java - 为什么我的实体管理器返回空结果列表？

Question

我的网络应用程序遇到了相当大的困境。

对于我的学校项目，我们必须从经典的 JDBC 集成迁移到 JPA 集成。就我自己而言，我决定使用 Hibernate JPA 框架。我已经在 SessionBean 内的 main 中尝试过，它在那里工作。但每当我将它集成到 Web Servlet 中时，我注意到它返回空列表。我尝试用 System.out.println() 显示列表的大小.

无论如何，我认为问题可能出在我的 persistence.xml 上，更具体地说，缺少 <jta-data-source>something here</jta-data-source>就在其中。

这是我的 persistence.xml ，也许你可以看到我遇到问题的地方:

<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence              http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
  <persistence-unit name="PERSISTENCE" transaction-type="JTA">
    <description>Hibernate JPA Configuration Example</description>
    <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
    <class>JavaBeans.Employee</class>
    <class>JavaBeans.User</class>
    <exclude-unlisted-classes>false</exclude-unlisted-classes>
    <properties>
      <property name="javax.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver"/>
      <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/JEEPRJ?serverTimezone=Europe/Paris"/>
      <property name="javax.persistence.jdbc.user" value="jee"/>
      <property name="javax.persistence.jdbc.password" value="jee"/>
      <property name="javax.persistence.jdbc.serverTimezone" value="Europe/Paris"/>
      <property name="javax.persistence.jdbc.useSSL" value="false"/>
      <property name="hibernate.show_sql" value="true"/>
      <property name="hibernate.hbm2ddl.auto" value="update"/>
      <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
      <property name="hibernate.transaction.jta.platform" value="org.hibernate.service.jta.platform.internal.SunOneJtaPlatform"/>
    </properties>
  </persistence-unit>
</persistence>

这是我的员工 session Bean:

package SessionBeans;

import JavaBeans.Employee;

import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import java.util.List;
import javax.persistence.Query;

/**
 *
 */
@Stateless
public class EmployeeSB {

    @PersistenceContext(name="PERSISTENCE")
    EntityManager em;

    public List<Employee> getAllEmployees(){

        String query = "SELECT e FROM Employee e ";
        Query q  = em.createQuery(query);
        List<Employee> employees = q.getResultList();
        if(employees!=null){
            System.out.println("it's not null list size : " + q.getResultList().size());
            for(Employee emp:employees){
                System.out.println("id  : " + emp.getId());
            }
            return employees;
        }
        System.out.println("it's null");
        return employees;
    }

还有我的 Employee 类:

@Entity
@Table(name = "EMPLOYE")
public class Employee implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Integer id;
    @Size(max = 255)
    @Column(name = "NOM")
    public String nom;
    @Size(max = 255)
    @Column(name = "PRENOM")
    public String prenom;
    @Size(max = 255)
    @Column(name = "TELDOMICILE")
    public String telDomicile;
    @Size(max = 255)
    @Column(name = "TELPORTABLE")
    public String telPortable;
    @Size(max = 255)
    @Column(name = "TELPRO")
    public String telPro;
    @Size(max = 255)
    @Column(name = "ADRESSE")
    public String adresse;
    @Size(max = 255)
    @Column(name = "CODEPOSTAL")
    public String codePostal;
    @Size(max = 255)
    @Column(name = "VILLE")
    public String ville;
    // @Pattern(regexp="[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?", message="Invalid email")//if the field contains email address consider using this annotation to enforce field validation
    @Size(max = 255)
    @Column(name = "EMAIL")
    public String email;


    public Employee() {

    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getNom() {
        return nom;
    }

    public void setNom(String nom) {
        this.nom = nom;
    }

    public String getPrenom() {
        return prenom;
    }

    public void setPrenom(String prenom) {
        this.prenom = prenom;
    }

    public String getTelDomicile() {
        return telDomicile;
    }

    public void setTelDomicile(String telDomicile) {
        this.telDomicile = telDomicile;
    }

    public String getTelPortable() {
        return telPortable;
    }

    public void setTelPortable(String telPortable) {
        this.telPortable = telPortable;
    }

    public String getTelPro() {
        return telPro;
    }

    public void setTelPro(String telPro) {
        this.telPro = telPro;
    }

    public String getAdresse() {
        return adresse;
    }

    public void setAdresse(String adresse) {
        this.adresse = adresse;
    }

    public String getCodePostal() {
        return codePostal;
    }

    public void setCodePostal(String codePostal) {
        this.codePostal = codePostal;
    }

    public String getVille() {
        return ville;
    }

    public void setVille(String ville) {
        this.ville = ville;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }





    public Employee(String nom, String prenom, String telDomicile, String telPortable, String telPro, String adresse, String codePostal, String ville, String email) {
        this.nom = nom;
        this.prenom = prenom;
        this.telDomicile = telDomicile;
        this.telPortable = telPortable;
        this.telPro = telPro;
        this.adresse = adresse;
        this.codePostal = codePostal;
        this.ville = ville;
        this.email = email;
    }



    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Employee)) {
            return false;
        }
        Employee other = (Employee) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "JavaBeans.Employee[ id=" + id + " ]";
    }
}

(别介意法语哈哈)。

无论如何，如果有人明白为什么实体管理器返回空列表，那将对我有很大帮助，并使我明白我在哪里犯了愚蠢的错误。

非常感谢大家，祝你有美好的一天，

票价。

Answer 1

如果您将上述 session bean 的相同代码复制/粘贴到 servlet 中，请注意事务管理。由于 session bean 将事务管理为其默认行为，因此您不必在 bean 中管理它们。然而 servlet 没有这样的行为，实现它们是您自己的责任。

请注意，在属于 View 层的 servlet 内实现与数据库相关的作业将是一种反模式。更好的方法是使用 session bean 来实现此功能，并在 View 层内调用它们。