[java] Must JDBC Resultsets and Statements be closed separately although the Connection is closed afterwards?

It is said to be a good habit to close all JDBC resources after usage. But if I have the following code, is it necessary to close the Resultset and the Statement?

Connection conn = null;
PreparedStatement stmt = null;
ResultSet rs = null;
try {
    conn = // Retrieve connection
    stmt = conn.prepareStatement(// Some SQL);
    rs = stmt.executeQuery();
} catch(Exception e) {
    // Error Handling
} finally {
    try { if (rs != null) rs.close(); } catch (Exception e) {};
    try { if (stmt != null) stmt.close(); } catch (Exception e) {};
    try { if (conn != null) conn.close(); } catch (Exception e) {};
}

The question is if the closing of the connection does the job or if it leaves some resources in use.

I'm now using Oracle with Java. Here my point of view :

You should close ResultSet and Statement explicitly because Oracle has problems previously with keeping the cursors open even after closing the connection. If you don't close the ResultSet (cursor) it will throw an error like Maximum open cursors exceeded.

I think you may encounter with the same problem with other databases you use.

Here is tutorial Close ResultSet when finished:

Close ResultSet when finished

Close ResultSet object as soon as you finish working with ResultSet object even though Statement object closes the ResultSet object implicitly when it closes, closing ResultSet explicitly gives chance to garbage collector to recollect memory as early as possible because ResultSet object may occupy lot of memory depending on query.

ResultSet.close();

Some convenience functions:

public static void silentCloseResultSets(Statement st) {
    try {
        while (!(!st.getMoreResults() && (st.getUpdateCount() == -1))) {}
    } catch (SQLException ignore) {}
}
public static void silentCloseResultSets(Statement ...statements) {
    for (Statement st: statements) silentCloseResultSets(st);
}

With Java 6 form I think is better to check it is closed or not before close (for example if some connection pooler evict the connection in other thread) - for example some network problem - the statement and resultset state can be come closed. (it is not often happens, but I had this problem with Oracle and DBCP). My pattern is for that (in older Java syntax) is:

try {
    //...   
    return resp;
} finally {
    if (rs != null && !rs.isClosed()) {
        try {
            rs.close();
        } catch (Exception e2) { 
            log.warn("Cannot close resultset: " + e2.getMessage());
        }
    }
    if (stmt != null && !stmt.isClosed()) {
        try {
            stmt.close();
        } catch (Exception e2) {
            log.warn("Cannot close statement " + e2.getMessage()); 
        }
    }
    if (con != null && !conn.isClosed()) {
        try {
            con.close();
        } catch (Exception e2) {
            log.warn("Cannot close connection: " + e2.getMessage());
        }
    }
}

In theory it is not 100% perfect because between the the checking the close state and the close itself there is a little room for the change for state. In the worst case you will get a warning in long. - but it is lesser than the possibility of state change in long run queries. We are using this pattern in production with an "avarage" load (150 simultanous user) and we had no problem with it - so never see that warning message.

Doesn't matter if Connection is poolable or not. Even poolable connection has to clean before returning to the pool.

"Clean" usually means closing resultsets & rolling back any pending transactions but not closing the connection. Otherwise pooling looses its sense.

As far as I remember, in the current JDBC, Resultsets and statements implement the AutoCloseable interface. That means they are closed automatically upon being destroyed or going out of scope.

The correct and safe method for close the resources associated with JDBC this (taken from How to Close JDBC Resources Properly – Every Time):

Connection connection = dataSource.getConnection();
try {
    Statement statement = connection.createStatement();

    try {
        ResultSet resultSet = statement.executeQuery("some query");

        try {
            // Do stuff with the result set.
        } finally {
            resultSet.close();
        }
    } finally {
        statement.close();
    }
} finally {
    connection.close();
}

Java 1.7 makes our lives much easier thanks to the try-with-resources statement.

try (Connection connection = dataSource.getConnection();
    Statement statement = connection.createStatement()) {
    try (ResultSet resultSet = statement.executeQuery("some query")) {
        // Do stuff with the result set.
    }
    try (ResultSet resultSet = statement.executeQuery("some query")) {
        // Do more stuff with the second result set.
    }
}

This syntax is quite brief and elegant. And connection will indeed be closed even when the statement couldn't be created.

If you want more compact code, I suggest using Apache Commons DbUtils. In this case:

Connection conn = null;
PreparedStatement stmt = null;
ResultSet rs = null;
try {
    conn = // Retrieve connection
    stmt = conn.prepareStatement(// Some SQL);
    rs = stmt.executeQuery();
} catch(Exception e) {
    // Error Handling
} finally {
    DbUtils.closeQuietly(rs);
    DbUtils.closeQuietly(stmt);
    DbUtils.closeQuietly(conn);
}

I created the following Method to create reusable One Liner:

public void oneMethodToCloseThemAll(ResultSet resultSet, Statement statement, Connection connection) {
    if (resultSet != null) {
        try {
            if (!resultSet.isClosed()) {
                resultSet.close();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
    if (statement != null) {
        try {
            if (!statement.isClosed()) {
                statement.close();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    if (connection != null) {
        try {
            if (!connection.isClosed()) {
                connection.close();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

I use this Code in a parent Class thats inherited to all my classes that send DB Queries. I can use the Oneliner on all Queries, even if i do not have a resultSet.The Method takes care of closing the ResultSet, Statement, Connection in the correct order. This is what my finally block looks like.

finally {
    oneMethodToCloseThemAll(resultSet, preStatement, sqlConnection);
}

From the javadocs:

When a Statement object is closed, its current ResultSet object, if one exists, is also closed.

However, the javadocs are not very clear on whether the Statement and ResultSet are closed when you close the underlying Connection. They simply state that closing a Connection:

Releases this Connection object's database and JDBC resources immediately instead of waiting for them to be automatically released.

In my opinion, always explicitly close ResultSets, Statements and Connections when you are finished with them as the implementation of close could vary between database drivers.

You can save yourself a lot of boiler-plate code by using methods such as closeQuietly in DBUtils from Apache.

No you are not required to close anything BUT the connection. Per JDBC specs closing any higher object will automatically close lower objects. Closing Connection will close any Statements that connection has created. Closing any Statement will close all ResultSets that were created by that Statement. Doesn't matter if Connection is poolable or not. Even poolable connection has to clean before returning to the pool.

Of course you might have long nested loops on the Connection creating lots of statements, then closing them is appropriate. I almost never close ResultSet though, seems excessive when closing Statement or Connection WILL close them.