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Java Forum / JavaBeans / September 2004serialized access (synchronized)
I know synchronized nor any other thread management thingies should be used in EJB's however. I have a piece of code in a session bean method that accesses a mail server. I must assure that only one thread at a time executes this method (serializable access), how can I assure that? > I know synchronized nor any other thread management thingies should be > used in EJB's however. > > I have a piece of code in a session bean method that accesses a mail > server. Why is that? SMTP servers can usually handle multiple concurrent requests. > I must assure that only one thread at a time executes this > method (serializable access), how can I assure that? If you want to access a POP or IMAP account then maybe you can have an EJB describing the connection and do all communication via this bean guarding access with a flag as persistent property. Regards robert > Why is that? SMTP servers can usually handle multiple concurrent > requests. I have some code that adds a message to the mail server (IMAP). And I have some other code that gets the count of messages. If one thread gets the count of messages and another one is adding a message i would have the wrong number of messages in my hand. > If you want to access a POP or IMAP account then maybe you can have an EJB > describing the connection and do all communication via this bean Thats exactly what I have. > guarding access with a flag as persistent property. Even if I do this then Thread1 Thread2 if (persistent.isFlag()); if (persisten.isFlag()); getMessagesCount(); addMessage(); And thread 1 gets an inconsistent messages count. > > I know synchronized nor any other thread management thingies should be > > used in EJB's however. [quoted text clipped - 15 lines] > > robert > > Why is that? SMTP servers can usually handle multiple concurrent > > requests. [quoted text clipped - 17 lines] > if (persistent.isFlag()); if (persisten.isFlag()); > getMessagesCount(); addMessage(); The pattern is rather // atomic operation that will block // until the lock is accessible aquireLock(); try { getMessageCount(); } finally { releaseLock(); } and aquireLock(); try { addMessage(); } finally { releaseLock(); } Regards robert > And thread 1 gets an inconsistent messages count. > [quoted text clipped - 12 lines] > > If you want to access a POP or IMAP account then maybe you can have an EJB > > describing the connection and do all communication via this bean guarding > > access with a flag as persistent property. > > > > Regards > > > > robert Is there a possibility to implement aquireLock() method without using the java synchronized keyword? > > > Why is that? SMTP servers can usually handle multiple concurrent > > > requests. [quoted text clipped - 71 lines] > > > > > > robert Possibly not. But maybe you can declare the method or the bean on EJB config level as not reentrant. robert > Is there a possibility to implement aquireLock() method without using > the java synchronized keyword? [quoted text clipped - 74 lines] > > > > > > > > robert And what if I do want my bean to be reentrant... It seems to me like this is a general problem that requires a general solution. Some "design pattern" that says : Requirement : You need to have some of your code to be single threaded and you have that piece of code in a session bean . General Solution : Thats what im looking for ;) > Possibly not. But maybe you can declare the method or the bean on EJB > config level as not reentrant. [quoted text clipped - 88 lines] > > > > > > > > > > robert tomerbd@barak-online.net (Tomer Ben-David) wrote in message > Is there a possibility to implement aquireLock() method without using > the java synchronized keyword? I don't think avoiding the synchronized keyword at all cost isn't really what the EJB calls for. Otherwise it wouldn't be possible to use any class inside an EJB that uses that keyword (e.g. IO functions, Hashtable). What the specification really means is that the developer doesn't have to care about multithreading in *general*. Competing for shared resources among threads is o.k. I would think. I can't see where the following implementation violates the specification. Of course locks like that will only work for EJB instances living in the same classloader space. private static Object lock=new Object(); private static boolean locked=false; private static void aquireLock() { synchronized(lock) { while(locked) lock.wait(); locked=true; } } private static void releaseLock() { synchronized(lock) { locked=false; lock.notify(); } } > > Why is that? SMTP servers can usually handle multiple concurrent > > requests. [quoted text clipped - 39 lines] > > > > robert Is it possible to use a message driven bean for the "sent" message, and subscribe to a JMS topic for the updated message count? Mark > > > Why is that? SMTP servers can usually handle multiple concurrent > > > requests. [quoted text clipped - 44 lines] > > Mark Even if its possible then I'm trying to find a general solution, not just for the sent messages count but for the general case of a need to access a resource in a serialized manner from EJB's --> for serialized access of entity beans to database data in a serialized manner all I need to do is to mark the transaction with isolation level serialized, so I'm trying to find something maybe a design pattern for the general case of trying to access a resource (any resource, in this example it was a mail server) in a serialized manner. Is there? > > > > Why is that? SMTP servers can usually handle multiple concurrent > > > > requests. [quoted text clipped - 53 lines] > case of trying to access a resource (any resource, in this example it > was a mail server) in a serialized manner. Is there? Hello ? I suspect this is an answerable question... (Has no solution in the specs etc) Is that so ? > Hello ? I suspect this is an answerable question... (Has no solution > in the specs etc) Is that so ? You would have to synchronize the access. Something like synchronize { get the count code } ----- synchronize { add the message }  SignatureIn remembrance -- http://www.ronsmits.org > > Hello ? I suspect this is an answerable question... (Has no solution > > in the specs etc) Is that so ? [quoted text clipped - 8 lines] > add the message > } But synchronization is something that should not be done in a container-j2ee environment! so do I have any option ? to synchronize - I shouldnt because of the j2ee environment, not to synchronize - I cant my application wont work, another example why it wont work is because generaly a user in my application can access his mailbox 10 times lets say per second, and if I use a javamail implementation with a store connection pool of size 5 (for example... and for the sake of the case) the user will get errors! because he tried to connect with more than 5 connection at the same time! (And I want to allow the user to do it...) what can I do ? > I know synchronized nor any other thread management thingies should be > used in EJB's however. > > I have a piece of code in a session bean method that accesses a mail > server. I must assure that only one thread at a time executes this > method (serializable access), how can I assure that? The ejb takes care of all that,thats what there for. Regards; > The ejb takes care of all that,thats what there for. From the EJB Spec (2.0): "...the EJB architecture does not define an API for managing isolation level.)..." More from the section : 17.3.2 "...The isolation level describes the degree to which the access to a resource manager by a transaction is isolated from the access to the resource manager by other concurrently executing transactions. The API for managing an isolation level is resource-manager specific. (Therefore, the EJB architecture does not define an API for managing isolation level.)..." Therefore I dont see any way in CMTD to control the isolation level of a stateless session bean method. What are my options? > > I know synchronized nor any other thread management thingies should be > > used in EJB's however. [quoted text clipped - 4 lines] > The ejb takes care of all that,thats what there for. > Regards; > I know synchronized nor any other thread management thingies should be > used in EJB's however. > > I have a piece of code in a session bean method that accesses a mail > server. I must assure that only one thread at a time executes this > method (serializable access), how can I assure that? Use cmt ,this will secure that only one threat is executing youre method. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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