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[Java教程]JDBC Tutorials: Commit or Rollback transaction in finally block


http://skeletoncoder.blogspot.com/2006/10/jdbc-tutorials-commit-or-rollback.html

 

JDBC Tutorials: Commit or Rollback transaction in finally block

 

In most of JDBC books, the transaction management idiom that is followed is, after executing the update statements commit, and if an SQLException is thrown, rollback.
That is,


Connection con = null;
try{
con = //...
con.setAutoCommit(false);

Statement stmt1 = ...
stmt1.executeUpdate();

// Some operations

Statement stmt2 = ...
stmt2.executeUpdate();

con.commit();
con.setAutoCommit(true);
}catch(SQLException e){
if(con!=null){
try{
con.rollback();
}catch(SQLException e){
// Log the error...
}
}
}


The similar structure is followed in the JDBC(TM) API
Tutorial and Reference from the Sun Microsystems. Have a look at theTransactions Tutorial and the Sample code provided.

There is a severe problem with this way of commiting and rollback. The problem is we are handling only the SQLException. What will happen if a RuntimeException occured after executing the first update statement but beforethe second update statement?

The transaction is opened, but neither commited nor rolled back. This will leave the data integrity into trouble. If we are reusing the same connection (as in most cases), and we commit the transaction in the next statements, we are into serious trouble. We have inconsitent data.

What is the solution?
Catch Exception instead of SQLException
A simpler and not recommended solution is, catch all the execeptions, including RuntimeException. Even now, what if an Error is thrown, say OutOfMemoryError or some VirtualMachineError or something else? What ever happens in the code, we should either the database should be committed or rolledback. So, the worst thing is we should catch the Throwable class, instead of Exception.

Doesn't this look awkward,Whenever we use transactions we should catch a Throwable class or atleast Exception class?

Use finally block
A clean solution and yet simple solution is, use finally block. Since it is always guaranteed that the finally block will be executed even when any Exception is thrown or even when the method is returned.



Connection con = null;
boolean success = false;
try{
con = //...
con.setAutoCommit(false);

Statement stmt1 = ...
stmt1.executeUpdate();

// Some operations

Statement stmt2 = ...
stmt2.executeUpdate();

success = true;

}catch(SQLException e){
success = false;
}finally{
if(con!=null){
try{
if(success){
con.commit();
con.setAutoCommit(true);
}else{
con.rollback();
}
}catch(SQLException e){
// Log the error...
}
}
}