通过递归展示树状结构

CREATE TABLE `tb_tree` (
  `cid` int(11) NOT NULL,
  `cname` varchar(255) DEFAULT NULL,
  `pid` int(11) DEFAULT NULL,
  PRIMARY KEY (`cid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert into tb_tree (CID, CNAME, PID) values (1, '中国', 0);
insert into tb_tree (CID, CNAME, PID) values (2, '北京市', 1);
insert into tb_tree (CID, CNAME, PID) values (3, '广东省', 1);
insert into tb_tree (CID, CNAME, PID) values (4, '上海市', 1);
insert into tb_tree (CID, CNAME, PID) values (5, '广州市', 3);
insert into tb_tree (CID, CNAME, PID) values (6, '深圳市', 3);
insert into tb_tree (CID, CNAME, PID) values (7, '海珠区', 5);
insert into tb_tree (CID, CNAME, PID) values (8, '天河区', 5);
insert into tb_tree (CID, CNAME, PID) values (9, '福田区', 6);
insert into tb_tree (CID, CNAME, PID) values (10, '南山区', 6);
insert into tb_tree (CID, CNAME, PID) values (11, '密云县', 2);
insert into tb_tree (CID, CNAME, PID) values (12, '浦东', 4);

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.5.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.dcsyun</groupId>
    <artifactId>spring-boot-09-mybatis</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>spring-boot-09-mybatis</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
            <version>1.3.2</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>
        
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

</project>

public class TreeNode implements Serializable {
    private static final long serialVersionUID = 1L;
    private Integer cid;
    private String cname;
    private Integer pid;
    private List<TreeNode> nodes = new ArrayList<TreeNode>();
        // getter/setter
  
}

@Mapper
public interface TreeMapper {
    @Select("SELECT * FROM tb_tree t WHERE t.cid=#{cid}")
    public TreeNode getTreeNode(Integer cid);
    @Select("SELECT * FROM tb_tree t WHERE t.pid=#{pid}")
    public List<TreeNode> queryTreeNode(Integer pid);
}

spring.datasource.url=jdbc:mysql://localhost:3306/spring_cache
spring.datasource.username=root
spring.datasource.password=123456
spring.datasource.driver-class-name=com.mysql.jdbc.Driver

mybatis.configuration.map-underscore-to-camel-case=true
logging.level.com.dcsyun.mapper=debug
debug=true

@RunWith(SpringRunner.class)
@SpringBootTest
public class SpringBoot09MybatisApplicationTests {
    @Autowired
    private TreeMapper treeDao;

    @Test
    public void contextLoads() throws Exception {
        TreeNode treeNode = recursiveTree(1);
        ObjectMapper om = new ObjectMapper();
        String str = om.writeValueAsString(treeNode);
        System.out.println(str);
    }

    public TreeNode recursiveTree(int cid) {
        // 根据cid获取节点对象
        TreeNode node = treeDao.getTreeNode(cid);
        // 查询cid下的所有子节点
        List<TreeNode> childTreeNodes = treeDao.queryTreeNode(cid); 
        // 遍历子节点
        for(TreeNode child : childTreeNodes){
            TreeNode n = recursiveTree(child.getCid()); // 递归
            node.getNodes().add(n);
        }
        return node;
    }

}