ResultSet占用内存资源特别高

使用JDBC向mysql发送查询时，有三种方式：

常规查询：JDBC驱动会阻塞的一次性读取全部查询的数据到 JVM 内存中，或者分页读取

流式查询：每次执行rs.next时会判断数据是否需要从mysql服务器获取，

    如果需要触发读取一批数据（可能n行）加载到 JVM 内存进行业务处理

游标查询：通过 fetchSize 参数，控制每次从mysql服务器一次读取多少行数据。

游标查询例子：

public static void cursorQuery() throws Exception {
    Connection connection = DriverManager.getConnection
    ("jdbc:mysql://localhost:3307/test?useSSL=false&useCursorFetch=true", "root", "123456");
    ((JDBC4Connection) connection).setUseCursorFetch(true); //com.mysql.jdbc.JDBC4Connection
    Statement statement = connection.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);    
    statement.setFetchSize(2000);    
    ResultSet rs = statement.executeQuery(sql);    
    while (rs.next()) {
        System.out.println(rs.getString(2));
        Thread.sleep(5000);
    }
    
    rs.close();
    statement.close();
    connection.close();
}

驱动mysql-connector-java-5.1.43-sources.jar!/com/mysql/jdbc/MysqlIO.java源码分析：

  protected ResultSetImpl getResultSet(StatementImpl callingStatement, long columnCount, int maxRows, int resultSetType, int resultSetConcurrency,
            boolean streamResults, String catalog, boolean isBinaryEncoded, Field[] metadataFromCache) throws SQLException {
        Buffer packet; // The packet from the server
        Field[] fields = null;

        // Read in the column information

        if (metadataFromCache == null /* we want the metadata from the server */) {
            fields = new Field[(int) columnCount];

            for (int i = 0; i < columnCount; i++) {
                Buffer fieldPacket = null;

                fieldPacket = readPacket();
                fields[i] = unpackField(fieldPacket, false);
            }
        } else {
            for (int i = 0; i < columnCount; i++) {
                skipPacket();
            }
        }

        // There is no EOF packet after fields when CLIENT_DEPRECATE_EOF is set
        if (!isEOFDeprecated() ||
        // if we asked to use cursor then there should be an OK packet here
                (this.connection.versionMeetsMinimum(5, 0, 2) && callingStatement != null && isBinaryEncoded && callingStatement.isCursorRequired())) {

            packet = reuseAndReadPacket(this.reusablePacket);
            readServerStatusForResultSets(packet);
        }

        //
        // Handle cursor-based fetch first
        //

        if (this.connection.versionMeetsMinimum(5, 0, 2) 
        && this.connection.getUseCursorFetch() && isBinaryEncoded && callingStatement != null
                && callingStatement.getFetchSize() != 0 && callingStatement.getResultSetType() == ResultSet.TYPE_FORWARD_ONLY) {
            ServerPreparedStatement prepStmt = (com.mysql.jdbc.ServerPreparedStatement) callingStatement;

            boolean usingCursor = true;

            //
            // Server versions 5.0.5 or newer will only open a cursor and set this flag if they can, otherwise they punt and go back to mysql_store_results()
            // behavior
            //

            if (this.connection.versionMeetsMinimum(5, 0, 5)) {
                usingCursor = (this.serverStatus & SERVER_STATUS_CURSOR_EXISTS) != 0;
            }

            if (usingCursor) {
                RowData rows = new RowDataCursor(this, prepStmt, fields);

                ResultSetImpl rs = buildResultSetWithRows(callingStatement, catalog, fields, rows, resultSetType, resultSetConcurrency, isBinaryEncoded);

                if (usingCursor) {
                    rs.setFetchSize(callingStatement.getFetchSize());
                }

                return rs;
            }
        }

        RowData rowData = null;

        if (!streamResults) {
            rowData = readSingleRowSet(columnCount, maxRows, resultSetConcurrency, isBinaryEncoded, (metadataFromCache == null) ? fields : metadataFromCache);
        } else {
            rowData = new RowDataDynamic(this, (int) columnCount, (metadataFromCache == null) ? fields : metadataFromCache, isBinaryEncoded);
            this.streamingData = rowData;
        }

        ResultSetImpl rs = buildResultSetWithRows(callingStatement, catalog, (metadataFromCache == null) ? fields : metadataFromCache, rowData, resultSetType,
                resultSetConcurrency, isBinaryEncoded);

        return rs;
    }