使用Jackson序列化/反序列化DynamoDB Record类遇构建方法错误求助

我需要用Jackson对DynamoDB的Record类进行序列化/反序列化操作，但无法修改该类，因此采用Jackson Mixin实现，具体代码如下：

@JsonDeserialize( builder = RecordBuilderMixIn.class )
private abstract static class RecordMixIn
{
}

@JsonPOJOBuilder( withPrefix = "" )
private abstract class RecordBuilderMixIn
{
}

var mapper = new ObjectMapper()
                .addMixIn( Record.Builder.class, RecordBuilderMixIn.class )
                .addMixIn( Record.class, RecordMixIn.class );

执行mapper.readValue(json, Record.class);时，出现如下错误：

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Builder class `com.surecloud.jackson.RecordObjectMapper$RecordBuilderMixIn` does not have build method (name: 'build')

解决方法 #

Jackson基于Builder模式反序列化时，要求Builder类必须提供一个生成目标实例的构建方法（默认名为build）。你的RecordBuilderMixIn是抽象类且未声明该方法，导致Jackson无法识别如何构建Record对象。

修改步骤： #

1. 在RecordBuilderMixIn中声明与原Record.Builder类里build()方法签名完全一致的抽象方法，让Jackson能找到构建入口。
2. 如果原Builder类的构建方法不是默认的build，可以通过@JsonPOJOBuilder的buildMethodName属性指定方法名。

修改后的代码示例：

@JsonDeserialize( builder = RecordBuilderMixIn.class )
private abstract static class RecordMixIn
{
}

@JsonPOJOBuilder( withPrefix = "" )
private abstract class RecordBuilderMixIn
{
    // 声明与原Record.Builder的build方法匹配的抽象方法
    public abstract Record build();
}

var mapper = new ObjectMapper()
                .addMixIn( Record.Builder.class, RecordBuilderMixIn.class )
                .addMixIn( Record.class, RecordMixIn.class );

特殊情况处理 #

如果原Record.Builder的构建方法名为create而非build，只需调整@JsonPOJOBuilder注解：

@JsonPOJOBuilder( withPrefix = "", buildMethodName = "create" )
private abstract class RecordBuilderMixIn
{
    public abstract Record create();
}

内容的提问来源于stack exchange，提问作者kylie.zoltan