Question

When using JDBC in Java, the generally accepted method of querying a database is to acquire a connection, create a statement from that connection, and then execute a query from that statement.

// load driver
Connection con = DriverManager.getConnection(..);
Statement stmt = con.createStatement();
ResultSet result = stmt.executeQuery("SELECT..");
// ...

However, I am unsure of how to treat a second query to the same database.

1. Can another query be executed safely on the same Statement object, or must another statement be created from the Connection object in order to execute another query?

2. If the same Statement object can be used for multiple queries, what is the purpose of the Statement class (since it would then make more sense for a Connection.executeQuery() method to exist)?

Answer 1

Yes you can reuse the Statement object, but the ResultSet objects returned by the executeQuery closes already opened resultsets.

See the javadoc for the explanation

By default, only one ResultSet object per Statement object can be open
at the same time. Therefore, if the reading of one ResultSet object is
interleaved with the reading of another, each must have been generated
by different Statement objects. All execution methods in the Statement
interface implicitly close a statment’s current ResultSet object if an
open one exists.

So the following occurs:

// load driver
Connection con = DriverManager.getConnection(..);
Statement stmt = con.createStatement();
ResultSet result = stmt.executeQuery("select ..");
// do something with result ... or not
ResultSet result2 = stmt.executeQuery("select ...");
// result is now closed, you cannot read from it anymore
// do something with result2
stmt.close(); // will close the resultset bound to it

For example you can find an open source implementation of Statement in the jTDS project.
In the Statement.executeQuery() method you can see a call to initialize() that closes all the resultsets already opened

protected void initialize() throws SQLException {
    updateCount = -1;
    resultQueue.clear();
    genKeyResultSet = null;
    tds.clearResponseQueue();
    // FIXME Should old exceptions found now be thrown instead of lost?
    messages.exceptions = null;
    messages.clearWarnings();
    closeAllResultSets();
}