MongoDB推荐使用ObjectId作为主键，但国内的开发都知道，事情往往不如人所愿，当我们真的出现了“_id”主键的类型为String时，且还必须想用mongoTemplate.findOne或findList时，直接使用该方法会导致查询结果为空。
因为mongoTemplate会在查询的时候将主键转换为ObjectId。
实际上这一步转换是MappingMongoConverter中做的，源码如下：
/*** Converts the given raw id value into either {@link ObjectId} or {@link String}.** @param id can be {@literal null}.* @param targetType must not be {@literal null}.* @return {@literal null} if source {@literal id} is already {@literal null}.* @since 2.2*/
@Nullabledefault Object convertId(@Nullable Object id, Class<?> targetType) {if (id == null || ClassUtils.isAssignableValue(targetType, id)) {return id;}if (ClassUtils.isAssignable(ObjectId.class, targetType)) {if (id instanceof String) {if (ObjectId.isValid(id.toString())) {return new ObjectId(id.toString());}// avoid ConversionException as convertToMongoType will return String anyways.return id;}}try {return getConversionService().canConvert(id.getClass(), targetType)? getConversionService().convert(id, targetType): convertToMongoType(id, (TypeInformation<?>) null);} catch (ConversionException o_O) {return convertToMongoType(id,(TypeInformation<?>) null);}}注意示例版本为spring-data-mongodb:3.3.5，一些更古早的版本，如2.1.9，需要重写其他部分，如果有人需要请留言，我会抽时间补充，但鉴于之前的版本有比较重大的漏洞，所以推荐升级版本。
if (ObjectId.isValid(id.toString())) { return new ObjectId(id.toString()); }
这段就是mongoTemplate将String转换为ObjectId的步骤。
所以我们要做的就是重写这部分，并且替换掉原有的：
1、新建一个Converter
public class MyMongoConverter extends MappingMongoConverter {public MyMongoConverter(DbRefResolver dbRefResolver, MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext) {super(dbRefResolver, mappingContext);}@Overridepublic Object convertId(@Nullable Object id, Class<?> targetType) {if (id == null || ClassUtils.isAssignableValue(targetType, id)) {return id;}if (ClassUtils.isAssignable(ObjectId.class, targetType)) {if (id instanceof String) {return id;}}try {return getConversionService().canConvert(id.getClass(), targetType)? getConversionService().convert(id, targetType): convertToMongoType(id, (TypeInformation<?>) null);} catch (ConversionException o_O) {return convertToMongoType(id,(TypeInformation<?>) null);}}
}2、在构建MongoTemplate时用我们自己写的替换
MongoTemolate mongoTemplate = new MongoTemplate(mongoDbFactory(), getDefaultMongoConverter(mongoDbFactory()))protected static MongoConverter getDefaultMongoConverter(MongoDatabaseFactory factory) {DbRefResolver dbRefResolver = new DefaultDbRefResolver(factory);MongoCustomConversions conversions = new MongoCustomConversions(Collections.emptyList());MongoMappingContext mappingContext = new MongoMappingContext();mappingContext.setSimpleTypeHolder(conversions.getSimpleTypeHolder());mappingContext.afterPropertiesSet();MyMongoConverter converter = new MyMongoConverter(dbRefResolver, mappingContext);converter.setCustomConversions(conversions);converter.afterPropertiesSet();converter.setTypeMapper(new DefaultMongoTypeMapper(null));return converter;}/*** @return {@link MongoDatabaseFactory}*/protected MongoDatabaseFactory mongoDbFactory() {ServerAddress serverAddress = new ServerAddress(host, port);MongoClientSettings.Builder setting = MongoClientSettings.builder().applyToClusterSettings(builder -> builder.hosts(Collections.singletonList(serverAddress))).retryWrites(false);if (ObjectUtil.isNotEmpty(username)) {MongoCredential credential = MongoCredential.createCredential(username, authenticationDatabase, password.toCharArray());setting.credential(credential);}MongoClient mongoClient = MongoClients.create(setting.build());return new SimpleMongoClientDatabaseFactory(mongoClient, database);}host,port,username,authenticationDatabase,password,database
分别为MongoDB的地址，端口号，用户名，认证数据库，密码，数据库名
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