1. Prepare the table structrue and corresponding data a. Table srtucture:

create table TB_TREE
(
CID NUMBER not null,
CNAME VARCHAR2(50),
PID NUMBER //parent node
)

b. Table data:

insert into tb_tree (CID, CNAME, PID) values (1, 'China', 0);
insert into tb_tree (CID, CNAME, PID) values (2, 'Beijing', 1);
insert into tb_tree (CID, CNAME, PID) values (3, 'Guangdong', 1);
insert into tb_tree (CID, CNAME, PID) values (4, 'Shanghai', 1);
insert into tb_tree (CID, CNAME, PID) values (5, 'Guangzhou', 3);
insert into tb_tree (CID, CNAME, PID) values (6, 'Shenzhen', 3);
insert into tb_tree (CID, CNAME, PID) values (7, 'Hai Zhu District', 5);
insert into tb_tree (CID, CNAME, PID) values (8, 'Tian He District', 5);
insert into tb_tree (CID, CNAME, PID) values (9, 'Fu Tian District', 6);
insert into tb_tree (CID, CNAME, PID) values (10, 'Nan Shan District', 6);
insert into tb_tree (CID, CNAME, PID) values (11, 'Mi Yun County', 2);
insert into tb_tree (CID, CNAME, PID) values (12, 'PuDong', 4);

2. TreeNode clas corresponding to tb_tree

public class Node implements Serializable {
private Integer id;
private String name;
private Integer parentId;
private List children = new ArrayList();

public Node() {
}

// getters and setters omitted
}

3. Test data

public class NodeTest {
@Test
public void loadTree() throws Exception{
System.out.println(JsonUtils.javaToJson(recursiveBuild(1)));
}

/**
* Recursively build a tree structure
*
* @param id
* @return
* @author jiqinlin
*/
public Node recursiveBuild(int id) {
// Retrieve the node object (SELECT * FROM tb_tree t WHERE t.id=?)
Node node = personService.getNode(id);
// Query all child nodes (SELECT * FROM tb_tree t WHERE t.parentId=?)
List childNodes = personService.queryNodes(id);
// Iterate through child nodes
for(Node child : childNodes){
Node n = recursiveBuild(child.getId()); // recursion
node.getChildren().add(n);
}

return node;
}
}

The output JSON format is as follows:

{
    "id": 1,
    "children": [
        {
            "id": 2,
            "children": [
                {
                    "id": 11,
                    "children": [],
                    "name": "Mi Yun County",
                    "parentId": 2
                }
            ],
            "name": "Beijing",
            "parentId": 1
        },
        {
            "id": 3,
            "children": [
                {
                    "id": 5,
                    "children": [
                        {
                            "id": 7,
                            "children": [],
                            "name": "Hai Zhu District",
                            "parentId": 5
                        },
                        {
                            "id": 8,
                            "children": [],
                            "name": "Tian He District",
                            "parentId": 5
                        }
                    ],
                    "name": "Guangzhou",
                    "parentId": 3
                },
                {
                    "id": 6,
                    "children": [
                        {
                            "id": 9,
                            "children": [],
                            "name": "Fu Tian District",
                            "parentId": 6
                        },
                        {
                            "id": 10,
                            "children": [],
                            "name": "Nan Shan District",
                            "parentId": 6
                        }
                    ],
                    "name": "Shenzhen",
                    "parentId": 3
                }
            ],
            "name": "Guangdong",
            "parentId": 1
        },
        {
            "id": 4,
            "children": [
                {
                    "id": 12,
                    "children": [],
                    "name": "PuDong",
                    "parentId": 4
                }
            ],
            "name": "Shanghai",
            "parentId": 1
        }
    ],
    "name": "China",
    "parentId": 0
}