转：java操纵主要数据库的lob类型数据

Clob和blob的操作主要分为3种：插入，更新和读取显示。

对于插入，可以分为两类。一类是可以直接按照正常的字段处理，一类为必须先插入空clob/blob再更新为真正的内容。

插入：
对于大部分的数据库，在插入lob时都可以通过PreparedStatement.setAsciiStream或PreparedStatement.setBinaryStream直接写入，查到的资料包括：
H2数据库：http://code.google.com/p/h2database/issues/detail?id=100#c3
Mysql数据库：http://lavasoft.blog.51cto.com/62575/64963
DB2：http://lavasoft.blog.51cto.com/62575/64683

而对于Oracle数据库则必须先插入空lob，重新查询再进行更新（查询时需要加for update）。方法如下：
Oracle数据库：http://www.iteye.com/topic/254
插入空Lob的方式很多，如可以插入长度为1的byte[1到blob中]或1个字符的字符串到clob中。也可以在创建表时通过default empty_clob()由数据库自动创建。

对于上面帖子的补充：robbin同志介绍说需要加for update行锁，是为了防止被其他连接并发更新，这种提法并不完整。oracle数据库要求进行lob更新时，必须加for update，否则报错，这是数据库一个强制要求。就算你知道不会有并发修改，也必须加。

对于Lob更新，方法都比较类似，查询到Lob字段然后直接更新（H2不支持Lob字段的更新）。需要注意的是，获取到Lob的Stream（如：Blob.setBinaryStream()）写入完毕后，必须调用flush()，否则可能会出现数据丢失（在Oracle10g + odbc14.jar下测试发现）。

对于Lob读取：可以使用传统的java.sql.Clob/java.sql.Blob的api，某些驱动也可以使用rs.getString()或rs.getBytes()（没有测试过，据说大部分都行）当成普通的varchar和字节字段处理，很是方便。

对于Clob/Blob的使用，大部分都纠结在Oracle上，Oracle有各种各样的限制，如经典的长度限制，最新的odbc14.jar驱动据说已经解决了所有问题，也就是增加一个连接参数SetBigStringTryClob的事情。相关的资料如下：
Oracle官方文档：
http://www.oracle.com/technology/sample_code/tech/java/codesnippet/jdbc/clob10g/handlingclobsinoraclejdbc10g.html
也可以看看一大堆跟贴的讨论：
http://www.iteye.com/topic/254

我没有测试过rs.getString()的方法，我测试过Oracle和H2的Clob api。通过clob api的方法，可以正常的写入和读取100万字符的大字符串。最理想，最节省内存，也最通用的应该还是JDBC为Lob设计的api。

 

 

JDBC操作MySQL Lob字段记实

 

虽然Java的持久化框架多如牛毛，但都离不开JDBC技术，JDBC在某些时候是其他框架难以取代的。也是java操作数据库最根本的技术。

 

上文写了JDBC操作DB2 Lob字段bug问题，为此，我还特意写了MySQL平台下的Lob字段操作，以便能得出更为准确的结论。

 

本文通过一个简单的Java类，就能增删改查MySQL的Lob字段。google一下，JDBC操作数据库Lob字段的完整代码一个也没找到。因此把这个测试代码也放在blog上，希望给正在用JDBC做MySQL开发的朋友们一点参考。

 

环境：

MySQL-5.0.45

mysql-connector-java-5.1.5.zip

 

测试的SQL脚本：

CREATE TABLE t_lob (
   NAME varchar(24) DEFAULT NULL,
  TXT text,
  IMG blob
) ENGINE=InnoDB DEFAULT CHARSET=gbk;

 

测试代码：

package lob;

import java.sql.*;
import java.io.*;

/**
* JDBC 读取MySQL lob字段测试
* File: TestLob4MySQL.java
* User: leizhimin
* Date: 2008-3-3 14:44:30
*/
public class TestLob4MySQL {
     public static final String url = "jdbc:mysql://localhost/testdb";
    public static final String username = "root";
    public static final String password = "leizhimin";
    public static final String driverClassName = "com.mysql.jdbc.Driver";


    /**
     * 数据库连接获取器
     *
     * @return 数据库连接
     */
    public static Connection makeConnection() {
        Connection conn = null;
        try {
            Class.forName(driverClassName);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        try {
            conn = DriverManager.getConnection(url, username, password);
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return conn;
    }

    /**
     * 测试数据库连接
     */
    public static void testConnection() {
        Connection conn = makeConnection();
        try {
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("SELECT * FROM mysql.user");
            while (rs.next()) {
                String s1 = rs.getString(1);
                System.out.println(s1);
            }
            rs.close();
            stmt.close();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 插入Lob字段
     */
    public static void testInsertlob() {
        Connection conn = makeConnection();
        try {
            conn.setAutoCommit(false);
            File txtFile = new File("C:\\txt.txt");
            File imgFile = new File("C:\\img.png");
            int txt_len = (int) txtFile.length();
            int img_len = (int) imgFile.length();
            try {
                InputStream fis1 = new FileInputStream(txtFile);
                InputStream fis2 = new FileInputStream(imgFile);
                PreparedStatement pstmt = conn.prepareStatement("INSERT INTO T_LOB(NAME,TXT,IMG) VALUES('G',?,?)");
                pstmt.setAsciiStream(1, fis1, txt_len);
                pstmt.setBinaryStream(2, fis2, img_len);
                pstmt.executeUpdate();
                conn.commit();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 读取lob字段
     */
    public static void testQueryLob() {
        Connection conn = makeConnection();
        try {
            conn.setAutoCommit(false);
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("SELECT TXT,IMG FROM T_LOB");
            int i = 1;
            while (rs.next()) {
                Clob clob = rs.getClob("TXT");
                Blob blob = rs.getBlob("IMG");
                InputStream txtIs = rs.getAsciiStream("TXT");
                InputStream imgIs = rs.getBinaryStream("IMG");

                InputStreamReader txtIsr = new InputStreamReader(txtIs);
                InputStreamReader imgIsr = new InputStreamReader(imgIs);

                BufferedReader buff_txtIsr = new BufferedReader(txtIsr);
                BufferedReader buff_imgIsr = new BufferedReader(imgIsr);

                String line = null;
                while (null != (line = buff_txtIsr.readLine())) {
                    System.out.println(line); //将其输出至屏幕，实际你可以按照需要处理
                }

                File fileOutput = new File("c:\\img_x" + i + ".png");
                FileOutputStream fo = new FileOutputStream(fileOutput);
                int c;
                while ((c = imgIs.read()) != -1)
                    fo.write(c);
                fo.close();
                System.out.println("img " + i + " retrieved!");
                i++;
            }
            conn.commit();
        } catch (SQLException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }

    }

    /**
     * 读取lob字段
     */
    public static void testQueryLob1() {
        Connection conn = makeConnection();
        try {
            conn.setAutoCommit(false);
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("SELECT TXT,IMG FROM T_LOB");
            while (rs.next()) {
                Clob clob = rs.getClob("TXT");
                Blob blob = rs.getBlob("IMG");
                InputStream txtIs = clob.getAsciiStream();
                InputStream imgIs = blob.getBinaryStream();

                InputStreamReader txtIsr = new InputStreamReader(txtIs);
                InputStreamReader imgIsr = new InputStreamReader(imgIs);

                BufferedReader buff_txtIsr = new BufferedReader(txtIsr);
                BufferedReader buff_imgIsr = new BufferedReader(imgIsr);

                String line = null;
                while (null != (line = buff_txtIsr.readLine())) {
                    System.out.println(line); //将其输出至屏幕，实际你可以按照需要处理
                }
            }
            conn.commit();
        } catch (SQLException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 删除lob字段
     */
    public static void testDeleteLob() {
        Connection conn = makeConnection();
        try {
            conn.setAutoCommit(false);
            Statement stmt = conn.createStatement();
            int row = stmt.executeUpdate("DELETE FROM T_LOB");
            conn.commit();
            System.out.println("删除 " + row + " 行数据！");
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 读取lob字段
     */
    public static void testUpdateLob() {
        Connection conn = makeConnection();

        try {
            String in_str="HAHAHAHAHAHA!!!";
            File in_file=new File("c:\\img_haha.png");
            InputStream txt_is = string2InputStream(in_str);
            InputStream img_is =new FileInputStream(in_file);

            conn.setAutoCommit(false);
            PreparedStatement pstmt = conn.prepareStatement("UPDATE T_LOB SET TXT=?, IMG=? WHERE NAME='G'");
            pstmt.setAsciiStream(1,txt_is,in_str.getBytes().length);
            pstmt.setBinaryStream(2,img_is,(int)in_file.length());

            int row = pstmt.executeUpdate();

            conn.commit();
            txt_is.close();
            img_is.close();

//            System.out.println("更新 " + row + " 行数据！");
        } catch (SQLException e) {
            e.printStackTrace();
        } catch (FileNotFoundException e) {
            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
        } catch (IOException e) {
            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
        } finally {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String args[]) {
//        testInsertlob();
//        testQueryLob();
//        testQueryLob1();
//        testDeleteLob();
        testUpdateLob();
    }

    public static InputStream string2InputStream(String str) {
        if (str == null) return null;
        return new ByteArrayInputStream(str.getBytes());
    }

    public static String inputStream2String(InputStream is) {
        StringBuffer sb = new StringBuffer();
        BufferedReader br = new BufferedReader(new InputStreamReader(is));
        String inputLine;
        try {
            while ((inputLine = br.readLine()) != null) {
                sb.append(inputLine).append("\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return sb.toString();
    }
}

 

一一运行各个测试方法，都没有问题。