Jsf EntityManager上的瞬态导致异常

我正在为我的JSF应用程序使用视图范围的托管bean。由于视图范围的bean需要序列化，所以我实现了Serialiazable。但问题在于EntityManager。我尝试使用transient关键字绕过它，但是我得到了以下异常

我使用了以下bean文件：

@ManagedBean
@ViewScoped
public class StatusBean2 implements Serializable  {

    /**
     * 
     */
    private static final long serialVersionUID = -7528998562821856315L;
    private transient EntityManager em;

    public StatusBean2() {
        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("FreeBird");
        em = emf.createEntityManager();
        FacesContext context = FacesContext.getCurrentInstance();
        HttpSession session = (HttpSession) context.getExternalContext()
                .getSession(true);
        int toIndex = 5;
       session.setAttribute("toIndex", toIndex);

    }
    public List<Status> getStatusList() {
        FacesContext context = FacesContext.getCurrentInstance();
        HttpSession session = (HttpSession) context.getExternalContext()
                .getSession(true);
        User user = (User) session.getAttribute("userdet");
        Query query = em.createQuery("SELECT s FROM Status s WHERE s.email='"//  line no 49
                + user.getEmail() + "' ORDER BY s.timeMillis desc",
                Status.class);
        List<Status> results = query.getResultList();
        Query query1 = em.createQuery("SELECT f FROM Friend f WHERE f.email='"
                + user.getEmail() + "'", Friend.class);
        List<Friend> results1 = query1.getResultList();
        Iterator<Friend> it = results1.listIterator();
        while (it.hasNext()) {
            String email = it.next().getFriendEmail();
            Query query2 = em.createQuery(
                    "SELECT s FROM Status s WHERE s.email='" + email
                            + "' ORDER BY s.timeMillis desc", Status.class);
            List<Status> results2 = query2.getResultList();
            results.addAll(results2);

        }
        Collections.sort(results);

        int toIndex = (int) session.getAttribute("toIndex");
        List<Status> subList = results.subList(0, toIndex);

        return subList;

    }
}

---

反序列化对象时不调用构造函数，因此

transient

字段保留为

null

您需要初始化字段：

@ManagedBean
@ViewScoped
public class StatusBean2 implements Serializable  {
    private static final long serialVersionUID = -7528998562821856315L;
    private transient EntityManager em;
    private static final EntityManagerFactory emf = Persistence.createEntityManagerFactory("FreeBird");

    public StatusBean2() {
        FacesContext context = FacesContext.getCurrentInstance();
        HttpSession session = (HttpSession) context.getExternalContext()
                .getSession(true);
        int toIndex = 5;
        session.setAttribute("toIndex", toIndex);   
    }

    private EntityManager em() {
        if(em == null)
            em = emf.createEntityManager();
        return em;
    }

    public List<Status> getStatusList() {
        FacesContext context = FacesContext.getCurrentInstance();
        HttpSession session = (HttpSession) context.getExternalContext()
                .getSession(true);
        User user = (User) session.getAttribute("userdet");
        Query query = em().createQuery("SELECT s FROM Status s WHERE s.email='"//  line no 49
                + user.getEmail() + "' ORDER BY s.timeMillis desc", Status.class);
        List<Status> results = query.getResultList();
        Query query1 = em().createQuery("SELECT f FROM Friend f WHERE f.email='"
                + user.getEmail() + "'", Friend.class);
        List<Friend> results1 = query1.getResultList();
        Iterator<Friend> it = results1.listIterator();
        while (it.hasNext()) {
            String email = it.next().getFriendEmail();
            Query query2 = em().createQuery(
                    "SELECT s FROM Status s WHERE s.email='" + email + 
                    "' ORDER BY s.timeMillis desc", Status.class);
            List<Status> results2 = query2.getResultList();
            results.addAll(results2);
        }
        Collections.sort(results);
        int toIndex = (int) session.getAttribute("toIndex");
        List<Status> subList = results.subList(0, toIndex);
        return subList;
    }
}

@ManagedBean
@视域
公共类StatusBean2实现可序列化{
私有静态最终长serialVersionUID=-7528998562821856315L；
私有瞬态实体管理器em；
私有静态最终EntityManagerFactory emf=Persistence.createEntityManagerFactory（“FreeBird”）；
公共状态bean2（）{
FacesContext context=FacesContext.getCurrentInstance（）；
HttpSession会话=（HttpSession）上下文。getExternalContext（）
.getSession（true）；
int-toIndex=5；
session.setAttribute（“toIndex”，toIndex）；
}
私有实体管理器em（）{
if（em==null）
em=emf.createEntityManager（）；
返回em；
}
公共列表getStatusList（）{
FacesContext context=FacesContext.getCurrentInstance（）；
HttpSession会话=（HttpSession）上下文。getExternalContext（）
.getSession（true）；
User=（User）session.getAttribute（“userdet”）；
Query Query=em（）.createQuery（“从状态s中选择s，其中s.email=”//第49行
+user.getEmail（）+“'ORDER BY s.timeMillis desc”，Status.class）；
List results=query.getResultList（）；
Query query1=em（）.createQuery（“从朋友f中选择f，其中f.email=”）
+user.getEmail（）+“”，Friend.class）；
List results1=query1.getResultList（）；
迭代器it=results1.listIterator（）；
while（it.hasNext（））{
字符串email=it.next（）.getFriendEmail（）；
Query query2=em（）.createQuery(
“从状态s中选择s，其中s.email=”+email+
“‘按s.TIMEMILIS desc订购’，状态类）；
List results2=query2.getResultList（）；
结果：addAll（结果2）；
}
收集。排序（结果）；
intToIndex=（int）session.getAttribute（“toIndex”）；
列表子列表=结果。子列表（0，toIndex）；
返回子列表；
}
}

异常表示您的对象为空：由以下原因引起：java.lang.NullPointerException您是否调试了

getStatusList（）

以查看最后返回的列表是否有内容或为空？尝试一下，然后发布您看到的内容。此外，将UI和后端（甚至是持久性）逻辑保持在同一代码中不是一个好做法。

@ManagedBean
@ViewScoped
public class StatusBean2 implements Serializable  {
    private static final long serialVersionUID = -7528998562821856315L;
    private transient EntityManager em;
    private static final EntityManagerFactory emf = Persistence.createEntityManagerFactory("FreeBird");

    public StatusBean2() {
        FacesContext context = FacesContext.getCurrentInstance();
        HttpSession session = (HttpSession) context.getExternalContext()
                .getSession(true);
        int toIndex = 5;
        session.setAttribute("toIndex", toIndex);   
    }

    private EntityManager em() {
        if(em == null)
            em = emf.createEntityManager();
        return em;
    }

    public List<Status> getStatusList() {
        FacesContext context = FacesContext.getCurrentInstance();
        HttpSession session = (HttpSession) context.getExternalContext()
                .getSession(true);
        User user = (User) session.getAttribute("userdet");
        Query query = em().createQuery("SELECT s FROM Status s WHERE s.email='"//  line no 49
                + user.getEmail() + "' ORDER BY s.timeMillis desc", Status.class);
        List<Status> results = query.getResultList();
        Query query1 = em().createQuery("SELECT f FROM Friend f WHERE f.email='"
                + user.getEmail() + "'", Friend.class);
        List<Friend> results1 = query1.getResultList();
        Iterator<Friend> it = results1.listIterator();
        while (it.hasNext()) {
            String email = it.next().getFriendEmail();
            Query query2 = em().createQuery(
                    "SELECT s FROM Status s WHERE s.email='" + email + 
                    "' ORDER BY s.timeMillis desc", Status.class);
            List<Status> results2 = query2.getResultList();
            results.addAll(results2);
        }
        Collections.sort(results);
        int toIndex = (int) session.getAttribute("toIndex");
        List<Status> subList = results.subList(0, toIndex);
        return subList;
    }
}