很多时候，，我们会把一棵树存放到数据库中，当前台需要展示一个树形列表时，将这棵树读取出来并显示，这个过程是怎么实现的呢？
这篇文章是以构造一棵easyui前台框架的一个树形列表为例，后台框架是spring MVC+JPA。
首先看一下数据库中这颗树是怎么存的：
树的结构一目了然，这是一棵表示部门的树。
下面是实体类：
/** * 部门类 用户所属部门（这里的部门是一个相对抽象的词） * 使用前缀编码，每级增加三个数字，如：第一级 001，第二级001001，第三级001001001 * @author Administrator * */@Entitypublic class Department {private String id;//部门id private String pid;//父idprivate String text;//部门名称private List<Department> children; //用于存储子节点@Idpublic String getId() {return id;}public void setId(String id) {this.id = id;}public String getPid() {return pid;}public void setPid(String pid) {this.pid = pid;}public String getText() {return text;}public void setText(String text) {this.text = text;}@OneToManypublic List<Department> getChildren() {return children;}public void setChildren(List<Department> children) {this.children = children;}}由于使用的是easyui ，为了树可以正确显示，必须包含属性id和text。
下面是控制类，用于读取数据库中的数据，生成一个List,该list返回到前台时会转成json格式的数据，重点需要理解的就是getTree()和buildTree()这两个方法，通过递归生成这棵树，基本实现原理是遍历树的某一层，获取每个节点的子节点们（一个list）,然后将它作为父节点的一个属性set进去
@Controller@RequestMapping("/department")public class DepartmentController {private static final String DEPARTEMNTMANAGE = "module/jsp/system/departmentManage.jsp";@Resource(name="departmentServiceImpl")private DepartmentService departmentService;/** * 获取部门管理页面. * @return */@RequestMapping("/getPage.do")public String getUserManagePage(){return DEPARTEMNTMANAGE;}/** * 获取部门树形列表 */@RequestMapping(value = "/getTree.do", method = RequestMethod.POST)@ResponseBody //此注解表明返回值跳过视图处理部分，直接写入 http response body中public List<Department> getTree(){List<Department> root = departmentService.findById("0"); //获取根节点（获取的值存到list中）net.sf.json.JSONArray jsonArray = net.sf.json.JSONArray.fromObject(buildTree(root));System.out.println(jsonArray.toString());return buildTree(root);}public List<Department> buildTree(List<Department> root){for(int i=0;i<root.size();i++){List<Department> children = departmentService.findByPid(root.get(i).getId()); //查询某节点的子节点（获取的是list）buildTree(children);root.get(i).setChildren(children);}return root;}}这里用查询的方法findById和findByPid是基于spring data JPA的方法，下面是相关类：
public interface DepartmentDao extends Repository<Department, Integer>{public List<Department> findByPid(String pid);public List<Department> findById(String id);}public interface DepartmentService {public List<Department> findByPid(String pid);public List<Department> findById(String id);}@Service("departmentServiceImpl")public class DepartmentServiceImpl implements DepartmentService{@Autowiredprivate DepartmentDao departmentDao;@Overridepublic List<Department> findByPid(String pid) {return departmentDao.findByPid(pid);}@Overridepublic List<Department> findById(String id) {return departmentDao.findById(id);}}前台调用如下：
$(function(){$('#tt').tree({url: 'department/getTree.do',loadFilter: function(data){if (data.d){return data.d;} else {return data;}}});});这是后台返回给前台一个如下的json格式的数据：
[{"children": [{"children": [{"children": [],"id": "001001","pid": "001","text": "部门一"},{"children": [],"id": "001002","pid": "001","text": "部门二"}],"id": "001","pid": "0","text": "一分公司"},{"children": [{"children": [],"id": "002001","pid": "002","text": "部门一"},{"children": [],"id": "002002","pid": "002","text": "部门二"}],"id": "002","pid": "0","text": "二分公司"},{"children": [],"id": "003","pid": "0","text": "三分公司"}],"id": "0","pid": "-1","text": "总公司"}]在页面上如下显示：
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