如何为不可变类创建默认构造函数 [英] How to create default constructor for immutable class

问题描述

我喜欢根据本文（为什么对象必须是不可变的）。

但是，我试图使用Jackson Object Mapper解析一个对象。我最初得到 JsonMappingException：找不到类型[simple type，class]的合适构造函数：无法从JSON对象实例化。

However, I am trying to parse an object using Jackson Object Mapper. I was initially getting JsonMappingException: No suitable constructor found for type [simple type, class ]: cannot instantiate from JSON object.

我可以按照这里，提供默认构造函数并使我的字段不是最终的。

I could fix it as mentioned here, by providing a default constructor and making my fields non-final.

import lombok.AccessLevel;
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.NonNull;

@AllArgsConstructor
// @NoArgsConstructor(access = AccessLevel.PRIVATE)
@Builder
@Data
public class School {

    @NonNull
    private final String schoolId;

    @NonNull
    private final String schoolName;
}

为了克服这个问题，我应该遵循什么样的编程风格？唯一的方法是让我的对象变得可变吗？

What is a good programming style that I should follow to overcome this problem? Is the only way around is to make my objects mutable?

我可以使用不使用默认构造函数的其他映射器吗？

Can I use a different mapper that does not use the default constructor?

推荐答案

您可以使用杰克逊工厂（使用 @ JsonCreator ）从地图上读取字段并调用非默认构造函数：

You can use a Jackson factory (method annotated with @JsonCreator) that reads fields off a map and calls your non-default constructor:

class School {
    //fields

    public School(String id, String name) {
        this.schoolId = id;
        this.schoolName = name;
    }

    @JsonCreator
    public static School create(Map<String, Object> object) {
        return new School((String) object.get("schoolId"), 
                          (String) object.get("schoolName"));
    }

    //getters
}

Jackson将使用json的 Map 版本调用 create 方法。这有效地解决了这个问题。

Jackson will call the create method with a Map version of the json. And this effectively solves the problem.

我相信你的问题是寻找杰克逊的解决方案，而不是新的模式/风格。

I believe your question looks for a Jackson solution, rather than a new pattern/style.

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