杰克逊反序列化绕过最终领域

Question

这是代码

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.Data;
import lombok.ToString;

public class Main {
    public static void main(String[] args) throws Exception {
        Fields f1 = new Fields(1);
        System.out.println(f1);

        ObjectMapper mapper = new ObjectMapper();
        String str = mapper.writeValueAsString(f1);
        System.out.println(str);

        Fields f2 = mapper.readValue(str, Fields.class);
        System.out.println(f2);
    }

    @Data
    @ToString
    public static class Fields {
        private final long value1;
        private final long value2;

        public Fields(@JsonProperty("blah") long value) {
            this.value1 = value++;
            this.value2 = value++;
            System.out.println(this);
        }
    }
}

输出

Main.Fields(value1=1, value2=2)
Main.Fields(value1=1, value2=2)
{"value1":1,"value2":2}
Main.Fields(value1=0, value2=1)
Main.Fields(value1=1, value2=2)

我的问题是：

为什么jackson在构造完成后修改了没有setter的private final fields？如果这是有意的，我该如何关闭它？
如果jackson可以直接设置字段，为什么还需要我用@JsonProperty注解构造函数？（从 Fields 中删除 @JsonProperty 会导致错误；我什至不需要使用正确的属性进行注释）

谢谢

Answer 1

Why is it required that I annotate the constructor with @JsonProperty?

不是。需要的是一个可访问的构造函数。你可以有一个无参数的构造函数

public Fields() {
    this.value1 = 0;
    this.value2 = 0;
    System.out.println("cons: " + this);
}

（这必然初始化字段，因为它们是 final）或者你可以有一个构造函数，Jackson 将尝试根据声明的 @JsonProperty 名称解析。请注意 JsonProperty#required 默认为 false。

Why did jackson modify private final fields that do not have setters after finish constructing it? If this is intended, how do I turn it off?

因为可以。因此，它允许您使用不可变类型进行反序列化。据我所知，没有任何内置方法可以禁用此功能。

Answer 2

Why did jackson modify private final fields that do not have setters after finish constructing it? If this is intended, how do I turn it off?

您可以在配置映射器时将 MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS 属性设置为 false（默认为 true）。

示例：

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.MapperFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Temp {

private static final String RAW = "{\"value1\": \"aabbcc\",\"value2\":\"zzzzz\"}";

public static void main(String[] args) throws Exception {
    System.out.println(new ObjectMapper().readValue(RAW, TestClass.class));

    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false); // you will receive UnrecognizedPropertyException without this line
    mapper.configure(MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS, false);

    System.out.println(mapper.readValue(RAW, TestClass.class));
}

public static class TestClass {

    private final String value1;
    private final String value2;

    @JsonCreator
    public static TestClass createTestClass(
            @JsonProperty("value1") String value1,
            @JsonProperty("blah") String value2) {

        return new TestClass(value1, value2);
    }

    private TestClass(String value1, String value2) {
        this.value1 = value1;
        this.value2 = value2;
    }

    public String getValue1() {
        return value1;
    }

    public String getValue2() {
        return value2;
    }

    @Override
    public String toString() {
        return "TestClass{" + "value1=" + value1 + ", value2=" + value2 + '}';
    }
}

}

输出：

TestClass{value1=aabbcc, value2=zzzzz}
TestClass{value1=aabbcc, value2=null}

杰克逊反序列化绕过最终领域

Jackson deserialization circumventing final fields

java

json

immutability

jackson

lombok