如何正确使用Jackson Mixin注释来实例化第三方类？

问题描述

我有一个第三方库类（来自Apache Axis），我想通过Jackson JSON序列化：

I have a third party library class (from Apache Axis) that I want to serialize by Jackson JSON:

public class NonNegativeInteger extends BigInteger {

    public NonNegativeInteger(byte[] val) {
        super(val);
        checkValidity();
    } // ctor

    public NonNegativeInteger(int signum, byte[] magnitude) {
        super(signum, magnitude);
        checkValidity();
    } // ctor

    public NonNegativeInteger(int bitLength, int certainty, Random rnd) {
        super(bitLength, certainty, rnd);
        checkValidity();
    } // ctor

    public NonNegativeInteger(int numBits, Random rnd) {
        super(numBits, rnd);
        checkValidity();
    } // ctor

    public NonNegativeInteger(String val) {
        super(val);
        checkValidity();
    }

    public NonNegativeInteger(String val, int radix) {
        super(val, radix);
        checkValidity();
    } // ctor

    /**
     * validate the value against the xsd definition
     */
    private BigInteger zero = new BigInteger("0");
    private void checkValidity() {
        if (compareTo(zero) < 0) {
            throw new NumberFormatException(
                    Messages.getMessage("badNonNegInt00")
                    + ":  " + this);
        }
    } // checkValidity

    /**
     * Work-around for http://developer.java.sun.com/developer/bugParade/bugs/4378370.html
     * @return BigIntegerRep
     * @throws ObjectStreamException
     */ 
    public Object writeReplace() throws ObjectStreamException {
        return new BigIntegerRep(toByteArray());
    }

    protected static class BigIntegerRep implements java.io.Serializable {
        private byte[] array;
        protected BigIntegerRep(byte[] array) {
            this.array = array;
        }
        protected Object readResolve() throws java.io.ObjectStreamException {
            return new NonNegativeInteger(array);
        }
    }
}

我的实体类包含我希望通过JSON序列化的 NonNegativeInteger 字段：

I have my entity class containing a NonNegativeInteger field that I want to serialize by JSON:

public class TestEntity {

    private NonNegativeInteger number;

    public NonNegativeInteger getNumber() {
        return number;
    }

    public void setNumber(NonNegativeInteger number) {
        this.number = number;
    }
}

当我通过Jackson JSON序列化上述对象时，我得到以下错误：

When I serialize the above object by Jackson JSON, I got the following error:

Can not instantiate value of type [simple type, class org.apache.axis.types.NonNegativeInteger] from Integral number; no single-int-arg constructor/factory method

然后我查看了POST请求实体，它是实际上 {number：10} 由杰克逊序列化。但由于 NonNegativeInteger 没有构造函数采用单个int，因此Jackson无法实例化 NonNegativeInteger 对象。所以我按照某人的建议为 NonNegativeInteger 添加了一个Mixin类，这样它就会有一个int为arg的构造函数：

Then I looked at the POST request entity, it is actually {"number" : 10} as serialized by Jackson. But since NonNegativeInteger doesn't have a constructor taking a single-int, Jackson can't instantiate a NonNegativeInteger object. So I followed someone's suggestions to add a Mixin class for NonNegativeInteger so that it will have a constructor with int as arg:

public abstract class NonNegativeIntegerMixin extends NonNegativeInteger {

    @JsonCreator
    public NonNegativeIntegerMixin(int val) {
        super(String.valueOf(val));
    }
}

然后我在我的JSON配置类中注册了它： / p>

Then I registered it in my JSON configuration class:

 objectMapper.addMixInAnnotations(NonNegativeInteger.class, NonNegativeIntegerMixin.class);

但它没有帮助，它仍然报告了同样的错误。我尝试手动编写JSON请求主体为 {number：10} 然后它运行良好。但我的客户端使用Jackson来序列化 NonNegativeInteger 。杰克逊自动转换为 {number：10} ，不带引号。如何解决此错误？

But it doesn't help, it still reported the same error. I tried manually writing the JSON request body to be {"number": "10"} then it worked well. But my client side used Jackson to serialize NonNegativeInteger. Jackson automatically converts to {"number": 10} without quotes. How can I fix this error?

编辑

NonNegativeInteger 类没有任何类字段（常量零字段除外）。 数字键来自我的 TestEntity 类。所以即使我在 NonNegativeIntegerMixin mixin类中添加 @JsonProperty 注释，Jackson JSON也不会实例化 NonNegativeInteger ，带有int类型arg。因此，我仍然遇到同样的错误。

The NonNegativeInteger class doesn't have any class field (except a constant zero field). The number key is from my TestEntity class. So I even if I add @JsonProperty annotation in my NonNegativeIntegerMixin mixin class, Jackson JSON will not instantiate a NonNegativeInteger with an int type arg. Therefore, I still got the same error.

推荐答案

您的混合注释对我来说似乎是正确的。所以这可能是一个错误;所以也许是文件错误：

Your mix-in annotation seems correct to me. So this could be a bug; so maybe file bug at:

问题的一个可能原因是类正在扩展 BigInteger ，它有现有的反序列化器。这可能导致注释被忽略，以及实例的反序列化。

One possible reason for problems is that class is extending BigInteger, which has existing deserializer. This could result in annotations being ignored, wrt how instances are deserialized.

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