Java之路

作為測試，我們新建一個TUser對象，其image屬性中，保存了一個圖片文件的二進制內容。而其resume屬性，我們以一個簡單的字符串作為填充。

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上面的代碼中，我們通過Hibemate.createBlob和Hibemate.createClob創建了對應的Blob和Clob對象。其中Blob對象基于一個FileInputStream構建，而Clob對象基于一個字符串構建。

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完成了寫入操作，對應的讀取操作代碼如下:

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通過上面的代碼，我們完成了針對SQLServer的Blob/Clob型字段操作.看起來非常簡單，不過，并非侮種數據庫都如此友善。讓我們接著來看看Oracle數據庫下的Blob/Clob字段讀寫，

通過修改hibernate.cfg.xml中的Dialect配置完成數據庫切換后，我們再次運行上面的TUser對象保存例程。

程序運行期間拋出異常:

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Hibernate:select hibernate_sequence.nextval from dual

Hibernate:insert into T_USER (name, age,? image,resume. id) values(?, ?, ?, ?, ?)

17:27:24,161 ERROR JDBCExceptionReporter:58 - - 不允許的操作： Streams type cannot be used in batching

17:27:24,171 ERROR Sessionlmpl:2399 - Could not synchronize database state with session

net.sf.hibernate.exception.GenericJDBCException:could not insert:[com.redsaga.hibernate.db.entity.TUser#6]

...

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觀察異常信息:streams type cannot be used in batching.這意味著Oracle JDBC不允許流操作以批量方式執行(Oracle CLOB采用流機制作為數據讀寫方式)。

這種錯誤一般發生在hibernate.cfg.xml中的hibernate jdbc.batch_size設定大于0的情況，將hibernate.jdbc.batch_size修改為0即可消除。

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<hibernate-configuration>

<session-factory>

???????????? ...

???????????? <property name='hibernate. jdbc.batch_size">0</property>

???????????? ...

</session-factory>

</hibernate-configuration>

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再次嘗試啟動代碼，發現依然無法成功運行，拋出異常如下:

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Hibernate: select hibernate_sequence.nextval from dual

Hibernate: insert into T--USER? (name,? age,? image,resume,id) values(?,?,?,?,?)

19:02:21,054 ERROR JDBCExceptionReporter:58 一IO異常:Connection reset bypeer: socket write error

19:02:21,054 ERROR JDBCExceptionReporter:58 一I。異常:Connection reset by peer:socket write error

19:02:21 。064 ERROR JDBCExceptionReporter:58一Io異常:Connection reset by peer: socket wr主to error

19:02:21,064 ERROR Sessionlrnpl:2399 一Could not synchronize database state with session

net.sf.hibernate.exception.GenericJDSCException:? could not insert:[com.redsaga.hibernate.db.entity.TUser#27]

...

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為什么會出現這樣的情況?

問題出在Oracce中Blob/Clob字段獨特的訪問方式，Oracle Blob/Clob字段本身擁有一個游標(cursor) , JDBC必須通過游標對Blob/ Clob字段進行操作，在Blob/Clob字段被創建之前，我們無法獲取其游標句柄，這也就意味著，我們必須首先創建一個空Blob/Clob字段，再從這個空Blob/Clob字段獲取游標，寫入我們所期望保存的數據。

如果用JDBC代碼來表示這個過程，則得到如下代碼:

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//... 獲取JDBC連接

dbconn.setAutoCommit(falee);

// ＝＝＝＝＝＝＝插入數據，BLOB CLOB字段插入空值

PreparedStatenent preStmt=

dbconn.prepareStatement(

??? "insert into T_USER (name, age,? id,image,resume) values

???? (?,?,?,?,?)");

preStmt.setString(1,"Shark");

preStmt.setInt(2,20);

preStmt.setInt(3,5);?

// 通過oracle.sgl.BLOB/CLOB.empty_lob()方法構造空Blob/Clob對象

preStmt.setBlob(4 ，oracle.sgl.BLOB.empty_lob());

preStmt.setClob(5,oracle.sgl.CLOB.empty_lob());

preStmt.executeUpdate();

preStmt.close():

//========== 再次從庫表讀出，獲得Blob/Clob句柄

preStmt=

? dbconn.prepareStatement(

????? "select? image,resume from T_USER where id=?for update');

preStmt.setint(l,5);

ResultSet rset=preStmt.executeQuery();

// 注意我們這里需要引用Oracle原生BLOB定義，如果使用了Weblogic JDBC Vendor

// 則應使用weblogic.jdbc.vendor.oracle. OracleThinBLob/OracleThinCLOb

rset.next();

oracle.sql.BLOB imqBlob = (oracle.sql.BLOB) rset.getBlob(1);

oracle.sql.CLOB resClob = (oracle.sql.CLOB) rset.getClob(2);

//======= 將二進創數據寫入Blob

FileInputStream inStream = new FileinputStream("c\\inimage.jpg");

OutputStream outStream = imgBlob.getBinaryOutputStream();

byte[] buf=new byte[10240];//10K 讀取緩存

int len;

while((len=inStream.read(buf))>0){

?? outStream.write(buf,0,len);

}

inStream.close();

outStream.close():

//======= 將字符串寫入Clob

resClob.putString(1 ，"This is my Glob"

//======= 將Blob/Clob字段更新到數據序

preStmt= dbconn.prepareStatement("update T_USER set? image=?,? resume=? where id=?");

preStmt.setBlob(1,imgBlob);

preStmt.setClob(2,resClob):

preStmt.setlnt(3 ，5);

preStmt.executeUpdate();

preStmt.close():

dbconn.commit();

dbconn.close():

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上面的代碼說明了Oracle中Blob/Clob字段操作的一般機制，那么，基于Hibernate的持久層實現中，應該如何對Blob/Clob字段進行處理?

我們知道，Hibernate底層數據訪問機制仍然是基于JDBC實現，那么也就意味著我們必須在Hibernate中模擬JDBC的訪問流程：

TUser user=new TUser();

user.setAge(new Integer(20));

user.setName("Shark');

user.setImage(Hibernate.createSlob(new byte [1])):

user.setResume(Hibernate.createClob(" "));// 注意這里的參教是一個空格

Transaction tx=session.beginTransaction();

session.save(user):

// 調用flush方法，強制Hibernate立即執行insert sql

session.flush();

// 通過refresh方法，強制Hibernate執行select for update

session.refresh(user, LockMode.UPGRADE);

// 向Blob寫入實際內容

oracle.sql.BLOB blob=(oracle.sql.BLOB)user.getImage();

OutputStream out=blob. getBinaryOutputStream();

FileInputStream imgis=new FileInputStream("C:\\inimage.jpg");

byte[] buf=new byte[10240];//10K 緩存

int len;

while((len=imgis.read(buf))>0){

? out.write(buf,0,len);

}

imgis.close();

out.close();

// 向Clob寫入實際內容

oracle.sql.CLOB clob=(oracle.sgl.CLOB)? user.getResume();

java.io.Writer writer = clob.getCharacterOutputStream();

writer.write("this is my? resume');

writer.close();

session.save(user);

tx.commit();

實際應用中,對于Clob字段，我們可以簡單地將其映射為String類型，不過在這種情況下需要注意，Oracle Thin Driver對Clob字段支持尚有欠缺，當Clob內容超出4000字節時將無法讀取，而Oracle OCI Driver(需要在本地安裝Oracle客戶端組件)則可以成功地完成大容量Clob字段的操作。

上面的代碼中，我們通過Session.save/flush/refresh方法的組合使用，實現了上面JDBC代碼中的Blob/Clob訪問邏輯。

Blob/Clob 字段的Hibernate保存實現如上所述，相對來講，讀取則沒有太多的障礙，之前的讀取代碼依然可以正常運行。

對于上面的實現，相信大家都感覺到了一些Bad Smell，如果Blob/Clob字段普遍存在，那么我們的持久層邏輯中可能遍布如此復雜的數據存儲邏輯、并與數據庫原生類緊密禍

如何解決這些問題?

回憶之前關于自定義數據類型的討論。通過自定義數據類型我們可以對數據的通用特征進行抽象，那么，對于Oracle的Blob/Clob字段，我們是否可以也對其進行抽象，并以其作為所有Oracle Blob/Clob字段的映射類型?

下面的StringClobType實現了這一目標:

public class StringClobType implements UserType{

??? private static final String ORACLE_DRIVER_NAME="Oracle JDBC driver";

?????? private static final int ORACLE_DRIVER_MAJOR_VERSION=9;

??? private static final int ORACLE_DRIVER_MINOR_VERSION=0;

??? public int[] sqlTypes(){

?????? return new int[] {Types.CLOB};

??? }

??? public Class returnedClass{

?????? return String.class;

??? }

??? public boolean equals(Object x, object y){

?????? return org.apache.commons.lang.ObjectUtils.equals(x, y);

??? }

??? public Object nullSafeGet(ResultSet rs, String[] names, Object owner)

??? throws HibernateException,SQLException{

?????? Clob clob=rs.getClob(names(O]);

?????? return(clob==null ? null:clob.getSubString(l,? (int) clob.length())):

??? }

??? public void nullSafeSet(PreparedStatement st ，Object value, int index)?

??? throws HibernateException, SQLException{

?????? DatabaseMetaData dbMetaData=st.getConnection().getMetaData();

?????? if (value==null)

?????????? st.setNull(index,? sqiTypes()(0));

?????? else

//???????? 本實現僅僅適用于Oracle數據序9.0以上版本

?????? ??? if

?????????? (ORACLE_DRIVER_NAME.equals(dbMetaData.getDriverName( ，))(

????????????? if((dbMetaData.getDriverMajorVersion()?

????????????????? ???>=ORACLE-DRIVER-MAJOR-VERSION)

????????????????? &&(dbMetaData.getDriverMinorVersion()??

???????????????????? >=ORACLE-DRIVER-MINOR-VERSION)) ｛

???????????????????????? try ｛

//??????????????? 通過動態加載方式進免編譯期對Oracle JDBC的依賴

???????????????????????? Class oracleClobClass=Class.forName('oracle.sgl.CLOB");

//????????????????????????????? 動態調用createTemporary方法

??????????????????????????????? Class partypes[]=new Class[3];

??????????????????????????????? partypes[0]=Connection.class;

??????????????????????????????? partypes[1]=Boolean.TYPE;

??????????????????????????????? partypes(2]=Integer.TYPE;

??????????????????????????????? Method createTemporaryMethod=

?????????????????????????????????? oracleClobClass.getDeclaredMethod(

????????????????????????????????????????? "createTemporaxy “，

????????????????????????????????????????? partypes);

??????????????????????????????? Field durationSessionField=

?????????????????????????????????? oracleClobClass.getField("DURATION-SESSION");

??????????????????????????????? Object arglist[]=new 0bject[3]:

?????????????????????????????????? Connection conn=

?????????????????????????????????????? st.getConnection().getMetaData().getConnection();

//????????????????????????????? 數據庫連接類型必須為OracleConnection

//????????????????????????????? 萊些應用服務器會使用自帶Oracle JDBC Wrapper，如Weblogic

//????????????????????????? ??? 這里需要特別注意

??????????????????????????????? Class oracleConnectionClass=

?????????????????????????????????? Class.forName("oracle.jdbc.OracleConnection");

??????????????????????????????? if(!oracleConnectionClass

?????????????????????????????????????? .isAssignableFrom(conn.getClass())){

?????????????????????????????????? throw new HibernateException(

????????????????????????????????????????? "Must be a oracle.jdbc.OracleConnection:.

????????????????????????????????????????? +conn.getClass().getName());

??????????????????????????????? }

??????????????????????????????? arglist[0] = conn;

??????????????????????????????? arglist(1] = Boolean.TRUE;

??????????????????????????????? arolist[2] = durationSessionField.get(null);

??????????????????????????????? Object tempClob =createTemporaryMethod.invoke(null,arglist);

??????????????????????????????? partypes=new Class[l];

??????????????????????????????? partypes[0]=Integer.TYPE;

??????????????????????????????? Method openMethod =oracleClobClass.getDeclaredMethod("open",partypes);

??????????????????????????????? Field modeReadWriteField =oracleClobClass.getField("MODE_READWRITE");

??????????????????????????????? arglist = new Object(l];

??????????????????????????????? arglis[0] = modeReadWriteField.get(null);

??????????????????????????????? openMethod.invoke(tempClob, arglist);

??????????????????????????????? Method getCharacterOutputStreamMethod=oracleClobClass.getDeclaredMethod("getCharacterOutputStream',null) ;

//???????????????????????????????????? call the getCharacterOutpitStream method

?????????????????????????????????????? Writer tempClobWriter =(Writer)getCharacterOutputStreamMethod.invoke(tempClob,null);

//???????????????????????????????????? 將參數寫入Clob

?????????????????????????????????????? tempClobwriter.write((String) value);

?????????????????????????????????????? tempClobWriter.flush();

?????????????????????????????????????? tempClobWriter.Close();

??????????????????????????????????????

//???????????????????????????????????? close? clob

?????????????????????????????????????? Method closeMethod=oracleClobClass.getDeclaredMethod("close", null);

?????????????????????????????????????? closeMethod.invoke(tempClob, null);

?????????????????????????????????????? st.setClob(index,? (Clob) tempClob);

??????????????????????????????? )catch? (ClassNotFoundException e){

?????????????????????????????????? throw new HibernateException("Unable to find a required class.\n"+e.getMessage()):

??????????????????????????????? }catch (NOSuchMethodException e){

?????????????????????????????????? throw new HibernateException("Unable to find a required method.\n"+e.getMessage()):

??????????????????????????????? }catch (NoSuchFieldException e){

?????????????????????????????????? throw new HibernateException("Unable to find a required field.\n"+e.getMessage());

??????????????????????????????? }catch (IllegalAccessException e){

?????????????????????????????????? throw new HibernateException("Unable to access a required method or field.\n"+e.getMessage());

?????????????????????????????????? catch (InvocationTargetException e){

?????????????????????????????????????? throw new HibernateException(e.getMessage());

?????????????????????????????????????? {? catch (IOException e){

????????????????????????????????????????? throw new HibernateException(e.getMessage());

?????????????????????????????????????? }

?????????????????????????????????????? else ｛

?????????????????????????????????????? throw new HibernateException(

????????????????????????????????????????????? "No CLOBS support.Use driver version"

????????????????????????????????????????????? +ORACLE_DRIVER_MAJOR_VERSION

????????????????????????????????????????????? +" ，minor"

????????????????????????????????????????????? +ORACLE_DRIVER_MINOR_VERSION);

?????????????????????????????????????? }

?????????????????????????????????? }else ｛

?????????????????????????????????? String str = (String)? value;

??? ??????????????????????????????? StrinaReader r = new StringReader(str);

?????????????????????????????????? St.setCharacterStream(index, r, str.length());

??????????????????????????????? }

??? }

??? public Object deepCopy(Object value){

?????? if(value==null)

?????????? return null;

?????? return new String((String)? value);

??? }

??? public boolean isMutable(){

?????? return false

??? }

}

上面這段代碼，重點在于nullSafeSet方法的實現，nullSafeSet中通過Java Reflection機制，解除了編譯期的Oralce JDBC原生類依賴。同時，借助Oracle JDBC提供的原生功能完成了Clob字段的寫入，Clob字段的寫入操作由于涉及特定數據庫內部實現細節，這里就不多費唇舌，大家可參見Oracle JDBC Java Doc.

這段代碼是由筆者根據Ali Ibrahim, Scott Miller的代碼修改而來的(原版請參見httpJ/www.hibemate， org /56.html )，支持Oracle 9以上版本，Oracle 8對應的實現請參見上述網址。

同樣的道理，讀者可以根據以上例程，編寫自己的ByteBlobType以實現byte[]到Blob的映射。

另外，此代碼必須運行在最新版的Oracle JDBC Driver上(筆者所用版本為Oracle9i9.2.0.5 for JDK1.4,如果使用9.2.0.3或之前版本則在新建l更卻刪除數據時可能會遇到“nomore data read from socket”錯誤)。

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