策略模式 | 易学教程

public interface HandlerMapResolver<K, V, R> {

  boolean support(K key, V value);

  R resolve(K key, V value);
}

import java.util.Collections;
import java.util.List;
import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;
import org.bson.types.ObjectId;

@Slf4j
@RequiredArgsConstructor
public class HandlerKafkaMessageResolverComposite {
  private final List<HandlerMapResolver> resolvers;

  public List<ObjectId> resolve(String key, String value) {
    for (HandlerMapResolver resolver : this.resolvers) {
      if (resolver.support(key, value)) {
        return (List<ObjectId>)resolver.resolve(key, value);
      }
    }
    log.warn("No resolver can handle this kafka message, key: {}, value: {}", key ,value);
    return Collections.emptyList();
  }
}

import java.util.Collections;
import java.util.List;
import org.bson.types.ObjectId;

public class OverrideEventResolver
    implements HandlerMapResolver<String, String, List<ObjectId>> {

  @Override
  public boolean support(String key, String value) {
    return true;
  }

  @Override
  public List<ObjectId> resolve(String key, String value) {
    return Collections.emptyList();
  }
}

策略模式
比如SpringMVC的HandlerMethodArgumentResolver接口，使用HandlerMethodArgumentResolverComposite(实现HandlerMethodArgumentResolver接口)，通过遍历内部所有的HandlerMethodArgumentResolver(当然有缓存机制)，选择support返回true的实例，并把接口的调用交给该实例处理

其实，我们在写业务代码的时候，很多童鞋喜欢用Map保存策略实现类，其实这样并不好，不够灵活，应该像spring这样提供一个support方法，可以支持更加复杂的逻辑判断

public class HandlerMethodArgumentResolverComposite implements HandlerMethodArgumentResolver {

    protected final Log logger = LogFactory.getLog(getClass());

    private final List<HandlerMethodArgumentResolver> argumentResolvers =
            new LinkedList<HandlerMethodArgumentResolver>();

    private final Map<MethodParameter, HandlerMethodArgumentResolver> argumentResolverCache =
            new ConcurrentHashMap<MethodParameter, HandlerMethodArgumentResolver>(256);

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return (getArgumentResolver(parameter) != null);
    }

    private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
        HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
        if (result == null) {
            for (HandlerMethodArgumentResolver methodArgumentResolver : this.argumentResolvers) {
                if (methodArgumentResolver.supportsParameter(parameter)) {
                    result = methodArgumentResolver;
                    this.argumentResolverCache.put(parameter, result);
                    break;
                }
            }
        }
        return result;
    }

    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

        HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
        if (resolver == null) {
            throw new IllegalArgumentException("Unknown parameter type [" + parameter.getParameterType().getName() + "]");
        }
        return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
    }

}

来源：https://www.cnblogs.com/tonggc1668/p/11454413.html

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策略模式