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Transaction management
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This chapter covers
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Conventional transaction management AspectJ-based transaction management using JDBC AspectJ-based transaction management using JTA
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Transaction management
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Consider the shopping cart example from chapter 5. When we add an item to the cart, we remove it from inventory. What would happen if the second operation failed The system would be in an inconsistent state, with the same item counted as being in the shopping cart and as part of inventory. To prevent this undesirable situation, we can execute both operations within a transaction. A transaction defines an atomic unit of work that ensures the system remains in a consistent state before and after its execution. Atomicity, consistency, isolation, and durability (ACID) are considered the four properties of a transaction. In this chapter, we focus on the first property atomicity because it is the most important and the hardest to achieve. Atomicity ensures that either all the operations within a transaction succeed as a single unit, or if one of the constituent operations fails, the system aborts the whole sequence of updates and rolls back any prior updates. If you want to read more on this topic, you can go to almost any JDBC or J2EE book (although the concept isn t limited to these areas). Transaction management is a crosscutting concern (by now, you probably saw it coming!). The operations under transaction control span multiple modules. Even in the simple case of the shopping cart example in chapter 5, the concern is spread across three classes: ShoppingCart, Inventory, and ShoppingCartOperator. In a real system, such operations touch many more classes and packages. The non-AOP solution causes the transaction management implementation to be integrated into all those modules, creating the usual set of problems associated with a lack of modularization of crosscutting concerns. The EJB application framework offers an elaborate and elegant system for transaction management. Transaction support for bean operations is expressed in a declarative form, in the deployment descriptor, separate from the core operation. This arrangement is similar in spirit to AOP separation of crosscutting concerns. But in many cases, you do not have the advantage of this built-in support. Using AspectJ in this situation extends the spirit of declarative transaction management to all your transaction management needs. In this chapter, we develop an aspect-oriented solution for transaction management in a simple JDBC-based system. This first version provides the essential concepts, and we will refine it as we go. We use just the basic commit and rollback functionality available with JDBC connections instead of using Java Transaction API (JTA) and transaction-aware connection objects. The JTA makes it possible to manage transactions that span multiple resources. Once you are familiar with the basic idea behind modularizing transaction management using AspectJ for a simple JDBC-based system, we briefly look at a template for a JTA-based transaction management system using AspectJ.
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Transaction management
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Figure 11.1 An overview of our banking example. The DatabaseHelper class is a result of refactoring to avoid duplicated code for creating a connection.
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11.1 Example: a banking system with persistence
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To illustrate the problem of transaction management and AspectJ-based solutions, let s develop a part of a banking system that must use transaction management. First, we will create the system without transaction management so that we can see which actions need to be within a transaction. We will also develop a logging aspect that allows us to observe the activities taking place. 11.1.1 Implementing the core concern In this section, we modify the banking system example introduced in chapter 10 to use JDBC for database persistence. Figure 11.1 shows our example system. The AccountJDBCImpl class is a concrete implementation of the Account interface from listing 10.1. Besides implementing all the methods specified in the interface, it contains a private method for setting balances. The InterAccountTransferSystem class, from listing 10.4, contains a single method for transferring an amount from one account to the other. Before we look at the JDBC implementation of this interface, let s build the helper class (listing 11.1) for creating database connections that we will use in our implementation.
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