Jackson，使用私有字段和 arg-constructor 反序列化类，没有注释 [英] Jackson, deserialize class with private fields and arg-constructor without annotations

问题描述

是否可以使用 Jackson 反序列化为具有私有字段和自定义参数构造函数的类，而无需使用注释和修改类?

It is possible to deserialize to a class with private fields and a custom argument constructor without using annotations and without modifying the class, using Jackson?

我知道在 Jackson 中使用这种组合是可能的:1) Java 8, 2) 使用-parameters"编译；选项，以及 3) 参数名称匹配 JSON.但默认情况下，在 GSON 中也可以没有所有这些限制.

I know it's possible in Jackson when using this combination: 1) Java 8, 2) compile with "-parameters" option, and 3) the parameters names match JSON. But it's also possible in GSON by default without all these restrictions.

例如:

public class Person {
    private final String firstName;
    private final String lastName;
    private final int age;

    public Person(String firstName, String lastName, int age) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }

    public static void main(String[] args) throws IOException {
        String json = "{firstName: "Foo", lastName: "Bar", age: 30}";
        
        System.out.println("GSON: " + deserializeGson(json)); // works fine
        System.out.println("Jackson: " + deserializeJackson(json)); // error
    }

    public static Person deserializeJackson(String json) throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES);
        mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
        return mapper.readValue(json, Person.class);
    }

    public static Person deserializeGson(String json) {
        Gson gson = new GsonBuilder().create();
        return gson.fromJson(json, Person.class);
    }
}

这对 GSON 来说效果很好，但 Jackson 抛出了:

Which works fine for GSON, but Jackson throws:

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `jacksonParametersTest.Person` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
 at [Source: (String)"{firstName: "Foo", lastName: "Bar", age: 30}"; line: 1, column: 2]
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67)

这在 GSON 中是可能的，所以我希望在 Jackson 中必须有某种方式而无需修改 Person 类，无需 Java 8，也无需显式自定义反序列化器.有人知道解决办法吗?

It's possible in GSON, so I would expect that there must be some way in Jackson without modifying the Person class, without Java 8, and without an explicit custom deserializer. Does anybody know a solution?

• 更新，附加信息

Gson 似乎跳过了参数构造函数，因此它必须使用反射在幕后创建一个无参数构造函数.

Gson seems to skip the argument constructor, so it must be creating a no-argument constructor behind the scenes using reflections.

此外，还有一个 Kotlin Jackson 模块 可以为Kotlin 数据类，即使没有-parameters"编译器标志.所以奇怪的是，Java Jackson 似乎不存在这样的解决方案.

Also, there exists a Kotlin Jackson module which is able to do this for Kotlin data classes, even without the "-parameters" compiler flag. So it is strange that such a solution doesn't seem to exist for Java Jackson.

这是 Kotlin Jackson 中可用的(漂亮且干净的)解决方案(IMO 也应该通过自定义模块在 Java Jackson 中可用):

This is the (nice and clean) solution available in Kotlin Jackson (which IMO should also become available in Java Jackson via a custom module):

val mapper = ObjectMapper()
    .enable(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES)
    .registerModule(KotlinModule())     
        
val person: Person = mapper.readValue(json, Person::class.java)

推荐答案

带有混合注解的解决方案

您可以使用混合注释.当修改类不是一种选择时，这是一个很好的选择.您可以将其视为一种在运行时添加更多注释的面向方面的方式，以增强静态定义的注释.

Solution with mix-in annotations

You could use mix-in annotations. It's a great alternative when modifying the classes is not an option. You can think of it as kind of aspect-oriented way of adding more annotations during runtime, to augment the statically defined ones.

假设您的 Person 类定义如下:

Assuming that your Person class is defined as follows:

public class Person {

    private final String firstName;
    private final String lastName;
    private final int age;

    public Person(String firstName, String lastName, int age) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }

    // Getters omitted
}

首先定义一个mix-in注解抽象类:

First define a mix-in annotation abstract class:

public abstract class PersonMixIn {

    PersonMixIn(@JsonProperty("firstName") String firstName,
                @JsonProperty("lastName") String lastName,
                @JsonProperty("age") int age) {
    }
}

然后配置 ObjectMapper 以使用定义的类作为 POJO 的混合:

Then configure ObjectMapper to use the defined class as a mix-in for your POJO:

ObjectMapper mapper = new ObjectMapper();
mapper.enable(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES);
mapper.addMixIn(Person.class, PersonMixIn.class);

并反序列化 JSON:

And deserialize the JSON:

String json = "{firstName: "Foo", lastName: "Bar", age: 30}";
Person person = mapper.readValue(json, Person.class);

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