What is statement has been closed, no further internal information availabl

Question

some times my search function get the following error, but ususally reflash the page, and the function work again )
Apr 6, 2009 4:10:05 PM Deferment.showResult doPost
SEVERE: null
java.lang.NullPointerException
at Deferment.showResult.doPost(showResult.java:92)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.servlets.InvokerServlet.serveRequest(InvokerServlet.java:402)
JDBC error:org.apache.tomcat.dbcp.dbcp.DelegatingCallableStatement with address: "com.mysql.jdbc.JDBC4CallableStatement@6e10d8: statement has been closed, no further internal information available" is closed.
java.sql.SQLException: org.apache.tomcat.dbcp.dbcp.DelegatingCallableStatement with address: "com.mysql.jdbc.JDBC4CallableStatement@6e10d8: statement has been closed, no further internal information available" is closed.
at org.apache.tomcat.dbcp.dbcp.DelegatingStatement.checkOpen(DelegatingStatement.java:137)
at org.apache.tomcat.dbcp.dbcp.DelegatingPreparedStatement.executeQuery(DelegatingPreparedStatement.java:91)
at Deferment.showResult.getUnderResult(showResult.java:346)
at Deferment.showResult.doPost(showResult.java:91)
at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:565)
at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1509)
at java.lang.Thread.run(Thread.java:619)

Answer 1

Are you still storing Connection objects in fields? Quit doing that. Servlets are automatically multithreaded, and one call will overwrite another call's variables while the first call is still trying to use them. Servlets should rarely have any kinds of fields (except maybe static final constants.)
The getConnection() method in reply 5 is still weird. A return statement inside finally is not a good thing to do; it behaves uncleanly when an exception occurs. And DeleteStudent() in reply 7 keeps getting new connections from the pool without returning them.
Here is some boilerplate code (does not necessarily compile):
class YourMeatAndPotatoesClass
    void whatever()
        Connection connection = null;
        PreparedStatement stmt = null;
        ResultSet res = null;
        try {
            String sql = "select ...";
            connection = SqlUtil.getConnection();
            stmt = connection.prepareStatement(sql);
            res = stmt.executeQuery();
            while (res.next()) {
        } finally {
            SqlUtil.close(connection, stmt, res);
public class SqlUtil
    public Connection getConnection()
        throws SQLException
        return pool.getConnection();
    public static void close(Connection c, Statement stmt, ResultSet res)
        throws SQLException
        try {
            if (res != null)
                res.close();
        } catch (SQLException e) {
            log.error(e, "error closing database result set");
            throw e;
        } finally {
            try {
                if (stmt != null)
                    stmt.close();
            } catch (SQLException e) {
                log.error(e, "error closing database statement");
                throw e;
            } finally {
                try {
                    if (c != null)
                        c.close(); // Maybe replace with "return to pool" call depending on your pool implementation
                } catch (SQLException e) {
                    log.error(e, "error closing database connection");
                    throw e;
}