使用JAX-RS框架搭建WebService使用Hibernate访问MySQL出现不能实例化类问题

Question

问题描述：大体思路是这样，我最近在做IOS开发想通过WebService访问服务器端数据库的数据，所以想使用Java搭建一个WebService于是在网上找资料使用Java实现的框架JAX-RS.代码如下：

com.hnu.hibernate.bean包下的Student.java类

package com.hnu.hibernate.bean;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
import javax.persistence.OneToMany;
import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement
@Entity
@Table(name="student")
public class Student {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Integer userid;

@Column(name="username")
private String username;

@Column(name="password")
private String password;

@Column(name="realname")
private String realname;

@Column(name="age")
private Integer age;

@Column(name="nationality")
private String nationality;

@Column(name="language")
private String language;

@Column(name="year")
private float year;

@Column(name="studyabroadschool")
private String studyabroadschool;

public Integer getUserid() {
return userid;
}

public void setUserid(Integer userid) {
this.userid = userid;
}

public String getUsername() {
return username;
}

public void setUsername(String username) {
this.username = username;
}

public String getPassword() {
return password;
}

public void setPassword(String password) {
this.password = password;
}

public String getRealname() {
return realname;
}

public void setRealname(String realname) {
this.realname = realname;
}

public Integer getAge() {
return age;
}

public void setAge(Integer age) {
this.age = age;
}

public String getNationality() {
return nationality;
}

public void setNationality(String nationality) {
this.nationality = nationality;
}

public String getLanguage() {
return language;
}

public void setLanguage(String language) {
this.language = language;
}

public float getYear() {
return year;
}

public void setYear(float year) {
this.year = year;
}

public String getStudyabroadschool() {
return studyabroadschool;
}

public void setStudyabroadschool(String studyabroadschool) {
this.studyabroadschool = studyabroadschool;
}
}

com.hnu.hibernate.dao包

package com.hnu.hibernate.dao;

import java.io.Serializable;
import java.util.List;

import org.hibernate.Session;

import com.hnu.hibernate.util.HibernateSessionFactory;
import com.hnu.hibernate.util.HibernateUtil;
public class BaseDAO {
/**
* 插入数据
*
* @param object
*/
public int create(T object) {

    Session session = HibernateSessionFactory.getSession();

    try {
        session.beginTransaction();

        session.persist(object);

        session.getTransaction().commit();

    } catch (Exception e) {
        session.getTransaction().rollback();
        return 0;
    } finally {
        session.close();
    }
    return 1;
}

/**
 * 更新数据库
 * 
 * @param object
 */
public int update(T object) {

    Session session = HibernateSessionFactory.getSession();
    try {
        session.beginTransaction();
        session.update(object);
        session.getTransaction().commit();
    } catch (Exception e) {
        session.getTransaction().rollback();
        return 0;
    } finally {
        session.close();
    }
    return 1;
}

/**
 * 从数据库中删除
 * 
 * @param object
 */
public int delete(T object) {

    Session session = HibernateSessionFactory.getSession();

    try {
        session.beginTransaction();
        session.delete(object);
        session.getTransaction().commit();
    } catch (Exception e) {
        session.getTransaction().rollback();
        return 0;
    } finally {
        session.close();
    }
    return 1;
}

/**
 * 查找单个Entity Bean
 * 
 * @param clazz
 * @param id
 * @return
 */
@SuppressWarnings("unchecked")
public T find(Class<? extends T> clazz, Serializable id) {

    Session session = HibernateSessionFactory.getSession();
    try {
        session.beginTransaction();
        return (T) session.get(clazz, id);
    } finally {
        session.getTransaction().commit();
        session.close();
    }
}

/**
 * 查找多个Entity Bean
 * 
 * @param hql
 * @return
 */
@SuppressWarnings("unchecked")
public List<T> list(String hql) {

    Session session =HibernateSessionFactory.getSession();
    try {
        session.beginTransaction();
        return session.createQuery(hql).list();
    } finally {
        session.getTransaction().commit();
        session.close();
    }
}

}

com.hnu.hibernate.bean包下生成的WebService Javabean StudentsResource.java类

package com.hnu.hibernate.bean;

import java.util.List;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import com.hnu.hibernate.dao.BaseDAO;

@Produces("application/xml")
@Path("students")
public class StudentsResource {
private BaseDAO baseDAO=new BaseDAO();
@GET
public List getStudents() {
return baseDAO.list("from Student");
}
}

com.hnu.hibernate.util包下Hibernate获取Session的类 HibernateSessionFactory.java类

package com.hnu.hibernate.util;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;

/**

• Configures and provides access to Hibernate sessions, tied to the
• current thread of execution. Follows the Thread Local Session

• pattern, see {@link http://hibernate.org/42.html }.
  */
  public class HibernateSessionFactory {

  /**

  • Location of hibernate.cfg.xml file.
  • Location should be on the classpath as Hibernate uses
    
  • #resourceAsStream style lookup for its configuration file.
  • The default classpath location of the hibernate config file is
  • in the default package. Use #setConfigFile() to update
  • the location of the configuration file for the current session.
    */ private static final ThreadLocal threadLocal = new ThreadLocal(); private static org.hibernate.SessionFactory sessionFactory;

  private static Configuration configuration = new Configuration();
  private static ServiceRegistry serviceRegistry;

  static {
  try {
  configuration.configure();
  serviceRegistry = new ServiceRegistryBuilder().applySettings(configuration.getProperties()).buildServiceRegistry();
  sessionFactory = configuration.buildSessionFactory(serviceRegistry);
  } catch (Exception e) {
  System.err.println("%%%% Error Creating SessionFactory %%%%");
  e.printStackTrace();
  }
  }
  private HibernateSessionFactory() {
  }

  /**

  • Returns the ThreadLocal Session instance. Lazy initialize
  • the SessionFactory if needed. *
  • @return Session

  • @throws HibernateException
    */
    public static Session getSession() throws HibernateException {
    Session session = (Session) threadLocal.get();

    if (session == null || !session.isOpen()) {
    if (sessionFactory == null) {
    rebuildSessionFactory();
    }
    session = (sessionFactory != null) ? sessionFactory.openSession()
    : null;
    threadLocal.set(session);
    }

    return session;
    }

  /**

  • Rebuild hibernate session factory * */ public static void rebuildSessionFactory() { try { configuration.configure(); serviceRegistry = new ServiceRegistryBuilder().applySettings(configuration.getProperties()).buildServiceRegistry(); sessionFactory = configuration.buildSessionFactory(serviceRegistry); } catch (Exception e) { System.err.println("%%%% Error Creating SessionFactory %%%%"); e.printStackTrace(); } }

  /**

  • Close the single hibernate session instance. *

  • @throws HibernateException
    */
    public static void closeSession() throws HibernateException {
    Session session = (Session) threadLocal.get();
    threadLocal.set(null);

    if (session != null) {
    session.close();
    }
    }

  /**

  • return session factory * / public static org.hibernate.SessionFactory getSessionFactory() { return sessionFactory; } /*
  • return hibernate configuration * */ public static Configuration getConfiguration() { return configuration; }

}

hibernate.cfg.xml部署文件

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<session-factory>
    <property name="hbm2ddl.auto">update</property>
    <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name="connection.url">jdbc:mysql://localhost:3306/vocabularyadaptivetestingsystem?characterEncoding=UTF-8</property>
    <property name="connection.username">root</property>
    <property name="connection.password">123</property>
    <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="show_sql">true</property>

    <property name="current_session_context_class">thread</property>
    <mapping class="com.hnu.hibernate.bean.Student"/>
</session-factory>

使用本地的ApplicationServer使用Hibernate访问数据库没有任何问题。但是一旦把访问Hibernate的方法部署到Tomcat服务器上，访问WebService方法没有问题，但是使用WebService调用如下代码：baseDAO.list("from Student");

就出现了如图所示的错误：

希望各位高手帮忙看一下，搞了好久没搞出来，重分酬谢！

Answer 1

 How to resolve java.lang.NoClassDefFoundError:

Class is not available in Java Classpath.
You might be running your program using jar command and class was not defined in manifest file's ClassPath attribute.
Any start-up script is overriding Classpath environment variable.
Because NoClassDefFoundError is a sub class of java.lang.LinkageError it can also come if one of it dependency like native library may not available.
Check for java.lang.ExceptionInInitializerError in your log file. NoClassDefFoundError due to failure of static initialization is quite common.
If you are working in J2EE environment than visibility of Class among multiple Classloaders can also cause java.lang.NoClassDefFoundError, see examples and scenario section for detailed discussion.

 static {
    //code for loading properties from file
  }
It would appear some uncaught exception occurred and propagated up to the actual ClassLoader attempting to load the class. We would need a stacktrace to confirm this though.