01/04/2017

今天遇到两个问题，都不知道是什么原因……

TypeReference读不出<T>

public interface Handler <I, O>{
  O handle (I input);
}

public abstract class AbstractHandler<T> implements Handler<Message, Response> {
  @Override
  public Response handler(Message message) throws Exception {
    ObjectMapper mapper = new ObjectMapper();
  // message.getData() returns List<Map<String, Object>>
  // What I am trying to do here is to convert 'List<Map<String, Object>>' to 'List<Client> or List<OtherType>'
    T domainObjects = mapper.readValue(mapper.writeValueAsString(message.getData(), new TypeReference<T>() {});
  // logic to build response
     return response;
  }
}

public abstract class ClientHandler extends AbstractHandler<List<Client>> {
}

public class FirstClientHandler extends ClientHandler {
  // logic related to FirstClientHandler
}

如果这样些的话，在调用FirstClientHandler的handle时候, TypeReference会在创建新对象的时候拿到T作为它的reference，显然这个T什么都不是，自然不能被jackson转化成功。只是不明白为什么这个T为什么没有从ClientHandler传递过来……

Paste_Image.png

但是如果把这部分code放到具体实现里就完全没有问题了。（这我暂时的解决办法）

public abstract class AbstractHandler implements Handler<Message, Response> {
}

public abstract class ClientHandler extends AbstractHandler<List<Client>> {
  @Override
  public Response handler(Message message) throws Exception {
    ObjectMapper mapper = new ObjectMapper();
  // Use actual data type instead of 'T'
    List<Client> domainObjects = mapper.readValue(mapper.writeValueAsString(message.getData(), new TypeReference<List<Client>() {});
  // logic to build response
     return response;
  }
}

所以问题在于我对继承GenericType的理解还是在于这个TypeReference的实现？不想把这段code放在ClientHandler里的原因是我还有别的各种handler都要用同样的逻辑，差别只在于List里的type。

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第二个问题大约google就可以了，只是上班的时间没有顾得上……
用低version的java跑高version的java compile出来的code会有问题吗？
Here is the answer from stackOverFlow

Short answer is yes, but in some cases (with new features that old version doesn't have), then no.

以上。