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I ve been using Hibernate for a while, and I really like it. But I find that there is a major, major issue. Synchronization. We lose data! We have a web based application where different people can simultaneously add or update data we have to synchronize! Don t lock or in any other way serialize access to objects in the application. You don t have to do anything to ensure that concurrent modification doesn t destroy your data; that is the job of Hibernate and the underlying database. This question is sometimes asked as follows: How does Hibernate ensure that an object modified in Session A is synchronized with the same loaded object in Session B It isn t the same object. Both objects may have the same database identity, but because they re loaded in different Sessions, they re two distinct instances in the JVM. This is called transaction-scoped identity, and there is no need to synchronize two objects that have only the database identity in common. In other words, all operations in concurrently running transactions are isolated from each other. So how do you solve this concurrent access to the same resource Hibernate uses existing mechanisms present in any database-management system to resolve the conflict. The Hibernate Session inherits the transaction semantics and transaction isolation level of the standard JDBC connection it s started with. This means you ll experience the typical problems of not having fully isolated access to a shared resource (dirty reads, nonrepeatable reads, phantom reads), depending on the transaction isolation level of your database connection and transaction. Conflicting updates, however, are resolved with an optimistic approach at commit time (using a version check). With Hibernate s automatic versioning, you can ensure that an object loaded and modified in Session A can t overwrite the changes made to an object in Session B if the database transaction of Session B (manipulating the same object) was committed earlier than Session A s. In rare cases, and when handling critical data, you may use a pessimistic approach. By setting a pessimistic exclusive lock (using a LockMode) in

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[telnet] init: [telnet] unload: [telnet] [get] Getting: http://127.0.0.1:8080/manager/remove path=/antbook [telnet] [echo] OK - Removed application at context path /antbook [telnet] clean: [telnet] [delete] Deleting: /home/tomcat4/tomcat4.0/webapps/antbook.war [telnet] [delete] Deleting directory /home/tomcat4/tomcat4.0/webapps/antbook [telnet] install-files: [telnet] [copy] Copying 1 file to /home/tomcat4/tomcat4.0/webapps [telnet] [unzip] Expanding: /home/tomcat4/tomcat4.0/webapps/antbook.war into /home/tomcat4/tomcat4.0/webapps/antbook [telnet] deploy: [telnet] [get] Getting: http://127.0.0.1:8080/manager/install path=/antbook &war=file:///home/tomcat4/tomcat4.0/webapps/antbook [telnet] [echo] OK - Installed application at context path /antbook [telnet] default: [telnet] BUILD SUCCESSFUL [telnet] Total time: 35 seconds [telnet] [done]

and to pass incoming messages by invoking implemented message listener methods. Using the messageListenerInterface parameter of the @MessageDriven annotation is just one way to specify a message listener; we could have done the following instead:

Public Sub LoadDatabaseSettings( ) ' ----- Get some system-level values from ' database storage. Dim holdText As String On Error Resume Next ' ----- Get the default location. holdText = GetSystemValue("DefaultLocation") If (holdText = "") Then holdText = "-1" DefaultItemLocation = CInt(holdText) ' ----- Get the maximum number of search matches. holdText = GetSystemValue("SearchLimit") If (holdText = "") Then holdText = "-1" SearchMatchLimit = CInt(holdText) ' ----- Get the number of days to wait before ' charging fines. holdText = GetSystemValue("FineGrace") If (holdText = "") Then holdText = "-1" FineGraceDays = CInt(holdText) End Sub

// Adapter Pattern Example Judith Bishop // Sets up a CoolBook // This is D-J's as changed for the book class AdapterPattern { // class SpaceBookSystem {

What would you like to do now Move Attack Stop Run Panic You start Panicking and begin seeing things What would you like to do now Move Attack Stop Run You run around in your panic What would you like to do now Move Attack Stop Run

using using using using System; System.Collections.Generic; System.Linq; System.Text;

The Mediator pattern is used in many modern systems that reflect a send/receive protocol, such as list servers and chat rooms. But in addition to supporting human interaction, the Mediator pattern has an important role to play as a means of centralizing interconnections between objects. If objects communicate directly with each other, it becomes difficult to move or reuse any one of them. Mediators can be set up to collect and disseminate information about who is connected to whom. For example, within a given application, GUI controls (or widgets) can appear in different forms and locations in different dialog boxes, and they may interact in different ways. For instance, pressing a button in a Continue dialog box may result in very different behavior than pressing the same button in a Print dialog box. This information can be kept in a Mediator for each type of dialog box. The Mediator collects the controls it needs and arranges their interconnections. The Mediator pattern can have a performance impact on a system. Because all communication must go through the mediator, it can become a bottleneck. Almost