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To: struts-user@jakarta.apache.org
From: Vic Cekvenich <cekvenich_vic@basebeans.com>
Subject: Re: Struts and Commons Chain
Date: Mon, 08 Dec 2003 08:15:47 -0500
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> This is very interesting I think.
> .V
> 
> Hookom, Jacob wrote:
> 
>> I was looking at chain's implementation, instead of executing the 
>> commands
>> with the visitor pattern, wouldn't a more powerful/flexible method be to
>> implement it using a filter pattern?
>>
>> See:
>> http://java.sun.com/blueprints/corej2eepatterns/Patterns/InterceptingFilter. 
>>
>> html
>>
>>
>> Sample Chain:
>> CommandServletStart
>> CommandExceptionHandler
>> CommandStoreState
>> CommandExceptionThrower
>>
>> -- 
>>
>> CommandStart.execute(Context ctx, Chain chain) throws Exception
>> {
>>     ServletContext srvCtx = (ServletContext) ctx;
>>     String usrId = srvCtx.getRequestScope().getParameter("id"));
>>     String newName = srvCtx.getRequestScope().getParameter("name"));
>>
>>     // get user and save state
>>     User user = SomeService.getUser(usrId);
>>     String oldName = user.getName();
>>     
>>     // set new state and execute
>>     try
>>     {
>>         user.setName(newName);
>>         ctx.setAttribute("user", user);
>>         chain.executeNext(ctx);
>>     }
>>     catch (Exception e)
>>     {
>>         // on error, rollback change
>>         user.setName(oldName);
>>         throw e;
>>     }
>> }
>>
>> -- 
>>
>> CommandExceptionHandler.execute(Context ctx, Chain chain) throws 
>> Exception
>> {
>>     try
>>     {
>>         chain.executeNext(ctx);
>>     }
>>     catch (SQLException e)
>>     {
>>         chain.executeNext(ctx);
>>     }
>>     // let unhandled exceptions go up
>> }
>>
>> CommandExceptionThrower.execute(Context ctx, Chain chain) throws 
>> Exception
>> {
>>     throw new SQLException();
>> }
>>
>> With this, you would not have to change your inheritance to add behavior
>> (going from a command to a filter).  The given command would already know
>> best on how to handle itself and since it calls the next item in the 
>> chain,
>> we can also wrap the context to take care of situations where we want to
>> have listener type behavior.
>>
>> The chain could also allow for runtime manipulation by the commands, to
>> insert additional commands to handle errors.
>>
>> chain.insert("SomeCommand").insert("AnotherCommand).executeNext(ctx);
>>
>> --------
>> Jacob Hookom
>> Senior Programmer/Analyst
>> McKesson Medical-Surgical
>> Golden Valley, MN
> 
> 


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