Nullpointexception using JSTL

Question

Hello.

Guys, I’m having a problem trying to pull up a list of users using a c:foreach. When requesting the list, a Nullpointexception is returned, which, although I understand the reason for the return, I cannot identify why. Testing with another code, I realize that my method that should return a list of users, identifies a certain difficulty when receiving the result of Resultlist in a List. The same warns through an exception, which cannot convert the User type to a List.

Below the DAO codes: package dao;

import model.Usuario;

import javax.inject.Inject;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.TypedQuery;
import javax.transaction.Transactional;
import java.util.List;

public class UsuarioDAO {

@Inject
Usuario usuario;

@PersistenceContext(unitName = "hemoapp-persistence-unit")
private EntityManager entityManager;

    @Transactional
    public void inserir(Usuario usuario) {

        if (usuario.getId_usuario() > 0) {
            entityManager.merge(usuario);
        } else {
            entityManager.persist(usuario);
        }
    }

    @Transactional
    public Usuario atualizar(Usuario usuario) {

        entityManager.merge(usuario);

        return usuario;
    }

    @Transactional
    public Usuario remover(String cpf) {

        this.usuario = verificarCpf(cpf);
        entityManager.remove(usuario);

        return this.usuario;
    }

    @Transactional
    public List<Usuario> getListaUsuarios(){

        String queryString = "select u.nome_usuario from Usuario u";

        TypedQuery<Usuario> query = entityManager.createQuery(queryString, 
        Usuario.class);

        //O problema na conversão dos tipos está aqui.
        List<Usuario> usuarios = query.getResultList();

        return usuarios;
    }
}

Code of the JSP page:

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>

<%@page pageEncoding="utf-8" %>
<%@include file="includes/cabecalho.jsp" %>
<%@include file="includes/main.html" %>

<jsp:useBean id="dao" class="dao.UsuarioDAO"/>

<c:forEach var="usuarios" items="${dao.usuarios}">
    ${usuarios.nome_usuario},
    ${usuarios.email_usuario}, 
    ${usuarios.cpf_usuario}
</c:forEach>

<%@include file="includes/rodape.html" %>

Stracktrace:

org.apache.jasper.JasperException: org.apache.jasper.el.JspELException: /relatorios.jsp(8,0) '${dao.listaUsuarios}' java.lang.NullPointerException
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:473)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:403)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:347)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at io.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:85)
at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:62)
at io.undertow.jsp.JspFileHandler.handleRequest(JspFileHandler.java:32)
at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36)
at org.wildfly.extension.undertow.security.SecurityContextAssociationHandler.handleRequest(SecurityContextAssociationHandler.java:78)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.servlet.handlers.security.SSLInformationAssociationHandler.handleRequest(SSLInformationAssociationHandler.java:131)
at io.undertow.servlet.handlers.security.ServletAuthenticationCallHandler.handleRequest(ServletAuthenticationCallHandler.java:57)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:46)
at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:64)
at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest(AuthenticationMechanismsHandler.java:60)
at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:77)
at io.undertow.security.handlers.NotificationReceiverHandler.handleRequest(NotificationReceiverHandler.java:50)
at io.undertow.security.handlers.AbstractSecurityContextAssociationHandler.handleRequest(AbstractSecurityContextAssociationHandler.java:43)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at org.wildfly.extension.undertow.security.jacc.JACCContextIdHandler.handleRequest(JACCContextIdHandler.java:61)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at org.wildfly.extension.undertow.deployment.GlobalRequestControllerHandler.handleRequest(GlobalRequestControllerHandler.java:68)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:292)
at io.undertow.servlet.handlers.ServletInitialHandler.access$100(ServletInitialHandler.java:81)
at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:138)
at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:135)
at io.undertow.servlet.core.ServletRequestContextThreadSetupAction$1.call(ServletRequestContextThreadSetupAction.java:48)
at io.undertow.servlet.core.ContextClassLoaderSetupAction$1.call(ContextClassLoaderSetupAction.java:43)
at org.wildfly.extension.undertow.security.SecurityContextThreadSetupAction.lambda$create$0(SecurityContextThreadSetupAction.java:105)
at org.wildfly.extension.undertow.deployment.UndertowDeploymentInfoService$UndertowThreadSetupAction.lambda$create$0(UndertowDeploymentInfoService.java:1508)
at org.wildfly.extension.undertow.deployment.UndertowDeploymentInfoService$UndertowThreadSetupAction.lambda$create$0(UndertowDeploymentInfoService.java:1508)
at org.wildfly.extension.undertow.deployment.UndertowDeploymentInfoService$UndertowThreadSetupAction.lambda$create$0(UndertowDeploymentInfoService.java:1508)
at org.wildfly.extension.undertow.deployment.UndertowDeploymentInfoService$UndertowThreadSetupAction.lambda$create$0(UndertowDeploymentInfoService.java:1508)
at org.wildfly.extension.undertow.deployment.UndertowDeploymentInfoService$UndertowThreadSetupAction.lambda$create$0(UndertowDeploymentInfoService.java:1508)
at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:272)
at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:81)
at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:104)
at io.undertow.server.Connectors.executeRootHandler(Connectors.java:326)
at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:812)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: org.apache.jasper.el.JspELException: /relatorios.jsp(8,0) '${dao.listaUsuarios}' java.lang.NullPointerException
at org.apache.jasper.el.JspValueExpression.getValue(JspValueExpression.java:123)
at org.apache.jsp.relatorios_jsp._jspx_meth_c_005fforEach_005f0(relatorios_jsp.java:209)
at org.apache.jsp.relatorios_jsp._jspService(relatorios_jsp.java:167)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:433)
... 43 more
Caused by: java.lang.NullPointerException
at dao.UsuarioDAO.getListaUsuarios(UsuarioDAO.java:51)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at javax.el.BeanELResolver.getValue(BeanELResolver.java:241)
at org.apache.jasper.el.JasperELResolver.getValue(JasperELResolver.java:110)
at com.sun.el.parser.AstValue.getValue(AstValue.java:139)
at com.sun.el.parser.AstValue.getValue(AstValue.java:203)
at com.sun.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:226)
at org.jboss.weld.el.WeldValueExpression.getValue(WeldValueExpression.java:50)
at org.apache.jasper.el.JspValueExpression.getValue(JspValueExpression.java:115)
... 48 more

Thank you.