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Next, we create a savepoint here because we don t want to lose the work done in logging in case of an error: savepoint = conn.setSavepoint(); Then, we carry out the transaction of inserting three constant numbers 1, 2, and 3 in table t1: // our real transaction begins pstmt = conn.prepareStatement( insertStmt ) ; pstmt.setInt( 1, 1 ); pstmt.executeUpdate(); pstmt.setInt( 1, 2 ); pstmt.executeUpdate(); pstmt.setInt( 1, 3 ); pstmt.executeUpdate(); } In case of an error (which is indicated by a SQLException being thrown), we roll back to the savepoint, log a failure message, issue a commit, and rethrow the exception: catch (SQLException e) { // an error occurred, we roll back to our savepoint conn.rollback( savepoint ); // and log the error message _log( pstmtLog, "demo_savepoint", "Failed with error: " + e.getMessage()); // we commit the log data conn.commit(); // and throw the exception throw e; } Otherwise, in case of success, we log a success message and commit the transaction: // if we reach here - it means transaction was successful // so we log the "success" message _log( pstmtLog, "demo_savepoint", "Successfully ended at: " + new Date() ); // commit the changes to the database including the log message conn.commit(); } finally { // release JDBC JDBCUtil.close( JDBCUtil.close( JDBCUtil.close( } }
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resources in the finally clause. pstmtLog ); pstmt ); conn );
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At the end of the program is the definition of the _log() method, which simply logs a message into transaction_log: private static void _log( PreparedStatement pstmtLog, String txnName, String logMessage ) throws SQLException { pstmtLog.setString( 1, txnName ); pstmtLog.setString( 2, logMessage ); pstmtLog.executeUpdate(); } } If we execute the program DemoSavepoint for the first time, we ll get the following results: B:\code\book\ch04>java DemoSavepoint ora10g URL:jdbc:oracle:thin:@(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp) (PORT=1521)(HOST=rmenon-lap))(CONNECT_DATA=(SID=ora10g))) Let s look at the data in the tables t1 and transaction_log: benchmark@ORA10G> column txn_name format a14 benchmark@ORA10G> column log_message format a30 benchmark@ORA10G> select * from transaction_log; TXN_NAME LOG_MESSAGE -------------- -----------------------------demo_savepoint starting the txn to demo savep oints at: Wed Dec 29 00:26:16 PST 2004 demo_savepoint Successfully ended at: Wed Dec 29 00:26:16 PST 2004
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benchmark@ORA10G> select * from t1; X ---------1 2 3 As you can see, the transaction was successful, and the transaction start and success messages were logged in the table transaction_log as per the requirements.
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If we rerun the DemoSavepoint program, we ll get an error since we re trying to reinsert the same value in column x of table t1, which is a primary key: B:\code\book\ch04>java DemoSavepoint ora10g URL:jdbc:oracle:thin:@(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(PORT=1521)(HOST=rmeno n-lap))(CONNECT_DATA=(SID=ora10g))) Exception in thread "main" java.sql.SQLException: ORA-00001: unique constraint ( BENCHMARK.SYS_C005868) violated ... When we execute a select from the tables t1 and transaction_log, we get the following: benchmark@ORA10G> select * from transaction_log; TXN_NAME LOG_MESSAGE -------------- -----------------------------demo_savepoint Failed with error: ORA-00001: unique constraint (BENCHMARK.S YS_C005868) violated demo_savepoint starting the txn to demo savep oints at: Wed Dec 29 00:26:16 PST 2004 demo_savepoint Successfully ended at: Wed Dec 29 00:26:16 PST 2004 demo_savepoint starting the txn to demo savep oints at: Wed Dec 29 00:29:33 PST 2004
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benchmark@ORA10G> select * from t1; X ---------1 2 3 As you can see, the failure messages in transaction_log were also logged as required. Note that from a coding perspective, re-executing the statements before the savepoint may not be an issue, but doing so may be undesirable from a performance standpoint if these statements do a lot of work. Without savepoints, you may be potentially undoing a lot of work, only to redo it immediately afterward.
CHAPTER 4 TRANSACTIONS
Summary
In this chapter, we briefly covered transactions, the different transaction isolation levels available, and the transaction isolation levels supported by Oracle. We discussed why we should commit a transaction based on business need, rather than on the amount of resources the transaction consumes. As you learned, breaking your transaction into smaller chunks with intermittent commits can lead to compromised data integrity, increased code complexity, and an overall slower system. You learned the importance of always turning off autocommit and explicitly executing a commit or rollback as required to end your transaction. You also examined transaction savepoints as applicable to JDBC applications, and you saw a use case illustrating savepoints. In the next two chapters, we will look at statements that enable you to do all the work within your transaction.
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