JAVA返回树结构（宇宙第一详细教程）

DROP TABLE IF EXISTS `demo`;
CREATE TABLE `demo` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `pid` int(11) NOT NULL,
  `name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
INSERT INTO `demo` VALUES ('1', '0', 'A');
INSERT INTO `demo` VALUES ('2', '1', 'B');
INSERT INTO `demo` VALUES ('3', '1', 'C');
INSERT INTO `demo` VALUES ('4', '2', 'D');
INSERT INTO `demo` VALUES ('5', '2', 'E');
INSERT INTO `demo` VALUES ('6', '3', 'F');
INSERT INTO `demo` VALUES ('7', '3', 'G');

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid</artifactId>
            <version>1.1.1</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.39</version>
        </dependency>
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
            <version>1.2.0</version>
        </dependency>

spring.datasource.url=jdbc:mysql://127.0.0.1:3306/tree
spring.datasource.username=root
spring.datasource.password=1234
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
TreeMapper
@Mapper
public interface TreeMapper {
    @Select("select * from demo")
    List<Tree> getAll();
}

public class Tree {
    private Integer id;
    private Integer pid;
    private String name;
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public Integer getPid() {
        return pid;
    }
    public void setPid(Integer pid) {
        this.pid = pid;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public String toString() {
        return "Tree{" +
                "id=" + id +
                ", pid=" + pid +
                ", name='" + name + '\'' +
                '}';
    }
}

public class NewTree {
    private String id;
    private String pid;
    private String name;
    public 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 getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public String toString() {
        return "Tree{" +
                "id=" + id +
                ", pid=" + pid +
                ", name='" + name + '\'' +
                '}';
    }
}

@Component
public class MenuTreeUtil {
    //已经被buildTree的list集合
    private List<NewTree> menuCommon;
    //返回给前端的NewTree List集合
    private List<Object> list = new ArrayList<Object>();
    public List<Object> menuList(List<NewTree> menu){
        this.menuCommon = menu;
        // 通过遍历menu，找到父节点为0的节点，它是顶级父节点
        // 然后调用menuChild，递归遍历所有子节点
        for (NewTree x : menu) {
            Map<String,Object> mapArr = new LinkedHashMap<String, Object>();
            if("0".equals(x.getPid())){
                mapArr.put("id", x.getId());
                mapArr.put("name", x.getName());
                mapArr.put("pid", x.getPid());
                //遍历开始
                mapArr.put("childList", menuChild(x.getId()));
                list.add(mapArr);
            }
        }
        return list;
    }
    private List<?> menuChild(String id){
        List<Object> lists = new ArrayList<Object>();
        //继续遍历menu
        for(NewTree a:menuCommon){
            Map<String,Object> childArray = new LinkedHashMap<String, Object>();
            //找到父ID等于父节点ID的子节点
            if(a.getPid().equals(id)){
                childArray.put("id", a.getId());
                childArray.put("name", a.getName());
                childArray.put("pid", a.getPid());
                //向下递归
                childArray.put("childList", menuChild(a.getId()));
                lists.add(childArray);
            }
        }
        return lists;
    }
}

@Controller
public class TreeController {
    @Resource
    TreeMapper treeMapper;
    @Resource
    MenuTreeUtil menuTreeUtil;
    @RequestMapping(value = "/menuList",method = {RequestMethod.GET})
    @ResponseBody
    public String getTree() throws Exception {
        List<NewTree> list = buildTree(treeMapper.getAll());
        List<Object> menuList = menuTreeUtil.menuList(list);
        System.out.println(menuList);
        return "YES";
    }
    //将实体类，变成标准的树结构，即NewTree类型
    private List<NewTree> buildTree(List<Tree> list){
        List<NewTree> newTrees = new LinkedList<>();
        for(Tree tree:list){
            NewTree newTree = new NewTree();
            newTree.setId(String.valueOf(tree.getId()));
            newTree.setPid(String.valueOf(tree.getPid()));
            newTree.setName(tree.getName());
            newTrees.add(newTree);
        }
        return newTrees;
    }
}

[{
  id = 1,
  name = A,
  pid = 0,
  childList = [{
  id = 2,
  name = B,
  pid = 1,
  childList = [{
    id = 4,
    name = D,
    pid = 2,
    childList = []
  }, {
    id = 5,
    name = E,
    pid = 2,
    childList = []
  }]
  }, {
  id = 3,
  name = C,
  pid = 1,
  childList = [{
    id = 6,
    name = F,
    pid = 3,
    childList = []
  }, {
    id = 7,
    name = G,
    pid = 3,
    childList = []
  }]
  }]
}]