获取子父节点，tree树状结构的方式--通过循环的方式

1.导包

<dependencies>
　　//将domain quary写到了这层
    <dependency>
        <groupId>cn.xxx.aigou</groupId>
        <artifactId>qqq_product_interface</artifactId>
        <version>1.0-SNAPSHOT</version>
    </dependency>

        <!--springboot相关的依赖-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
        </dependency>

        <!--mp的依赖-->
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>2.2.0</version>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.20</version>
        </dependency>
    <!--eureka的客户端-->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
    </dependency>

    <!--swagger2接口文档依赖-->
    <dependency>
        <groupId>io.springfox</groupId>
        <artifactId>springfox-swagger2</artifactId>
        <version>2.9.2</version>
    </dependency>
    <!-- https://mvnrepository.com/artifact/io.springfox/springfox-swagger-ui -->
    <dependency>
        <groupId>io.springfox</groupId>
        <artifactId>springfox-swagger-ui</artifactId>
        <version>2.9.2</version>
    </dependency>
    <!--配置中心支持-->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-config</artifactId>
    </dependency>
</dependencies>

2.contrlloer实现

/**
 * 树状结构数据查询：
 * @return
 */
@RequestMapping(value = "/treeData",method = RequestMethod.GET)
public List<ProductType> treeData()
{

    return productTypeService.treeData();
}

3.service写逻辑

@Override
public List<ProductType> treeData() {
    /*
        使用循环的方式获取不同层次的节点
     */
    return treeDataLoop();
}
private List<ProductType> treeDataLoop() {
    //1：首先查询出所有的数据
    List<ProductType> allProductTypes = productTypeMapper.selectList(null);

    //获取所有的数据的key--value的值
    Map<Long,ProductType> map=new HashMap<>();
    for (ProductType pt : allProductTypes) {
        map.put(pt.getId(),pt);
    }

    //最终返回的结果：一级菜单
    List<ProductType> result=new ArrayList<>();

    //2：遍历：封装父节点和子节点
    for (ProductType currentProductType : allProductTypes) {
        //判断是否是一级菜单
        if(currentProductType.getPid()==0){
            // 如果是一级菜单则返回
            result.add(currentProductType);
        }else{
            ProductType parent =map.get(currentProductType.getPid());
            parent.getChildren().add(currentProductType);
        }

    }

    return result;
}

3.前台写好后，直接测试