无法反序列化我的Springboot POST映射函数上从Angular http发布请求发送的POJO

问题描述

对于上下文，我的应用程序是一家咖啡店，我想将一系列商品发送到我的springboot后端。但是杰克逊给出了例外：

Cannot construct instance of `me.andrewq.coffeeshop.menu_items.Menu` 
(no Creators,like default constructor,exist): cannot deserialize from Object value 
(no delegate- or property-based Creator)
at [Source: (pushbackinputstream); line: 1,column: 3] (through reference chain: 
java.util.ArrayList[0])] with root cause
com.fasterxml.jackson.databind.exc.InvalidDeFinitionException: 
Cannot construct instance of `me.andrewq.coffeeshop.menu_items.Menu` 
(no Creators,column: 3] (through reference chain: 
java.util.ArrayList[0]).

这是项目的类的样子（省略了setter和getter之后）：

public class Menu {

    private int productId;

    private String name;

    private double price;

    private String[][] productOptions;

    private String type;

    // These 3 variables belong to drinks. The creams and sugars more so for coffees
    private String currentSize;

    private Integer creams;

    private Integer sugars;


    public Menu(int productId,String name,double price,String productOptions,String type){
        this.productId = productId;
        this.name = name;
        this.price = price;
        this.productOptions = convertOptions(productOptions);
        this.type = type;
    }

    /**
     * Used for converting the product options which is a key-value pair seperated by a ',' in the DB,into a 2D array in this class.
     * @param options
     * @return
     */
    private String[][] convertOptions(String options) {
        String[] optionPairs = options.split(",");

        //hard coded b/c I kNow that these are pairs 
        String retVal[][] = new String[optionPairs.length][2];

        for(int i = 0; i < optionPairs.length; ++i){
            String[] temp = optionPairs[i].split(":");
            retVal[i] =  temp;
        }

        return retVal;
    }
 
    @Override
    public String toString(){

        return String.format("{productId: %i,name: %s}",this.productId,this.name);
    }

}

请求在控制器类中的接收方式为：

@RestController
public class OrderController {
    
    @CrossOrigin(origins = "http://localhost:4200")
    @PostMapping(path = "/guestOrder")
    public String order(@RequestBody List<Menu> order){
        
        for(Menu item: order){
            System.out.println(item.toString());
        }
        
        return "Sending order worked";
    }
}

在Angular中，项目定义为：

export interface Menu {
    productId: number;

    name: string;
    
    price: number;
    
    productOptions: string[][];
    
    type: string;

    // additional field for drinks and coffees
    currentSize: string;

    creams: number;

    sugars: number;
}

http请求调用为： this.http.post<string>(`${this.url}/guestOrder`,this.orderItems);，其中http: HttpClient和orderItems: Menu[]。

在不格式化JSON的情况下，JSON字符串的第65列会发生错误：

[{"productId":1,"name":"Iced Coffee","price":2,"productOptions":[["S","2.00"],["M","2.50"],["L","3.00"]],"type":"IC","currentSize":"S","creams":0,"sugars":0}]

这是productOptions的第一个括号

解决方法

该异常实际上说得很好-您需要在POJO类中添加一个默认构造函数。

JSON解析器的工作方式是，首先创建一个空实例，然后为JSON文本中遇到的每个属性调用setter方法。 JSON中未包含的属性保持不变，因此具有默认构造函数为其分配的值（通常为null，除非您将其设置为其他值）。

我希望您说的为清晰起见而省略的getter和setter方法确实存在，否则将无法正常工作。