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Exceptions Related to Data Access
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As if managing database resources correctly and in a flexible way weren t difficult enough, the problems with JDBC coding don t stop there. As mentioned earlier, SQLException causes leakage in applications for two important reasons: Passing SQLException to calling code is unacceptable and is clearly an example of a dataaccess detail that leaks into other parts of the application. It s a checked exception, and calling code has no way of responding in a meaningful way. The only sensible thing to do would be to declare SQLException in the throws clause of method signatures. This would have disastrous consequences, since your entire application would become dependent on the JDBC API. Wrapping SQLException in a generic unchecked exception such as GenericDataAccess Exception is equally fruitless. It solves the problem of the checked exception, but this approach is equally uninformative. You will still need to consider the error codes inside SQLException to understand the root cause of the exception. The problems are relevant to applications that use JDBC, as well as to applications that use tools and frameworks that depend on JDBC behind the scenes. The single most important question is, What has caused this error And to learn the cause, you need to look at the information inside SQLException. SQLException does contain two error codes that you can sometimes use to get information: the vendor code and the SQLState code. However, the exact same error condition will return different error codes for different database vendors. Therefore, application code that relies on these error codes is not portable. This constitutes a serious leakage related to data access. Another problem comes when debugging SQLException occurrences. Consider the SQLException retrieved from the following log trace: java.sql.SQLException: ORA-00942: table or view does not exist The underlying database is Oracle. We re being informed that a table or view does not exist, but we don t know the table name or the SQL statement that caused the exception. We can look at the stack trace to identify where the exception originated. But database administrators or other maintenance personnel who see this exception won t have much use for a stack trace if they either don t have access to the source code or don t have the Java skills to look at the source code and find the erroneous SQL statement. So, even if we understand the codes, we don t always get sufficient information to understand the cause of the problem, and an application that runs in a production environment and throws data-access exceptions that can t immediately be solved is a serious problem.
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CHAPTER 5 INTRODUCTION TO DATA ACCESS
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An article on the Oracle website titled Add Some Spring to Your Oracle JDBC Access (http://www. Tip oracle.com/technology/pub/articles/marx_spring.html) shows some interesting uses of the Spring
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data-access exception hierarchy.
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Database Transactions
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We ve already covered the problem of inappropriate connection life cycles and how this can prevent applications from working with database transactions. We concluded that we can t expect developers to come up with a solution that fixes this problem because it s too much work and very challenging to get it exactly right. But even if developers do manage to fix this problem, they will have solved only one piece of the database transaction puzzle. Data-access details can be leaked in three areas related to transactions, which apply to all dataaccess frameworks in Java, not just JDBC: Connection life cycles: As discussed earlier, data-access code must be able to transparently work with and without database transactions to avoid leakage. This leads us to the next point. Transaction demarcation: We need to decide where database transactions must be declared in the flow of the application. In other words, the boundaries need to be marked. However, application code should preferably not decide on this in order to keep maximum flexibility. Transaction management: Transactions need to be started and ended. In the case of JDBC, we need to call specific methods on the java.sql.Connection class. However, application code should not be aware of how this works; otherwise, we re faced with leakage of data-access details. Let s take a closer look at the areas of transaction demarcation and transaction management.
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