Basically I have:

String query = "SELECT * FROM table WHERE UNIQUEID=? AND DIR IS NOT NULL AND NAME IS NOT NULL AND PAGETYPE IS NOT NULL";
DBConnect Database = new DBConnect();
Connection con = null;
PreparedStatement ps = null;
ResultSet rs=null;
try {
    con = Database.getcon();
    ps = con.prepareStatement(query);
    ps.setString(1, URI);
    rs=ps.executeQuery();
    if(rs.next()){
    }
} finally {
    if(ps != null)
        ps.close();
    if(rs != null)
        rs.close();
    if(con != null)
        con.close();
}
query = "SELECT COUNTCOMMENTS FROM videosinfos WHERE UNIQUEID=?";
try {
    con = Database.getcon();
    ps = con.prepareStatement(query); // Getting error here
    rs=ps.executeQuery();
    ps.setString(1, URI);
    rs=ps.executeQuery();
    if(rs.next()){
        comments = rs.getInt(1);
    }
} finally {
    if(ps != null)
        ps.close();
    if(rs != null)
        rs.close();
    if(con != null)
        con.close();
}

Note: The line I get the error, have a comment on it.

Connecting to Database:

public DBConnect(){
    try{
        Class.forName("com.mysql.jdbc.Driver");

        String unicode="useSSL=false&autoReconnect=true&useUnicode=yes&characterEncoding=UTF-8";
        con = DriverManager.getConnection("jdbc:mysql://localhost:15501/duckdb?"+unicode, "root", "_PWD");
        st = con.createStatement();
    }catch(Exception ex){
        System.out.println(ex.getMessage());
        System.out.println("couldn't connect!");
    }
}
public Connection getcon(){
    DBConnect condb = new DBConnect();
    Connection connect = con;
    return con;
}

But in compilation I get this error:

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: No operations allowed after connection closed. at sun.reflect.GeneratedConstructorAccessor18.newInstance(Unknown Source) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) at java.lang.reflect.Constructor.newInstance(Constructor.java:423) at com.mysql.jdbc.Util.handleNewInstance(Util.java:404) at com.mysql.jdbc.Util.getInstance(Util.java:387) at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:917) at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:896) at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:885) at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:860) at com.mysql.jdbc.ConnectionImpl.throwConnectionClosedException(ConnectionImpl.java:1246) at com.mysql.jdbc.ConnectionImpl.checkClosed(ConnectionImpl.java:1241) at com.mysql.jdbc.ConnectionImpl.prepareStatement(ConnectionImpl.java:4102) at com.mysql.jdbc.ConnectionImpl.prepareStatement(ConnectionImpl.java:4071) at duck.reg.pack.DBConnect.getitemfull_details(DBConnect.java:686) at duck.reg.pack.index.doPost(index.java:73) at javax.servlet.http.HttpServlet.service(HttpServlet.java:661) at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799) at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455) at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:748)

I'm Using Debian with MariaDB v10.x/Mysql

It's because of two reasons:

• Connection is closed after the execution of first PreparedStatement
• getcon uses an instance variable instead of local variable. Due to this, same conn variable (the one that was closed earlier) gets returned when getcon is called next time.

To fix this, getcon and DBConnect need to be modified to declare a local conn variable and return it (in fact, you don't need DBConnect at all), e.g.:

public Connection getcon(){
    try{
        Class.forName("com.mysql.jdbc.Driver");
        String unicode="useSSL=false&autoReconnect=true&useUnicode=yes&characterEncoding=UTF-8";
        return DriverManager.getConnection("jdbc:mysql://localhost:15501/duckdb?"+unicode, "root", "_PWD");
    }catch(Exception ex){
        System.out.println(ex.getMessage());
        System.out.println("couldn't connect!");
        throw new RuntimeException(ex);
    }
}