Spring Data Reactive Mongo：类类型转换器

Question

我正在使用带有反应式 mongo 存储库的 Spring boot 2 webflux。我正在尝试使用 Class<> 字段类型来持久化和查询实体。但我不知道如何配置我的应用程序以使用这种类型。 这是我想存储在 mongo 中的实体：

import lombok.Data;
import org.springframework.data.annotation.Id;

@Data
public class ServiceEntity {

  @Id
  private String id;
  private String name;
  private Class inputClass;
}

这是我的 mongo 配置：

import static java.util.Arrays.asList;

import com.mongodb.MongoClient;
import com.mongodb.MongoException;
import com.mongodb.async.client.MongoClientSettings;
import lombok.extern.slf4j.Slf4j;
import org.bson.BsonReader;
import org.bson.BsonWriter;
import org.bson.codecs.Codec;
import org.bson.codecs.DecoderContext;
import org.bson.codecs.EncoderContext;
import org.bson.codecs.configuration.CodecProvider;
import org.bson.codecs.configuration.CodecRegistries;
import org.bson.codecs.configuration.CodecRegistry;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.convert.converter.Converter;
import org.springframework.data.mongodb.core.convert.MongoCustomConversions;

@Configuration
@Slf4j
public class MongoConfiguration {

  @Bean
  public MongoClientSettings settings() {
    log.debug("Configure mongo settings");
    return MongoClientSettings.builder()
        .codecRegistry(CodecRegistries.fromRegistries(
            CodecRegistries.fromProviders(new ClassCodecProvider()),
            MongoClient.getDefaultCodecRegistry()))
        .build();
  }

  //  @Bean
  public MongoCustomConversions customConversions() {
    log.debug("Configure mongo custom conversions");
    return new MongoCustomConversions(asList(new StringToClassConverter()));
  }

  private static class ClassCodec implements Codec<Class> {

    @Override
    public Class decode(BsonReader reader, DecoderContext decoderContext) {
      try {
        return Class.forName(reader.readString());
      } catch (ClassNotFoundException e) {
        throw new MongoException("Couldn't read value as class type", e);
      }
    }

    @Override
    public void encode(BsonWriter writer, Class value, EncoderContext encoderContext) {
      writer.writeString(value.getName());
    }

    @Override
    public Class<Class> getEncoderClass() {
      return Class.class;
    }
  }

  private static class ClassCodecProvider implements CodecProvider {

    @Override
    public <T> Codec<T> get(Class<T> clazz, CodecRegistry registry) {
      if (clazz == Class.class) {
        return (Codec<T>) new ClassCodec();
      }
      return null;
    }
  }

  private static class StringToClassConverter implements Converter<String, Class> {

    @Override
    public Class convert(String source) {
      try {
        return Class.forName(source);
      } catch (ClassNotFoundException e) {
        throw new MongoException("Couldn't read string as class type", e);
      }

    }
  }

}

问题是当我尝试从 mongo 读取实体时出现错误：

org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.String] to type [java.lang.Class<?>]
    at org.springframework.core.convert.support.GenericConversionService.handleConverterNotFound(GenericConversionService.java:321)
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:194)
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:174)
    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.getPotentiallyConvertedSimpleRead(MappingMongoConverter.java:887)
    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.readValue(MappingMongoConverter.java:1392)

我还创建了字符串到类转换器（请参阅配置中的StringToClassConverter）并尝试将其用作自定义转换器。但是当我将这样的配置放到上下文中时（在 customConversions() 方法上取消注释 @Bean 注释），它已被用于任何字符串转换。结果，在保存实体时：

org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [java.lang.Class<?>] for value 'service1'; nested exception is com.mongodb.MongoException: Couldn't read string as class type
    at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:46)
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:191)
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:174)
    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.getPotentiallyConvertedSimpleWrite(MappingMongoConverter.java:849)
    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.writeSimpleInternal(MappingMongoConverter.java:829)
    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.writeProperties(MappingMongoConverter.java:488)

完整代码可在github 获得。转载请见com.example.mongoclasspertist.ServiceRepositoryTest#test。

通过将 PR 提交到 Spring 存储库，我看到了如何解决这个问题的下一个方法（但它们可能会导致另一个问题）： * 在org.springframework.data.mongodb.core.convert.MongoConverters 中创建ClassToStringConverter 和StringToClassConverter * 更改org.springframework.data.mongodb.core.convert.MappingMongoConverter#getPotentiallyConvertedSimpleWrite 方法逻辑，仅当源和目标类型适用于自定义转换器时进行转换。

请帮我解决这个问题。

Answer 1

在转换器上使用注释@ReadingConverter 似乎可以完成这项工作。请试一试。

@ReadingConverter
private static class StringToClassConverter implements Converter<String, Class> 
{