如何使 Guice 正确绑定此泛型？

Question

我正在处理消息的平台上工作，我运行遇到了 Guice 绑定问题。我已经编写了一个示例项目来显示我遇到的问题，这是一个看起来像这样的异常：

1) com.icacarealign.worker_example.PreprocessorMiddleware<MessageType> cannot be used as a key; It is not fully specified.

代码来了

接口

public interface Middleware {
  void configure();
}

public interface PreprocessorMiddleware<MessageType> extends Middleware {
  void prework(Context<MessageType> contextObj);
}

public interface PostprocessorMiddleware<MessageType> extends Middleware {
  void postwork(Context<MessageType> contextObj);
}

Guice 绑定

public class MiddlewareModule<MessageType> extends AbstractModule {
  private final List<Class<? extends PostprocessorMiddleware<MessageType>>> _postprocessors;
  private final List<Class<? extends PreprocessorMiddleware<MessageType>>> _preprocessors;

  public MiddlewareModule(
    List<Class<? extends PreprocessorMiddleware<MessageType>>> preprocessors,
    List<Class<? extends PostprocessorMiddleware<MessageType>>> postprocessors
  ) {
    _preprocessors = preprocessors;
    _postprocessors = postprocessors;
  }

  @Override
  protected void configure() {
    bindAllMiddleware(_preprocessors, new TypeLiteral<PreprocessorMiddleware<MessageType>>() {});
    bindAllMiddleware(_postprocessors, new TypeLiteral<PostprocessorMiddleware<MessageType>>() {});
  }

  private <T extends Middleware> void bindAllMiddleware(List<Class<? extends T>> middleware, TypeLiteral<T> type) {
    Multibinder<T> multibinder = Multibinder.newSetBinder(binder(), type);

    middleware.forEach(middlewareType -> bindMiddleware(multibinder, middlewareType));
  }

  private <T extends Middleware> void bindMiddleware(Multibinder<T> binder, Class<? extends T> type) {
    binder().bind(type).in(Singleton.class);
    binder.addBinding().to(type);
  }
}

主要方法

public class Main {
  public static void main(String[] args) {
    List<Class<? extends PreprocessorMiddleware<Message>>> preprocessorMiddlewares = new ArrayList<>();
    List<Class<? extends PostprocessorMiddleware<Message>>> postprocessorMiddlewares = new ArrayList<>();

    preprocessorMiddlewares.add(ArbitraryPrepreprocessorMiddleware.class);
    postprocessorMiddlewares.add(ArbitraryPostprocessorMiddleware.class);

    MiddlewareModule<Message> module = new MiddlewareModule<>(preprocessorMiddlewares, postprocessorMiddlewares);

    Injector injector = Guice.createInjector(module);
  }
}

我做错了什么？

Answer 1

您不能出于 Guice 绑定的目的在 TypeLiteral 中使用类型变量（在本例中为 MessageType）。相反，您需要重构以传递在不使用任何类型变量的情况下构造的 TypeToken（可能这意味着从调用站点传入它）。

是对类似情况的更复杂的回答。结果是，如果你真的需要避免在每个调用点都创建类型字面量，你可以再抽象一点，但它的复杂性成本相当高。

Answer 2

正如 jacobm 所描述的，您需要解决泛型问题，并且可能需要传入您要绑定的类型。幸运的是，您可以使用 Guice 的 Types.newParameterizedType 来创建完全指定的 TypeLiteral。

public class MiddlewareModule<MessageType> extends AbstractModule {
  private final Class<MessageType> _clazz;
  private final List<Class<? extends PostprocessorMiddleware<MessageType>>> _postprocessors;
  private final List<Class<? extends PreprocessorMiddleware<MessageType>>> _preprocessors;

  public MiddlewareModule(
    // Accept the message type in a way that survives erasure.
    Class<MessageType> clazz,
    List<Class<? extends PreprocessorMiddleware<MessageType>>> preprocessors,
    List<Class<? extends PostprocessorMiddleware<MessageType>>> postprocessors
  ) {
    _clazz = clazz;
    _preprocessors = preprocessors;
    _postprocessors = postprocessors;
  }

  @Override
  protected void configure() {
    // Use the Class to create your fully-specified TypeLiteral.
    bindAllMiddleware(_preprocessors,
        Types.newParameterizedType(PreprocessorMiddleware.class, _clazz));
    bindAllMiddleware(_postprocessors,
        Types.newParameterizedType(PostprocessorMiddleware.class, _clazz));
  }

  private <T extends Middleware> void bindAllMiddleware(List<Class<? extends T>> middleware, TypeLiteral<T> type) {
    Multibinder<T> multibinder = Multibinder.newSetBinder(binder(), type);

    middleware.forEach(middlewareType -> bindMiddleware(multibinder, middlewareType));
  }

  private <T extends Middleware> void bindMiddleware(Multibinder<T> binder, Class<? extends T> type) {
    bind(type).in(Singleton.class);  // Don't call binder() explicitly.
    binder.addBinding().to(type);
  }
}

请注意，这是未经测试的代码，因为我没有完整的 SSCCE；您可能需要调整类型参数，以便让 Guice 相信您在泛型中使用通配符是安全的。

如何使 Guice 正确绑定此泛型？

How do I make Guice bind this generic correctly?

java

guice