org.postgresql.util.PSQLException: This connection has been closed

Boa tarde, senhores!

Seguindo a apostila “FJ_21 - Java para Desenvolvimento Web” da Caelum, travei num exercício onde explica como criar um @WebFilter para conexões com banco de dados. Tenho um formulário para gravar dados de contato, porém quando é enviado os dados pelo request e entra no filtro e logo apos a execução do metodo doFilter da classe FilterChain, o fluxo nao passa para a servlet controladora e sim para a proxima linha do metodo doFilter da classe que implementa Filter que é o fechamento da conexão com o banco de dados, dai onde apresenta a exceção no console:

set 07, 2016 12:46:26 PM org.apache.catalina.core.StandardWrapperValve invoke

GRAVE: Servlet.service() for servlet [br.com.caelum.mvc.servlet.ControllerServlet] in context with path [/fj21-agenda] threw exception [A lógica de negócios causou uma exceção] with root cause

org.postgresql.util.PSQLException: This connection has been closed.

at org.postgresql.jdbc.PgConnection.checkClosed(PgConnection.java:796)

at org.postgresql.jdbc.PgConnection.prepareStatement(PgConnection.java:1633)

at org.postgresql.jdbc.PgConnection.prepareStatement(PgConnection.java:420)

at br.com.caelum.servlet.dao.ContatoDao.doConsultarContato(ContatoDao.java:57)

at br.com.caelum.mvc.logica.ListaContatosLogic.executa(ListaContatosLogic.java:21)

at br.com.caelum.mvc.servlet.ControllerServlet.service(ControllerServlet.java:27)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:747)

at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:485)

at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:410)

at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:337)

at br.com.caelum.mvc.servlet.ControllerServlet.service(ControllerServlet.java:30)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

at br.com.caelum.agenda.filtro.FiltroTempoDeExecucao.doFilter(FiltroTempoDeExecucao.java:36)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

at br.com.caelum.agenda.filtro.FiltroConexao.doFilter(FiltroConexao.java:45)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:218)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)

at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)

at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:442)

at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1082)

at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:623)

at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:318)

at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)

at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)

at java.lang.Thread.run(Unknown Source)

Gostaria de informações para entender o fluxo corretamente.

Implementação do filtro:

@WebFilter("/*")
public class FiltroConexao implements Filter {

    private Connection connection;

    @Override
    public void destroy() {
        // TODO Auto-generated method stub

    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-generated method stub

    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain filter)
            throws IOException, ServletException {

        try {

            connection = ConnectionFactory.getConnection();

            // Pendurando a connection na requisição
            request.setAttribute("conexao", connection);

            /**De acordo com a explicação, depois da execução desse metodo, o fluxo da requisição 
            continuaria, nesse caso para a classe controladora, e depois no final de toda a
            requisição, iria executar o fechamento com o banco de dados.*/
            filter.doFilter(request, response); 

            connection.close();

        } catch (SQLException | ClassNotFoundException ex) {
            throw new ServletException(ex);
        }

    }

}

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