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An Example of Using Savepoints
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Savepoints have limited use in a JDBC application. This is because, in most cases, the requirement is to either commit the entire transaction or undo the effect of the entire transaction. However, savepoints may come in handy in some cases. Consider the requirement to log a transaction s identifying name and its success or failure in a table, transaction_log, which is created as follows: benchmark@ORA10G> create table transaction_log 2 ( 3 txn_name varchar2(15), 4 log_message varchar2(500) 5 ); Table created. If a transaction is successful, we want to insert a success message in this table. If a transaction fails, we want to log a message to the effect that the transaction has failed with a message indicating what caused the failure. Before any transaction, we also want to log a message indicating when the transaction began.
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Consider the following pseudo code that tries to do this without savepoints: Step 1: Log the message "beginning transaction at <timestamp>". Step 2: Do the transaction. If there is an error then go to step 3. Otherwise go to Step 4. Step 3: Since a rollback will undo the changes done by Step 1, we do the following: Step 3a. Redo the step 1 hopefully, we have saved the original timestamp. Step 3b. Log the transaction failure message. Step 3c. Commit and raise an exception. Step 4: Log the transaction success message. Step 5: Commit the transaction. Notice how in the case of a transaction failure, we need to redo the work done by step 1. Now if we use savepoints, the pseudo code changes to the following: Step 1: Log the message "beginning transaction at <timestamp>". Step 2: Create a savepoint save_point1. Step 3: Do the transaction. If there is an error then go to Step 4. Otherwise go to Step 5. Step 4a. Roll back to save_point1. Step 4b. Log the transaction failure message. Step 4c. Commit and raise an exception. Step 5: Log the transaction success message. Step 6: Commit the transaction. In case of success, the basic steps are the same. In case of an error in the transaction, we can simply roll back to the savepoint, log a failure message, commit, and raise an exception. Essentially, we did not have to redo the work done in step 1 because we used a savepoint. Let s see the same example in a working JDBC program. The transaction involves inserting three numbers into the table t1 created as follows: benchmark@ORA10G> create table t1 2 ( 3 x number primary key 4 ); Table created. Notice that column x in table t1 is a primary key. Thus, we cannot insert the same number twice in this table. The following DemoSavepoint program shows how to execute this simple transaction and use a savepoint to implement the pseudo code presented earlier. The class definition begins with the imports before the definition of the main() method: /*This program demonstrates how to use the Savepoint feature * that has been introduced in JDBC 3.0. * COMPATIBLITY NOTE: * runs successfully on 10.1.0.2.0 and 9.2.0.1.0 */ import java.util.Date; import java.sql.SQLException; import java.sql.Savepoint;
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import java.sql.PreparedStatement; import java.sql.Connection; import book.util.JDBCUtil; public class DemoSavepoint { public static void main(String args[]) throws SQLException { Along with the usual connection variable, we also declare two PreparedStatement variables: one to insert the log statement and the other to insert into table t1 as part of the main transaction. We also declare a Savepoint variable: Connection conn = null; PreparedStatement pstmtLog = null; PreparedStatement pstmt = null; Savepoint savepoint = null; The following insert statement would be used to log messages into the transaction_log table: String insertTxnLogStmt = "insert into transaction_log(txn_name, log_message) " + "values( , )"; The following insert statement would be used to insert records into table t1: String insertStmt = "insert into t1(x) values( )"; try { try { Inside the try catch block, we first get the connection: conn = JDBCUtil.getConnection("benchmark", "benchmark", args[0]); Then we prepare the statement to insert the log messages and invoke the method _log() (defined later), which simply inserts a given transaction name and log message into the transaction_log table: pstmtLog = conn.prepareStatement( insertTxnLogStmt ) ; _log( pstmtLog, "demo_savepoint", "starting the txn to demo savepoints at: " + new Date() );
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