创建design-demo项目

项目代码：https://gitee.com/java_wxid/java_wxid/tree/master/demo/design-demo
项目结构如下（示例）：
在这里插入图片描述

创建ItemController

代码如下（示例）：

package com.example.designdemo.controller;

import com.example.designdemo.items.node.ProductItem;
import com.example.designdemo.service.ItemService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;
/**
 * @Author: zhiwei Liao
 * @Date: 2022/9/29 21:29
 * @Description:
 */
@RestController
public class ItemController {
    @Autowired
    private ItemService itemService;

    @GetMapping("get")
    public ProductItem getItem() {
        return itemService.getItem();
    }

    @PostMapping("del")
    public ProductItem delItem(@RequestBody ProductItem productItem) {
        return itemService.delItem(productItem);
    }

    @PostMapping("add")
    public ProductItem addItem(@RequestBody ProductItem productItem) {
        return itemService.addItem(productItem);
    }

}

创建ItemService

代码如下（示例）：

package com.example.designdemo.service;

import com.example.designdemo.items.node.ProductItem;

/**
 * @Author: zhiwei Liao
 * @Date: 2022/9/29 21:29
 * @Description:
 */
public interface ItemService {

    ProductItem getItem();

    ProductItem delItem(ProductItem productItem);

    ProductItem addItem(ProductItem productItem);

}

创建ItemServiceimpl

代码如下（示例）：

package com.example.designdemo.service.impl;

import com.example.designdemo.MockDb;
import com.example.designdemo.items.node.ProductItem;
import com.example.designdemo.items.visitor.AddItemVisitor;
import com.example.designdemo.items.visitor.DelItemVisitor;
import com.example.designdemo.service.ItemService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
/**
 * @Author: zhiwei Liao
 * @Date: 2022/9/29 21:29
 * @Description:
 */
@Service
public class ItemServiceimpl implements ItemService {
    @Autowired
    private DelItemVisitor delItemVisitor;

    @Autowired
    private AddItemVisitor addItemVisitor;

    //这部分只有初始化的时候获取一次 或者 直接预热到缓存中
    public ProductItem getItem() {
        System.out.println("从DB 获取所有的目录");
        System.out.println("将数据组装为 ProductItem");
        System.out.println("将组装好的 ProductItem 放入缓存中，永不过期 ");
        return MockDb.ProductItem;
    }

    public ProductItem delItem(ProductItem productItem) {
        ProductItem item = delItemVisitor.visitor(productItem);
        MockDb.ProductItem = item;
        System.out.println("update db");
        return item;
    }

    public ProductItem addItem(ProductItem productItem) {
        ProductItem item = addItemVisitor.visitor(productItem);
        MockDb.ProductItem = item;
        System.out.println("update db");
        return item;
    }
}

创建ItemVisitor

代码如下（示例）：

package com.example.designdemo.items.visitor;


import com.example.designdemo.items.node.ProductItem;
/**
 * @Author: zhiwei Liao
 * @Date: 2022/9/29 21:29
 * @Description:
 */
public interface ItemVisitor<T> {
    T visitor(ProductItem productItem);
}

创建DelItemVisitor

代码如下（示例）：

package com.example.designdemo.items.visitor;

import com.example.designdemo.MockDb;
import com.example.designdemo.items.node.ProductItem;
import org.springframework.stereotype.Component;
/**
 * @Author: zhiwei Liao
 * @Date: 2022/9/29 21:29
 * @Description:
 */
@Component
public class DelItemVisitor implements ItemVisitor<ProductItem>{
    // 入参是 id 2, pid为 1
    @Override
    public ProductItem visitor(ProductItem productItem) {
        ProductItem currentItem = MockDb.ProductItem; //  从缓存来的 to do
        if(productItem.getId() == currentItem.getId()) {
            throw new UnsupportedOperationException("根节点不能删。");
        }
        if(productItem.getPid() == currentItem.getId()) {
            currentItem.removeChild(productItem);
            return currentItem;
        }
        delChild(productItem, currentItem);
        return currentItem;
    }

    private void delChild(ProductItem productItem, ProductItem currentItem) {
        for(ProductItem item : currentItem.getChild()) {
            if(item.getId() == productItem.getPid()) {
                item.removeChild(productItem);
                break;
            } else {
                delChild(productItem, item);
            }
        }
    }
}

创建AddItemVisitor

代码如下（示例）：

package com.example.designdemo.items.visitor;

import com.example.designdemo.MockDb;
import com.example.designdemo.items.node.ProductItem;
import org.springframework.stereotype.Component;
/**
 * @Author: zhiwei Liao
 * @Date: 2022/9/29 21:29
 * @Description:
 */
@Component
public class AddItemVisitor implements ItemVisitor<ProductItem>{
    // 入参是 id 2, pid为 1
    @Override
    public ProductItem visitor(ProductItem productItem) {
        ProductItem currentItem = MockDb.ProductItem; //  从缓存来的 to do
        if(productItem.getId() == currentItem.getId()) {
            throw new UnsupportedOperationException("根节点是唯一的。");
        }
        if(productItem.getPid() == currentItem.getId()) {
            currentItem.addChild(productItem);
            return currentItem;
        }
        addChild(productItem, currentItem);
        return currentItem;
    }

    private void addChild(ProductItem productItem, ProductItem currentItem) {
        for(ProductItem item : currentItem.getChild()) {
            if(item.getId() == productItem.getPid()) {
                item.addChild(productItem);
                break;
            } else {
                addChild(productItem, item);
            }
        }
    }
}

创建ProductItem

代码如下（示例）：

package com.example.designdemo.items.node;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
/**
 * @Author: zhiwei Liao
 * @Date: 2022/9/29 21:29
 * @Description:
 */
public class ProductItem extends AbstractProductItem{
    private int id;
    private int pid;
    private String name;
    private List<ProductItem> child = new ArrayList<>();

    @Override
    public void removeChild(AbstractProductItem item) {
        ProductItem removeItem = (ProductItem) item;
        this.child = child.stream().filter(x->x.getId() != removeItem.getId()).collect(Collectors.toList());
    }

    @Override
    public void addChild(AbstractProductItem item) {
        this.child.add((ProductItem) item);
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getPid() {
        return pid;
    }

    public void setPid(int pid) {
        this.pid = pid;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<ProductItem> getChild() {
        return child;
    }

    public void setChild(List<ProductItem> child) {
        this.child = child;
    }
}

创建AbstractProductItem

代码如下（示例）：

package com.example.designdemo.items.node;
/**
 * @Author: zhiwei Liao
 * @Date: 2022/9/29 21:29
 * @Description:
 */
public abstract class AbstractProductItem {
    public abstract void removeChild(AbstractProductItem item);
    public abstract void addChild(AbstractProductItem item);
}

创建MockDb

代码如下（示例）：

package com.example.designdemo;


import com.example.designdemo.items.node.ProductItem;

import java.util.ArrayList;
import java.util.List;
/**
 * @Author: zhiwei Liao
 * @Date: 2022/9/29 21:29
 * @Description:
 */
public class MockDb {
    public static com.example.designdemo.items.node.ProductItem ProductItem = new ProductItem();
    static {
        ProductItem.setId(1);
        ProductItem.setPid(0);
        ProductItem.setName("书籍");
        List<ProductItem> child = new ArrayList<>();
        ProductItem c1 = new ProductItem();
        c1.setId(2);
        c1.setPid(1);
        c1.setName("技术书籍");
        ProductItem c2 = new ProductItem();
        c2.setId(3);
        c2.setPid(1);
        c2.setName("历史书籍");
        List<ProductItem> child1 = new ArrayList<>();
        ProductItem c3 = new ProductItem();
        c3.setId(4);
        c3.setPid(2);
        c3.setName("并发编程");
        ProductItem c4 = new ProductItem();
        c4.setId(5);
        c4.setPid(2);
        c4.setName("JVM");
        child1.add(c3);
        child1.add(c4);
        c1.setChild(child1);
        child.add(c1);
        child.add(c2);
        ProductItem.setChild(child);
    }
}