无法反序列化我的 POJO 从我的 Springboot POST 映射函数上的 Angular http post 请求发送

Question

就上下文而言，我的应用程序是一家咖啡店，我想将一系列项目发送到我的springboot后端。然而，杰克逊给出了一个例外：

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, 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]).

这就是项的类的样子(在省略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);
}
}

该请求在控制器类中被接收为：

@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[]。

在没有格式化JSON的情况下，JSON字符串的第65列出现错误：

[{"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

Answer 1

这个异常实际上说明得很好——您需要向POJO类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中不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(。

我希望你所说的为了清晰起见而省略的getter和setter确实存在，否则就不会起作用。