读取BLOB字段会因为文件字段太大导致connect连接超时关闭而不能读全的情况的解决方案

方案一：

通过委托模式（类似这种），在Controller层的请求方法中实现事务层上的读取blob并转化成流

上代码：

控制层

/*** BLOB下载*/@RequestMappingpublic String downloadBlob(HttpServletRequest request,final HttpServletResponse response, String param, String fileName)throws Exception {String[] params = param.split(";");// 数据集IDString dataset_id = params[0];// 字段名String columnName = params[1];// 唯一键值UniqueKey uk = UniqueKey.parseUniqueKey(params[2]);if (fileName == null) {// 文件名为空的时候以主键值作为文件名fileName = uk.firstEntry().getValue().toString();}final String destFileName = fileName;DataSetMeta dataSetMeta = datasetService.retrieveDataSetMeta(dataset_id);DBDataAccessorWithBlob accessor = new DBDataAccessorWithBlob(dataSetMeta);accessor.handleBlob(columnName, uk, new BlobHandler() {// 处理BLOB对象，以流的形式写入response实现下载public void handle(Blob blob) throws Exception {InputStream inputStream = blob.getBinaryStream();// 设置response类型、文件名、大小等。response.setContentType("application/octet-stream");response.setCharacterEncoding("UTF-8");String downLoad = new String(destFileName.getBytes("GBK"),"ISO-8859-1");response.setHeader("Content-disposition","attachment;filename=\"" + downLoad + "\"");response.setHeader("Content-Length", String.valueOf(blob.length()));// 写入responseOutputStream output = null;try {byte[] bytes = new byte[1024];output = response.getOutputStream();int byteRead;while ((byteRead = inputStream.read(bytes)) != -1) {output.write(bytes, 0, byteRead);}output.flush();} finally {IOUtils.closeQuietly(inputStream);IOUtils.closeQuietly(output);response.flushBuffer();}}});return null;}

事务层：

public class DBDataAccessorWithBlob extends DBDataAccessor {public DBDataAccessorWithBlob(DataSetMeta dataSetMeta) {super(dataSetMeta);}public void handleBlob(String columnName, UniqueKey uk, BlobHandler blobHandler)throws Exception {BaseDBAccessor accessor = (BaseDBAccessor) getDataAccessor();if (dataSetMeta == null || dataSetMeta.getDataColumn() == null|| columnName == null || columnName.trim().equals("")) {return;}String DBDriver="";String DBurl=""；String DBusername="";String DBpassword="";//通过jdbc创建并获取连接DriverManager.registerDriver((Driver) Class.forName(DBDriver).newInstance());Connection conn=DriverManager.getConnnection(DBurl,DBusername,DBpassword);通过连接池获取连接//Connection con = accessor.getConnection(dataSetMeta);try {// 查询结果字段String[] result_column_array = new String[] { columnName };// 构建SQL语句String sql = "";sql += accessor.buildSelectStament(dataSetMeta.getFullName(),result_column_array, true);sql += accessor.buildUniqueKeyWhere(uk);PreparedStatement ps = con.prepareStatement(sql);// 设置唯一键值String[] uk_column_array = uk.getFields();for (int i = 0; i < uk_column_array.length; i++) {String column = uk_column_array[i];ps.setObject(i + 1, uk.get(column));}ResultSet rs = ps.executeQuery();while (rs.next()) {Blob blob = rs.getBlob(columnName);blobHandler.handle(blob);}rs.close();ps.close();} catch (SQLException e) {throw e;} finally {try {con.close();} catch (SQLException e) {throw e;}}return;}public interface BlobHandler {public void handle(Blob blob) throws Exception;}
}

看出来上面的情况木有，在控制层的实现事务层的接口BlobHandler，而在事务层调用接口的方法。这样可以在未关闭之前已经将blob字段转换成流了，不用担心连接因读的时间过长而关闭的情况了。

第二种方案：

不能过jdbc的动态创建连接，DriverManager.getConnection()来获取连接，而是取连接池中存在的连接，因为连接池中的连接在代码手动关闭的时候并不是实际关闭了，而是将连接放入到连接池中去，程序仍然可以读取blob字段，因为blob并不是读取数据本事，而是数据的引用，因此这种方式，也能读全文件内容。

控制层：

/*** BLOB下载*/@RequestMappingpublic String downloadBlob(HttpServletRequest request,final HttpServletResponse response, String param, String fileName)throws Exception {String[] params = param.split(";");// 数据集IDString dataset_id = params[0];// 字段名String columnName = params[1];// 唯一键值UniqueKey uk = UniqueKey.parseUniqueKey(params[2]);if (fileName == null) {// 文件名为空的时候以主键值作为文件名fileName = uk.firstEntry().getValue().toString();}final String destFileName = fileName;DataSetMeta dataSetMeta = datasetService.retrieveDataSetMeta(dataset_id);DBDataAccessorWithBlob accessor = new DBDataAccessorWithBlob(dataSetMeta);Blob blob=accessor.handleBlob(columnName, uk, null);   // 处理BLOB对象，以流的形式写入response实现下载InputStream inputStream = blob.getBinaryStream();// 设置response类型、文件名、大小等。response.setContentType("application/octet-stream");response.setCharacterEncoding("UTF-8");String downLoad = new String(destFileName.getBytes("GBK"),"ISO-8859-1");response.setHeader("Content-disposition","attachment;filename=\"" + downLoad + "\"");response.setHeader("Content-Length", String.valueOf(blob.length()));// 写入responseOutputStream output = null;try {byte[] bytes = new byte[1024];output = response.getOutputStream();int byteRead;while ((byteRead = inputStream.read(bytes)) != -1) {output.write(bytes, 0, byteRead);}output.flush();} finally {IOUtils.closeQuietly(inputStream);IOUtils.closeQuietly(output);response.flushBuffer();}};

事务层：

public class DBDataAccessorWithBlob extends DBDataAccessor {public DBDataAccessorWithBlob(DataSetMeta dataSetMeta) {super(dataSetMeta);}public Blob handleBlob(String columnName, UniqueKey uk, BlobHandler blobHandler)throws Exception {BaseDBAccessor accessor = (BaseDBAccessor) getDataAccessor();if (dataSetMeta == null || dataSetMeta.getDataColumn() == null|| columnName == null || columnName.trim().equals("")) {return;}通过连接池获取连接Connection con = accessor.getConnection(dataSetMeta);try {// 查询结果字段String[] result_column_array = new String[] { columnName };// 构建SQL语句String sql = "";sql += accessor.buildSelectStament(dataSetMeta.getFullName(),result_column_array, true);sql += accessor.buildUniqueKeyWhere(uk);PreparedStatement ps = con.prepareStatement(sql);// 设置唯一键值String[] uk_column_array = uk.getFields();for (int i = 0; i < uk_column_array.length; i++) {String column = uk_column_array[i];ps.setObject(i + 1, uk.get(column));}ResultSet rs = ps.executeQuery();while (rs.next()) {blob = rs.getBlob(columnName);}rs.close();ps.close();} catch (SQLException e) {throw e;} finally {try {con.close();} catch (SQLException e) {throw e;}}return blob;}
}