英文:
Jackson: deserialize to different type
问题
我知道已经有一个类似的问题存在（https://stackoverflow.com/questions/10427929/jackson-deserializing-to-a-different-type），但那里的答案对我没有帮助。旧的 JSON 文件已经在生产中使用，无法更改。
我有一个常规的应用程序，使用旧的 POJO（普通Java对象）。现在我有了新的 POJO，目前为了支持旧文件，我正在反序列化为旧的 POJO，然后手动转换为新的 POJO。我希望直接将转换编写到Jackson反序列化器中，但我遇到了问题。过去我曾编写过一个简单的Jackson模块，用于序列化和反序列化特定类型，并且我对Jackson有一些基础知识。
我有类似以下的代码：
public class MyModule extends com.fasterxml.jackson.databind.module.SimpleModule {
public MyModule() {
super("MyModule", Version.unknownVersion());
addDeserializer(OldPojo.class, new JsonDeserializer<NewPojo>() {
@Override
public NewPojo deserialize(JsonParser p, DeserializationContext ctxt) {
// 转换并返回新的POJO
}
});
// 其他的反序列化器...
}
}
这种方法可行吗？我做错了什么？非常感谢。
英文:
I know a similar question already exists (https://stackoverflow.com/questions/10427929/jackson-deserializing-to-a-different-type) but the answer there does not help me. The old JSON files are already in production and cannot be changed.
I have a regular application with old POJOs. Now I have new POJOs and currently to support the old files I am deserializing to the old POJOs and converting manually to the new ones. I would like to write the conversion directly into a Jackson deserializer but I am having trouble. In the past I have writen a simple Jackson module for serializing and deserializing certain types and I have some basic knowledge of Jackson.
I have something like:
public class MyModule extends com.fasterxml.jackson.databind.module.SimpleModule
public MyModule() {
super("MyModule", Version.unknownVersion());
addDeserializer(OldPojo.class, new JsonDeserializer<NewPojo>() {
@Override
public NewPojo deserialize(JsonParser p, DeserializationContext ctxt) {
// convert and return new pojo
}
}
// others deserializers ...
}
}
Would this approach be possible? And what am I doing wrong? Lots of thanks.
答案1
得分: 0
SimpleModule.addDeserializer()] [1] 的文档为该方法提供了以下签名：
public <T> SimpleModule addDeserializer(Class<T> type, JsonDeserializer<? extends T> deser)
其中 <T> 是 JsonDeserializer.deserialize() 的返回类型（好的，<? extends T> 就是该类型……）。
所以在您的情况下，您必须能够将 NewPojo 实例强制转换为 OldPojo 引用：
NewPojo o = new NewPojo();
OldPojo r = (OldPojo) o;
如果这样的转换没有引发 ClassCastException，那么您的方法可能会成功……
您说过您不能更改 JSON 文件 - 但是您是否必须保留 OldPojo 类？为什么下面的代码段不起作用：
…
addDeserializer( NewPojo.class, new JsonDeserializer<NewPojo>() )
{
…
英文:
The [documentation for `SimpleModule.addDeserializer()] 1 gives the signature for that method like this:
public <T> SimpleModule addDeserializer(Class<T> type, JsonDeserializer<? extends T> deser)
where <T> is the return type of JsonDeserializer.deserialize() (ok, <? extends T> is that type …).
So in your case, you must be able to case a NewPojo instance to an OldPojo reference:
NewPojo o = new NewPojo();
OldPojo r = (OldPojo) o;
If that works without a ClassCastException, you approach may be successful …
You said that you cannot change the JSON files – but do you have to keep the OldPojo class as well? Why does
…
addDeserializer( **NewPojo.class**, new JsonDeserializer<NewPojo>() )
{
…
do not work for you?
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