FetchType.LAZY得到空OpenJPA中

Question

我有應用程序使用OpenJPA和實體User和UserHasRoles：

public class User implements Serializable { 
@Id 
@GeneratedValue 
@Basic(optional = false) 
@Column(name = "id_user") 
private Integer idUser; 
@Size(max = 8) 
@Column(name = "login") 
private String login; 
@Size(max = 64) 
@Column(name = "password") 
private String password; 
@OneToMany(cascade = CascadeType.ALL, mappedBy = "user", fetch=FetchType.LAZY) 
private List<UserHasRoles> userHasRolesList; 

//getters, setters ... 
} 

public class UserHasRoles implements Serializable { 
@EmbeddedId 
protected UserHasRolesPK userHasRolesPK; 
@Basic(optional = false) 
@NotNull 
@Size(min = 1, max = 45) 
@Column(name = "role_name") 
private String roleName; 
@JoinColumn(name = "id_user", referencedColumnName = "id_user", insertable = false, updatable = false) 
@ManyToOne(optional = false) 
private User user; 

//getters, setters ... 
} 

當我運行這段代碼我得到NullPointerException異常：

User user = new User(); //utworzenie użytkownika i dodanie do bazy  
user.setLogin(registerLog); 
user.setPassword(registerPass); 
userFacade.create(user); 

UserHasRoles userHasRoles = new UserHasRoles(user.getIdUser(), new Short("2")); 
userHasRoles.setRoleName("admin"); 
userHasRoles.setUser(user); 
userHasRolesFacade.create(userHasRoles); 

ArrayList<UserHasRoles> list = new ArrayList<UserHasRoles>();  //1 
list.add(userHasRoles);            //1  
user.setUserHasRolesList(list);          //1 
userFacade.edit(user);            //1 


User u = userFacade.find(user.getIdUser()); 
List l = u.getUserHasRolesList(); 

l.get(0); //i get there NullPointerException, because l is null 

爲什麼堅持和合並對象後（在上面註釋中添加UserHasRoles - 1的列表）當我找到對象時，我得到空？

---

我的persistence.xml：只有

<?xml version="1.0" encoding="UTF-8"?> 
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"> 
<persistence-unit name="CollDocPU" transaction-type="JTA"> 
<jta-data-source>java:openejb/Resource/myDatabase</jta-data-source> 
<non-jta-data-source>java:openejb/Resource/myDatabaseUnmanaged</non-jta-data-source> 
<provider>org.apache.openjpa.persistence.PersistenceProviderImpl</provider> 
<class>model.entity.Course</class> 
<class>model.entity.CourseHasLecturer</class> 
<class>model.entity.File</class> 
<class>model.entity.Lecturer</class> 
<class>model.entity.Mail</class> 
<class>model.entity.Roles</class> 
<class>model.entity.Solution</class> 
<class>model.entity.Student</class> 
<class>model.entity.StudentAddSolution</class> 
<class>model.entity.StudentAtCourse</class> 
<class>model.entity.Task</class> 
<class>model.entity.User</class> 
<class>model.entity.UserHasRoles</class> 
<exclude-unlisted-classes>false</exclude-unlisted-classes> 
<properties> 
    <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:11080/me?zeroDateTimeBehavior=convertToNull"/> 
    <property name="javax.persistence.jdbc.password" value="pass"/> 
    <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/> 
    <property name="javax.persistence.jdbc.user" value="me"/> 
    <property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema(ForeignKeys=true)"/> 
</properties> 

Answer 1

問題在於物件的生命週期。所有實體必須重新連接到持久化上下文。

要解決它，我不得不做出getUserHasRolesList方法在userFacade：

public List<UserHasRoles> getUserHasRolesList(User us){ 

    us = em.merge(us);  //here i re-attached entity 

    return us.getUserHasRolesList(); 
} 

同樣的問題在這裏：OpenJPA - lazy fetching does not work這裏：What's the lazy strategy and how does it work?

Answer 2

FetchType.LAZY加載列表，當你調用方法getUserHasRolesList()和這種方法似乎是在 「用戶」 類

@OneToMany(cascade = CascadeType.ALL, mappedBy = "user", fetch=FetchType.LAZY) 
private List<UserHasRoles> userHasRolesList; 

您可以將提取更改爲EAGER，然後將lis t將與其餘的田地一起裝載。