndex.getString("COLUMN_NAME");
                ColumnMeta col = tm.getAllColumns().get(colName);
                if (tm.getAllIndexes().containsKey(indexName)) {
                    IndexMeta index = tm.getAllIndexes().get(indexName);
                    index.getValues().add(col);
                } else {
                    IndexMeta index = new IndexMeta();
                    index.setIndexName(indexName);
                    index.setNonUnique(rsIndex.getBoolean("NON_UNIQUE"));
                    index.setIndexQualifier(rsIndex.getString("INDEX_QUALIFIER"));
                    index.setIndexName(rsIndex.getString("INDEX_NAME"));
                    index.setType(rsIndex.getShort("TYPE"));
                    index.setOrdinalPosition(rsIndex.getShort("ORDINAL_POSITION"));
                    index.setAscOrDesc(rsIndex.getString("ASC_OR_DESC"));
                    index.setCardinality(rsIndex.getInt("CARDINALITY"));
                    index.getValues().add(col);
                    if ("PRIMARY".equalsIgnoreCase(indexName)) {
                        index.setIndextype(IndexType.PRIMARY);
                    } else if (!index.isNonUnique()) {
                        index.setIndextype(IndexType.UNIQUE);
                    } else {
                        index.setIndextype(IndexType.NORMAL);
                    }
                    tm.getAllIndexes().put(indexName, index);
                }
            }
            if (tm.getAllIndexes().isEmpty()) {
                throw new ShouldNeverHappenException("Could not found any index in the table: " + tableName);
            }
        }
        return tm;
    }