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Extending iBATIS
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dataSource = new SimpleDataSource(map); } public DataSource getDataSource() { return dataSource; } }
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As we ve said before, more complex DataSource implementations might take a lot more work, but we hope that never becomes an issue you need to worry over. The final topic we will cover in extending iBATIS is customizing your transaction management.
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12.5 Customizing transaction management
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iBATIS offers a number of transaction options, as you ve read in earlier chapters.
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However, there is always room for customization with today s wide range of application servers and custom approaches to transaction management. From the outside, transactions seem simple, offering only a few functions: start, commit, roll back, and end. But on the inside, transactions are very complex and are one of the behaviors of application servers that tend to deviate from the standard. For that reason, iBATIS allows you to customize your own transaction management system. If you ve had any experience in the area, that statement probably sent shivers down your spine and so it should. Implementing a transaction manager correctly is a terribly difficult thing to do. For that reason, we won t even bother tackling a true implementation here. Instead, we ll discuss the interfaces in detail, which will help you gain a head start should you ever be tasked with implementing them. If you do want an example, iBATIS comes with three implementations: JDBC, JTA, and EXTERNAL. Table 12.4 summarizes these, in case you missed them in previous chapters.
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Table 12.4 Built-in transaction manager configurations Description Uses the transaction facilities provided by the JDBC Connection API Starts a global transaction, or joins an existing global transaction No-op implementation of commit and rollback, thus leaving commit and rollback to some external transaction manager
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JdbcTransactionConfig JtaTransactionConfig ExternalTransactionConfig
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Customizing transaction management
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In most cases, one of the options in table 12.4 should work for you. However, if your application server or transaction manager is nonstandard (or buggy), iBATIS provides interfaces for you to build your own transaction management adapter: TransactionConfig and Transaction. You will generally need to implement both to have a complete implementation, unless your situation allows you to reuse one of the Transaction classes from one of the other implementations.
12.5.1 Understanding the TransactionConfig interface
The TransactionConfig interface is a factory of sorts, but is mostly responsible for configuring the transaction facilities for the implementation. The interface is as follows:
public interface TransactionConfig { public void initialize(Properties props) throws SQLException, TransactionException; public Transaction newTransaction(int transactionIsolation) throws SQLException, TransactionException; public int getMaximumConcurrentTransactions(); public void setMaximumConcurrentTransactions(int max); public DataSource getDataSource(); public void setDataSource(DataSource ds); }
The first method is initialize(). As we ve seen with other parts of the framework that can be extended, this method is used to configure the transaction facilities. It takes a Properties instance as its only parameter, which can contain any number of configuration options. For example, the JTA implementation requires a UserTransaction instance that is retrieved from a JNDI tree. So one of the properties passed to the JTA implementation is the JNDI path to the UserTransaction it needs. Next is the newTransaction() method. This is a factory method for creating new instances of transactions. It takes an int parameter (unfortunately; it should be a type-safe enumeration) that describes the transaction isolation level within which the transaction should behave. Available transaction isolation levels are defined on the JDBC Connection class as constants as follows:
Extending iBATIS
TRANSACTION_READ_UNCOMMITTED TRANSACTION_READ_COMMITTED TRANSACTION_REPEATABLE_READ TRANSACTION_SERIALIZABLE TRANSACTION_NONE
Each of these is documented in the JDBC Connection API, and you can learn more in chapter 7. The important thing to note here is that if your transaction manager implementation does not support one or more of these, you should be sure to throw an exception to let the developer know. Otherwise, there could be unexpected consequences that are difficult for your users to debug. The next pair of methods are getDataSource() and setDataSource(). These methods describe a JavaBeans property for the DataSource associated with this TransactionConfig instance. Usually you won t have to do anything special with the DataSource, but it is provided here so that you can decorate it with additional behavior if you need to. Many transaction manager implementations wrap the DataSource and the Connection objects it provides, to add transaction related functionality to each of them. The final pair of methods makes up another JavaBeans property that allows the framework to configure a maximum number of concurrent transactions supported. Your implementation may or may not be configurable, but it is important to ensure that you throw an appropriate exception if the number set is too high for your system to handle.
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