当我使用JPA2和Hibernate时，清空DataBase

Question

我有一个简单的控制台应用程序与JPA。

package com.concretepage.entity;

import javax.persistence.*;

@Entity
@Table(name="farmer")
@NamedQuery(name = "farmer.getAll", query = "SELECT a from Farmer a")
public class Farmer {
    @Id
    @Column(name="id")
    private int id;
    @Column(name="name")
    private String name;
    @Column(name="village")

    private String village;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getVillage() {
        return village;
    }
    public void setVillage(String village) {
        this.village = village;
    }

}

package com.concretepage;

import com.concretepage.entity.Farmer;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.TypedQuery;
import java.util.List;

public class JPADemo {
    private static final EntityManagerFactory emFactory;
    static {
           emFactory = Persistence.createEntityManagerFactory("com.concretepage");
    }
    public static EntityManager getEntityManager(){
        return emFactory.createEntityManager();
    }
    public static void main(String[] args) {
        EntityManager em = getEntityManager();  
        em.getTransaction().begin();

        Farmer farmer = em.find(Farmer.class, 3);
        System.out.println(farmer);
        TypedQuery<Farmer> namedQuery = em.createNamedQuery("farmer.getAll",     Farmer.class);

        List<Farmer> farmers = namedQuery.getResultList();
        System.out.println(farmers);

        System.out.println("Done");
    }
}

<persistence 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"
             version="2.0">
    <persistence-unit name="com.concretepage">
        <description>JPA Test</description>
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <class>com.concretepage.entity.Farmer</class>
        <properties>
            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
            <property name="hibernate.hbm2ddl.auto" value="validate"/>

            <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
            <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost/test"/>
            <property name="javax.persistence.jdbc.user" value="root"/>
            <property name="javax.persistence.jdbc.password" value="root"/>
        </properties>
    </persistence-unit>
</persistence>

方法em.find（...）和namedQuery.getResultList（）返回true响应！ Everythink'is OK :) 但是当我在Web服务应用程序中使用此代码时：

@Stateless
public class TestService {

    @PersistenceContext(unitName = "MainDB")
    private EntityManager em;

    public Farmer add(Farmer farmer){
        farmer = em.merge(farmer);
        return farmer;
    }

    public Farmer get(Integer id){
        return em.find(Farmer.class, id);
    }

    public List<Farmer> getAll(){
        TypedQuery<Farmer> namedQuery = em.createNamedQuery("Farmer.getAll", Farmer.class);
        return namedQuery.getResultList();
    }
}
<persistence-unit name="MainDB" transaction-type="JTA">
    <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
    <class>model.Farmer</class>
    <properties>
        <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
        <property name="hibernate.hbm2ddl.auto" value="validate"/>

        <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
        <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost/test"/>
        <property name="javax.persistence.jdbc.user" value="root"/>
        <property name="javax.persistence.jdbc.password" value="root"/>
    </properties>
</persistence-unit>

如果我使用：

<property name="hibernate.hbm2ddl.auto" value="validate"/> or not use

收到Exeption：org.hibernate.HibernateException：缺少表 如果我使用：

<property name="hibernate.hbm2ddl.auto" value="update"/>

收到空响应em.find（...）和namedQuery.getResultList（），但表和数据都存在！ 我如何使用curent数据库和数据？