使用通过反射创建的对象的问题

Question

我正在尝试编写一个程序，可用于向一组订阅者发送消息。用于传递消息的订阅者和技术并不重要。消息类型由XML模式定义，并作为Java API实现。有一个抽象的超类型，所有的消息类型都可以扩展。API定义了静态方法来允许XML和Java之间的转换，反之亦然(fromXML()和toXML())。

我很难实现这一点。我已经包括了一个示例的代码，我试图得到下面的工作。它不按编写的方式编译。抽象超类型是MessageType类，我的计划是传递"真实"消息类型的名称，并让此代码创建必要的对象来执行发送指定子类型消息所需的操作。

public class Writer {
  public static void runExample(String[] args) throws Exception {
    UUID serviceID = UUID.fromString(args[1]);
    ServiceBus bus = ServiceBus.getServiceBus();
    bus.init(serviceID);
    // The intention is to pass the message type name so that this code can be used for any
    // "message" type.
    String msgName = args[0];
    // Get the message type class from the passed name.
    Class<? extends MessageType> msgClass = (Class<? extends MessageType>) Class.forName(msgName);
    // Create a writer for the specified message type.
    MessageWriter<? extends MessageType> writer = bus.createWriter(msgName, msgClass);
    // Read in some XML content from a file.
    String xml = loadMsg(args[2]);
    // Want to do something like this:
    msgClass object = msgClass.fromXML(xml);
    // Of course, this does not compile. Is there a way to do this?
    // Create a java object of the message type from the XML read in.
    MessageType object = MessageType.fromXML(xml);
    // With the line above the statement below fails to compile with the error:
    //   The method write(capture#4-of ? extends MessageType) in the type
    //   MessageWriter<capture#4-of ? extends MessageType> is not applicable for
    //   the arguments (MessageType)
    writer.write(object);
  }
  private static String loadMsg(String fileName) throws IOException {
    BufferedReader reader = new BufferedReader(new FileReader(new File(fileName)));
    String line = null;
    StringBuilder sb = new StringBuilder();
    do {
    line = reader.readLine();
    if (line != null)
      sb.append(line);
    } while (line != null);
    reader.close();
    return sb.toString();
  }
}

有谁知道我上面描述的是否有可能做到?

Answer 1

由于您的Writer是MessageWriter<? extends MessageType>，这意味着它可以是任何MessageWriter，它采用MessageType的子类。所以实际的实例可以是MessageWriter<MySuperMessageType>。由于MessageType实例不能被编译器确认为MySuperMessageType，编译器会使其失败。

如果写入器的类型为MessageWriter<MessageType>或MessageWritier<? super MessageType>，则可以工作。另一个选择是创建一个泛型方法…

private <T extends MessageType> myMethod(Class<T> type){
   MessageWriter<T> writer = bus.createWriter(msgName, msgClass);
   String xml = loadMsg(args[2]);
   // you could use reflection to get the public static methods from the class instance
   // msgClass object = msgClass.fromXML(xml);
   T object = (T) MessageType.fromXML(xml);
   writer.write(object);
}