PO即 Persistence Object
VO即 Value Object
VO和PO的主要區(qū)別在于：
VO是獨立的Java Object。
PO是由Hibernate納入其實體容器（Entity Map）的對象，它代表了與數(shù)據(jù)庫中某條記錄對應(yīng)的Hibernate實體，PO的變化在事務(wù)提交時將反應(yīng)到實際數(shù)據(jù)庫中。
實際上，這個VO被用作Data Transfer Object，即所謂的DTO。想必，VO就是Data Access Object ---DAO了啦。為什么要有這二者之分呢？如在傳統(tǒng)的MVC架構(gòu)中，位于Model層的PO，是否允許被傳遞到其他層面。由于PO的更新最終將被映射到實際數(shù)據(jù)庫中，如果PO在其他層面（如View層）發(fā)生了變動，那么可能會對Model層造成意想不到的破壞。
主要想說的還是如何進行二者之間的轉(zhuǎn)換：
屬性復(fù)制可以通過Apache Jakarta Commons Beanutils（http://jakarta.apache.org/commons/beanutils/）組件提供的屬性批量復(fù)制功能，避免繁復(fù)的get/set操作。down下來之后，里面的API DOC一應(yīng)俱全。
對于一些無需處理其它處理（如過濾）直接用BeanUtilsBean.copyProperties方法，其參考如下：
public static void copyProperties(java.lang.Object dest,
????????????????????????????????? java.lang.Object orig)
?????????????????????????? throws java.lang.IllegalAccessException,
????????????????????????????????? java.lang.reflect.InvocationTargetExceptioCopy property values from the origin bean to the destination bean for all cases where the property names are the same.
?
范例１：
TUser?user?
=
?
new
?TUser();
TUser?anotherUser?
=
?
new
?TUser();
user.setName(
"
Emma
"
);
user.setUserType(
1
);
try
?
{
BeanUtils.copyProperties(anotherUser,user);
System.out.println(
"
UserName?=>?
"
+
anotherUser.getName()
);
System.out.println(
"
UserType?=>?
"
+
?anotherUser.getUserType()
);
}
?
catch
?(IllegalAccessException?e)?
{
e.printStackTrace();
}
?
catch
?(InvocationTargetException?e)?
{
e.printStackTrace();
}
也可以利用其中的一些方法在copy屬性的時候達到自己的要求，如：
范例２
/**?*/
/**
//*
?*?Created?on?2006-4-26
?
*/
package
?com.util;
import
?java.beans.PropertyDescriptor;
import
?java.util.Collection;
import
?org.apache.commons.beanutils.PropertyUtils;
/**?*/
/**
?
*/
/**?*/
/**
?*?CopyUtil
?*?
@author
?Jkallen
?
*/
public
?
class
?CopyUtil?
{
????
????
/**?*/
/**
?
*/
/**?*/
/**
?????*?Copy?properties?of?orig?to?dest
?????*?Exception?the?Entity?and?Collection?Type
?????*?
@param
?dest
?????*?
@param
?orig
?????*?
@return
?the?dest?bean
?????
*/
????
public
?
static
?Object?copyProperties(Object?dest,?Object?orig)?
{
????????
if
?(dest?
==
?
null
?
||
?orig?
==
?
null
)?
{
????????????
return
?dest;
????????}
????????
????????PropertyDescriptor[]?destDesc?
=
?PropertyUtils.getPropertyDescriptors(dest);
????????
try
?
{
????????????
for
?(
int
?i?
=
?
0
;?i?
<
?destDesc.length;?i
++
)?
{
????????????????Class?destType?
=
?destDesc[i].getPropertyType();
????????????????Class?origType?
=
?PropertyUtils.getPropertyType(orig,?destDesc[i].getName());
????????????????
if
(destType?
!=
?
null
?
&&
?destType.equals(origType)
????????????????????????
&&
?
!
destType.equals(Class.
class
))?
{
????????????????????
if
(
!
Collection.
class
.isAssignableFrom(origType))
{????????????????????
????????????????????????
try
{
????????????????????????????Object?value?
=
?PropertyUtils.getProperty(orig,?destDesc[i].getName());
????????????????????????????PropertyUtils.setProperty(dest,?destDesc[i].getName(),?value);
????????????????????????}
catch
(Exception?ex)
{????????????????????????????
????????????????????????}
????????????????????}
????????????????}
????????????}
????????????
????????????
return
?dest;
????????}
catch
(Exception?ex)?
{
????????????
throw
?
new
?CopyException(ex);
//
????????????return?dest;
????????}
????}
????
????
????
/**?*/
/**
?
*/
/**?*/
/**
?????*?Copy?properties?of?orig?to?dest
?????*?Exception?the?Entity?and?Collection?Type
?????*?
@param
?dest
?????*?
@param
?orig
?????*?
@param
?ignores
?????*?
@return
?the?dest?bean
?????
*/
????
public
?
static
?Object?copyProperties(Object?dest,?Object?orig,?String[]?ignores)?
{
????????
if
?(dest?
==
?
null
?
||
?orig?
==
?
null
)?
{
????????????
return
?dest;
????????}
????????
????????PropertyDescriptor[]?destDesc?
=
?PropertyUtils.getPropertyDescriptors(dest);
????????
try
?
{
????????????
for
?(
int
?i?
=
?
0
;?i?
<
?destDesc.length;?i
++
)?
{
????????????????
if
?(contains(ignores,?destDesc[i].getName()))?
{
????????????????????
continue
;
????????????????}
????????????????
????????????????Class?destType?
=
?destDesc[i].getPropertyType();
????????????????Class?origType?
=
?PropertyUtils.getPropertyType(orig,?destDesc[i].getName());
????????????????
if
(destType?
!=
?
null
?
&&
?destType.equals(origType)
????????????????????????
&&
?
!
destType.equals(Class.
class
))?
{
????????????????????
if
(
!
Collection.
class
.isAssignableFrom(origType))
{
????????????????????????Object?value?
=
?PropertyUtils.getProperty(orig,?destDesc[i].getName());
????????????????????????PropertyUtils.setProperty(dest,?destDesc[i].getName(),?value);
????????????????????}
????????????????}
????????????}
????????????
????????????
return
?dest;
????????}
catch
(Exception?ex)?
{
????????????
throw
?
new
?CopyException(ex);
????????}
????}
????
????
static
?
boolean
?contains(String[]?ignores,?String?name)?
{
????????
boolean
?ignored?
=
?
false
;
????????
for
?(
int
?j?
=
?
0
;?ignores?
!=
?
null
?
&&
?j?
<
?ignores.length;?j
++
)?
{
????????????
if
?(ignores[j].equals(name))?
{
????????????????ignored?
=
?
true
;
????????????????
break
;
????????????}
????????}
????????
????????
return
?ignored;
????}
}
可以看到，在范例１中通過方法copyProperties的時候，二者之間在的屬性名必須相同（Copy property values from the origin bean to the destination bean for all cases where the property names are the same）。而在范例２中通過
? Object value = PropertyUtils.getProperty(orig, destDesc[i].getName());
? PropertyUtils.setProperty(dest, destDesc[i].getName(), value);
也是將源與目的之間copy相同的屬性名。而VO是在前臺顯示，所以難免會用到PO中所不存在的屬性值。比如PO中可能是一個對象，而VO中則可能是此對象的全部屬性。其中的一些轉(zhuǎn)換則需要依據(jù)前臺需?要針對性地處理啦！
?????????在copy的過程中,若實體中存在一對多,多對多等關(guān)系,則DTO中也應(yīng)該存在此關(guān)系,此時不能直接將內(nèi)部的DTO or List(一對多)里面的數(shù)據(jù)一次性全部拷過去,這時應(yīng)該對每個DTO進行copy,當(dāng)然,若在前臺你無需用到相關(guān)的DTO則可以跳過。而對于一對多（LIST），而應(yīng)將實體里面的每個一對多轉(zhuǎn)換成對應(yīng)的DTO，再依次放到LSIT里面，再將此LIST賦值給（父）DTO，這里面的關(guān)系如同（一個簡單的）遞歸關(guān)系。---更新于5-25-2006
Reference:? Apache DOC and <<Hibernate 開發(fā)指南>>???????????????