昨天还在用Torque,今天听说Hibernate更好,明天说不准又有更好的持久层工具...

如何使应用程序不变化的情况下选择不同的持久层呢？

看看Finetix LLC的资深顾问Charles Chan给我们现身说法。

Charles Chan在文章 Object-Relational Mapping with Apache Jakarta OJB

中介绍了Apache OJB的使用,并在文章附录中提到一种屏蔽不同持久层的设计思路,他采用了工厂方法模式(Factory Method),并通过实例Apache OJB来说明他的设计。

这个设计有两个产品，一个是PersistenceManager，另一个是Transaction 。

OJBPersistenceManager和OJBTransaction 分别是两个产品的实现。

It is often desirable to decouple your application from the specifics of a persistence framework, so that you can easily switch frameworks in the future. I recommend creating custom PersistenceManager and Transaction classes for this purpose. For example:

Example

13

: PersistenceManager and Transaction Interfaces

/** */

/**

* The abstract base class for all PersistenceManager. It provides

* method to persist and object and to obtain the current

* transaction.

*/

public

abstract

class

PersistenceManager

{

public

static

PersistenceManager getInstance()

{

return

new

OJBPersistenceManager();

}

public

abstract

void

makePersistent(Object o)

throws

PersistenceException;

public

abstract

Transaction currentTransaction()

throws

PersistenceException;

}

/** */

/**

* A Transaction interface. It provides minimal support for

* beginning, committing, and rolling back a transaction.

*/

public

interface

Transaction

{

public

void

begin()

throws

PersistenceException;

public

void

commit()

throws

PersistenceException;

public

void

rollback()

throws

PersistenceException;

}

/** */

/**

* An OJB specific implementation of the PersistenceManager

* class.

*/

public

class

OJBPersistenceManager

extends

PersistenceManager

{

private

PersistenceBroker broker

=

null

;

public

OJBPersistenceManager()

{

broker

=

PersistenceBrokerFactory.defaultPersistenceBroker();

}

public

void

makePersistent(Object o)

throws

PersistenceException

{

try

{

broker.store(o);

}

catch

(PersistenceBrokerException e)

{

//

rethrow PersistenceException

}

}

public

Transaction currentTransaction()

throws

PersistenceException

{

return

new

OJBTransaction(broker);

}

}

/** */

/**

* An OJB specific implementation of the Transaction

* interface.

*/

public

class

OJBTransaction

implements

Transaction

{

private

PersistenceBroker broker

=

null

;

public

OJBTransaction(PersistenceBroker broker)

{

this

.broker

=

broker;

}

public

void

begin()

throws

PersistenceException

{

try

{

broker.beginTransaction();

}

catch

(TransactionAbortedException e)

{

//

rethrow PersistenceException

}

catch

(TransactionInProgressException e)

{

//

rethrow PersistenceException

}

}

public

void

commit()

throws

PersistenceException

{

try

{

broker.commitTransaction();

}

catch

(TransactionAbortedException e)

{

//

rethrow PersistenceException

}

catch

(TransactionNotInProgressException e)

{

//

rethrow PersistenceException

}

}

public

void

rollback()

throws

PersistenceException

{

try

{

broker.abortTransaction();

}

catch

(TransactionNotInProgressException e)

{

//

rethrow PersistenceException

}

}

}

After the above classes are created, it is very simple to convert the test application to use them:

下面是这个Factory的使用：

Example

14

: Example Using PersistenceManager and Transaction Interfaces

PersistenceManager pm

=

PersistenceManager.getInstance();

Transaction tx

=

pm.currentTransaction();

tx.begin();

Manager manager

=

newManager();

manager.setName(

"

John Smith

"

);

manager.setProjectNumber(

new

Integer(

10

));

pm.makePersistent(manager);

Employee employee

=

newEmployee();

employee.setName(

"

David Cosby

"

);

pm.makePersistent(employee);

tx.commit();

It looks surprisingly like JDO, doesn't it? With a little more effort, you can also hide the OJB query mechanism.

改进建议：

1. 改用抽象工厂方法, 一套持久层产品提供一个工厂,用于创建不同的产品(如:PersistenceManager,Transaction....)

2.考虑从不同的持久层产品中抽取一套通用的持久层接口