我正在使用Hibernate注释开发一个项目，并且我在实现EntityManager时遇到问题..在尝试EntityManager.find（）时我得到了一个nullpointerexception 我不明白为什么！有什么帮助吗？ 这是一些代码
Users.java
package com.rimweb.users.persistance;
// Generated 6 mars 2014 12:00:34 by Hibernate Tools 3.4.0.CR1
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import static javax.persistence.GenerationType.IDENTITY;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
/**
* Users generated by hbm2java
*/
@Entity
@Table(name = "users", catalog = "user")
public class Users implements java.io.Serializable {
private Integer userid;
private Address address;
private String username;
private String password;
private String firstname;
private String lastname;
private String email;
public Users() {
}
public Users(Address address, String username, String password,
String firstname, String lastname, String email) {
this.address = address;
this.username = username;
this.password = password;
this.firstname = firstname;
this.lastname = lastname;
this.email = email;
}
@Id
@GeneratedValue(strategy =GenerationType.IDENTITY)
@Column(name = "userid", unique = true, nullable = false)
public Integer getUserid() {
return this.userid;
}
public void setUserid(Integer userid) {
this.userid = userid;
}
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "addressid")
public Address getAddress() {
return this.address;
}
public void setAddress(Address address) {
this.address = address;
}
@Column(name = "username")
public String getUsername() {
return this.username;
}
public void setUsername(String username) {
this.username = username;
}
@Column(name = "password", length = 64)
public String getPassword() {
return this.password;
}
public void setPassword(String password) {
this.password = password;
}
@Column(name = "firstname")
public String getFirstname() {
return this.firstname;
}
public void setFirstname(String firstname) {
this.firstname = firstname;
}
@Column(name = "lastname")
public String getLastname() {
return this.lastname;
}
public void setLastname(String lastname) {
this.lastname = lastname;
}
@Column(name = "email")
public String getEmail() {
return this.email;
}
public void setEmail(String email) {
this.email = email;
}
@Override
public String toString() {
return "Users [userid=" + userid + ", username=" + username
+ ", password=" + password + ", firstname=" + firstname
+ ", lastname=" + lastname + ", email=" + email + "]";
}
/* (non-Javadoc)
* @see java.lang.Object#hashCode()
*/
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((address == null) ? 0 : address.hashCode());
result = prime * result + ((email == null) ? 0 : email.hashCode());
result = prime * result
+ ((firstname == null) ? 0 : firstname.hashCode());
result = prime * result
+ ((lastname == null) ? 0 : lastname.hashCode());
result = prime * result
+ ((password == null) ? 0 : password.hashCode());
result = prime * result + ((userid == null) ? 0 : userid.hashCode());
result = prime * result
+ ((username == null) ? 0 : username.hashCode());
return result;
}
/* (non-Javadoc)
* @see java.lang.Object#equals(java.lang.Object)
*/
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Users other = (Users) obj;
if (address == null) {
if (other.address != null)
return false;
} else if (!address.equals(other.address))
return false;
if (email == null) {
if (other.email != null)
return false;
} else if (!email.equals(other.email))
return false;
if (firstname == null) {
if (other.firstname != null)
return false;
} else if (!firstname.equals(other.firstname))
return false;
if (lastname == null) {
if (other.lastname != null)
return false;
} else if (!lastname.equals(other.lastname))
return false;
if (password == null) {
if (other.password != null)
return false;
} else if (!password.equals(other.password))
return false;
if (userid == null) {
if (other.userid != null)
return false;
} else if (!userid.equals(other.userid))
return false;
if (username == null) {
if (other.username != null)
return false;
} else if (!username.equals(other.username))
return false;
return true;
}
}
UsersHome.java:
package com.rimweb.users.dao;
// Generated 6 mars 2014 16:42:25 by Hibernate Tools 3.4.0.CR1
import java.util.ArrayList;
import java.util.List;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.PersistenceContext;
import javax.persistence.PersistenceUnit;
import javax.persistence.Query;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import com.rimweb.users.persistance.Users;
/**
* Home object for domain model class Users.
* @see com.rimweb.users.dao.Users
* @author Hibernate Tools
*/
@Stateless
public class UsersHome {
private static final Log log = LogFactory.getLog(UsersHome.class);
@PersistenceContext(unitName="manager1")
//@PersistenceUnit
//EntityManagerFactory entityManagerFactory= Persistence.createEntityManagerFactory("manager1");
//private EntityManager entityManager = entityManagerFactory.createEntityManager();
private EntityManager entityManager;
public void persist(Users transientInstance) {
log.debug("persisting Users instance");
try {
entityManager.persist(transientInstance);
log.debug("persist successful");
} catch (RuntimeException re) {
log.error("persist failed", re);
throw re;
}
}
public void remove(Users persistentInstance) {
log.debug("removing Users instance");
try {
entityManager.remove(persistentInstance);
log.debug("remove successful");
} catch (RuntimeException re) {
log.error("remove failed", re);
throw re;
}
}
public Users merge(Users detachedInstance) {
log.debug("merging Users instance");
try {
Users result = entityManager.merge(detachedInstance);
log.debug("merge successful");
return result;
} catch (RuntimeException re) {
log.error("merge failed", re);
throw re;
}
}
public Users findById(Integer id) {
log.debug("getting Users instance with id: " + id);
try {
Users instance = entityManager.find(Users.class, id);
log.debug("get successful");
return instance;
} catch (RuntimeException re) {
log.error("get failed", re);
throw re;
}
}
public List<Users> findAll() {
log.debug("getting all User instances");
try {
Query query = entityManager.createQuery("select u from Users u");
List<Users> usersList = (List<Users>) query.getResultList();
log.debug("findAll successful");
return usersList;
} catch (RuntimeException re) {
log.error("findAll failed", re);
return new ArrayList<Users>();
}
}
}
MainTest.java
package com.rimweb.users.dao;
import java.math.BigDecimal;
import java.util.List;
import org.hibernate.Session;
import org.hibernate.criterion.Order;
import com.rimweb.users.persistance.Users;
public class MainTest {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
UsersHome dao = new UsersHome() ;
Users us = dao.findById(new Integer(1));
System.out.println("-Livre :"+us);
//////////////////////////////////////////
List<Users> list = dao.findAll();
for(Users l : list)
{ System.out.println("##"+l);}
}
}
答案 0 :(得分：0)
在您的UserHome.java类中有实体管理器，但它是null，因为您正在创建刚刚定义的EntityManager。
因此，创建实体管理器，请创建Util类...
例如HibernateUtil.java
public class HibernateUtil {
private static final EntityManagerFactory entityManagerFactory;
static {
try {
entityManagerFactory = Persistence.createEntityManagerFactory("abc");
} catch (Throwable ex) {
System.err.println("Initial SessionFactory creation failed." + ex);
throw new ExceptionInInitializerError(ex);
}
}
public static EntityManagerFactory getEntityManagerFactory() {
return entityManagerFactory;
}
}
并将持久性单元名称提供给以下方法而不是abc。
entityManagerFactory = Persistence.createEntityManagerFactory("abc");
然后使用事务在每个方法中创建实体管理器。
例如......
public Users merge(Users detachedInstance) {
log.debug("merging Users instance");
try {
em = HibernateUtil.getEntityManagerFactory().createEntityManager();
em.getTransaction().begin();
Users result = entityManager.merge(detachedInstance);
em.getTransaction().commit();
em.close();
log.debug("merge successful");
return result;
} catch (RuntimeException re) {
em.getTransaction().rollback();
log.error("merge failed", re);
throw re;
}
}
这足以避免空指针异常....