1. 树实现通过pid进行指向上一层来实现，实体类代码如下

package com.test.model;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;

@Entity
public class Org {
private int id;
private String name;
private Set children = new HashSet();
private Org parent;

@Id  
@GeneratedValue  
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;  
}

@OneToMany(mappedBy="parent", cascade=CascadeType.ALL, fetch=FetchType.EAGER)  
public Set<Org> getChildren() {  
    return children;  
}  
public void setChildren(Set<Org> children) {  
    this.children = children;  
}

@ManyToOne  
@JoinColumn(name="parent\_Id")  
public Org getParent() {  
    return parent;  
}  
public void setParent(Org parent) {  
    this.parent = parent;  
}  

}

2.测试类代码如下，   查询时可通过递归进行查询

package com.test.test;

import org.hibernate.classic.Session;

import com.test.Util.HibernateUtil;
import com.test.model.Org;

public class StudentManager {
public static void main(String[] args) {
new StudentManager().testSave();
}

public void testSave() {  
    Session session = HibernateUtil.getSessionFactory().getCurrentSession();  
    session.beginTransaction();

    Org o = new Org();  
    o.setName("总公司");

    Org o1 = new Org();  
    o1.setName("分公司1");

    Org o2 = new Org();  
    o2.setName("分公司2");

    Org o21 = new Org();  
    o21.setName("分公司2\_1");

    Org o22 = new Org();  
    o22.setName("分公司2\_2");

    o.getChildren().add(o1);  
    o.getChildren().add(o2);

    o1.setParent(o);

    o2.setParent(o);  
    o2.getChildren().add(o21);  
    o2.getChildren().add(o22);

    o21.setParent(o2);  
    o22.setParent(o2);

    session.save(o);  
    testLoad();  
    session.getTransaction().commit();  
}

public void testLoad() {  
    Session session = HibernateUtil.getSessionFactory().getCurrentSession();  
    session.beginTransaction();  
    Org org = (Org)session.load(Org.class, 1);  
    print(org, 0);

}

private void print(Org org, int level) {  
    String str = "----";  
    for(int i=0; i<level; i++) {  
        System.out.print(str);  
    }

    System.out.println(org.getName());  
    level++;  
    for(Org o:org.getChildren()) {  
        print(o, level);  
    }  
}  

}