Member Avatar for Xessa

Hello. I have a PreparedStatement/CallableStatement, a ResultSet and a Connection...

Now after execution finishes, i want to release my resources.


First I close my connection...
I also want to release my ResultSet and my PS

Will assigning null to my Connection object after closing the Connection object also assign null to my PS and ResultSet.

Do I need to assign null to my PS and RS.

myConnection.close();
myConnection = null;
myConnection.close();
myConnection = null;
myResultSet = null;
myPreparedStatement = null;

What is the difference between these two ?

Which is best for performance?

Does assigning null value to a connection also assigns null to PS and RS?

Will assigning null to the connection trigger the garbage collector?

Is code 2 better than code 1 when it comes releasing the resources?

Thanks...

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  • Latest Post Latest Post by JamesCherrill

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Generated

A short, practical follow-up for that builds on ’s point: assigning a variable to null does not close the JDBC resource or force the garbage collector. The reliable approach is to explicitly close ResultSet and PreparedStatement (and then Connection). Best practice is to let language constructs manage closing so you never forget.

Use try-with-resources (Java 7+) to close everything automatically:

try (Connection conn = dataSource.getConnection();
     PreparedStatement ps = conn.prepareStatement(sql);
     ResultSet rs = ps.executeQuery()) {

    while (rs.next()) {
        // process row
    }

} // rs, ps, conn are closed automatically (or returned to pool)

If you cannot use try-with-resources, close in a finally block and close in reverse order of creation, swallowing nothing silently (log exceptions):

ResultSet rs = null;
PreparedStatement ps = null;
Connection conn = null;
try {
    conn = dataSource.getConnection();
    ps = conn.prepareStatement(sql);
    rs = ps.executeQuery();
    // process
} finally {
    if (rs != null) try { rs.close(); } catch (SQLException e) { /* log */ }
    if (ps != null) try { ps.close(); } catch (SQLException e) { /* log */ }
    if (conn != null) try { conn.close(); } catch (SQLException e) { /* log */ }
}

Extra notes and gotchas:

  • Close order: ResultSet -> Statement -> Connection. Closing the Connection may close children on some drivers, but that behavior is not guaranteed across all drivers—explicitly close each.
  • Connection pools: calling close() typically returns the connection to the pool; setting the reference to null does nothing useful for pooling.
  • Performance: assigning null rarely improves performance; prompt close() reduces resource pressure.
  • Always log or handle SQLExceptions from close(); try-with-resources handles suppressed exceptions cleanly.

In short: close resources explicitly (prefer try-with-resources), don’t rely on null assignments, and be mindful of pooling and driver differences.

Member Avatar for JamesCherrill

To understand this you need to be clear about the difference between an object and a reference variable.
myConnection and myResultSet are not objects, they are reference variables. Either they hold a reference to an object of the appropriate type, or their value is null.
There is no way you can make an object equal to null. Objects are created by the "new" statement, and continue to exist until there are no references left to them. So when you say myConnection = null, you are making the variable myConnection not refer to the connection object anymore. This does not trigger garbage collection, which happens whenever the garbage collecter choses according to its own very complex rules.
Whether that causes the object to garbage collected or not dependes on whether there are any other active references to it anywhere in your code OR in the Java API objects that you have created/used.
So what youi need to do is to ensure that every variable you have that refers to these objects (or refers to another object that has a reference to them) either goes out of scope or is set to null. You will no longer have anyway to access these objects, and some time later ther garbage collecter will garbage collect them.
In general, worrying too much about garbage collection in Java is a waste of time; trying to optimise it doubly so.

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