刚刚接触学习泛型DAO，按照网上的一个例子来写，运行就提示，Exception in thread "main" java.lang.NullPointerException。开发环境是myeclipse6.5,jdk5.0，只好来这里请教，代码如下：
[code="java"]文件列表1：
文件名 说明
IDao.java dao的顶层抽象接口
AbstractDao.java IDao的实现类，实现了get/update/delete等基本操作
User.java USER表对应的entity定义
IUserDao.java USER表操作DAO接口定义
UserDao.java USER表操作DAO实现
HelloGenerics.java 测试类
IDao.java
// super dao interface
public interface IDao {
public T get(PK pk);
public List getAll();
public PK save(T entity);
public void update(T entity);
public void saveOrUpdate(T entity);
public void delete(T entity);
}
AbstractDao.java
//super abstract dao class
public abstract class AbstractDao implements IDao {
private Class clz;
public AbstractDao(Class clz) {
this.clz = clz;
}
public T get(PK pk) {
return (T)getSession().get(clz, pk);
}
public List <T>getAll() {
return getSession().createCriteria(clz).list();
}
public PK save(T entity) {
return (PK)getSession().save(entity);
}
public void update(T entity) {
getSession().update(entity);
}
public void saveOrUpdate(T entity) {
getSession().saveOrUpdate(entity);
}
public void delete(T entity) {
getSession().delete(entity);
}
private Session session;
protected Session getSession() {
//wrap session in a class such as HibernateUtils, then you can use HibernateUtils.getCurrentSession() for getting a session.
return session;
}
}
User.java
//persist entity
public class User {
private String id;
private String name;
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
//... setter/getter HERE
}
IUserDao.java
//User Dao interface
public interface IUserDao extends IDao {
// all are empty
}
UserDao.java
//UserDao Implementation
public class UserDao extends AbstractDao implements IUserDao {
public UserDao() {
super(User.class);
}
// or use the following constructor
//public UserDao(Class type) {
// super(type);
//}
}
HelloGenerics.java
public class HelloGenerics {
public static void main(String[] args) {
IUserDao userDao = new UserDao();
User user = userDao.get("1");
if (user != null) {
System.out.println(user.getName());
}
List<User> userList = userDao.getAll();
for (User u:userList) {
System.out.println(user.getName());
}
}
}[/code]
