一、背景
spring-data-mongo 实现了基于 MongoDB 的 ORM-Mapping 能力，
通过一些简单的注解、Query 封装以及工具类，就可以通过对象操作来实现集合、文档的增删改查；
在 SpringBoot 体系中，spring-data-mongo 是 MongoDB Java 工具库的不二之选。
二、问题产生
在一次项目问题的追踪中，发现 SpringBoot 应用启动失败，报错信息如下：
Error creating bean with name 'mongoTemplate' defined in class path resource [org/bootfoo/BootConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.data.mongodb.core.MongoTemplate]: Factory method 'mongoTemplate' threw exception; nested exception is org.springframework.dao.DataIntegrityViolationException: Cannot create index for 'deviceId' in collection 'T_MDevice' with keys '{ "deviceId" : 1}' and options '{ "name" : "deviceId"}'. Index already defined as '{ "v" : 1 , "unique" : true , "key" : { "deviceId" : 1} , "name" : "deviceId" , "ns" : "appdb.T_MDevice"}'.; nested exception is com.mongodb.MongoCommandException: Command failed with error 85: 'exception: Index with name: deviceId already exists with different options' on server 127.0.0.1:27017. The full response is { "createdCollectionAutomatically" : false, "numIndexesBefore" : 6, "errmsg" : "exception: Index with name: deviceId already exists with different options", "code" : 85, "ok" : 0.0 } at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:588) at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88) at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:366) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1264) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553) ... Caused by: org.springframework.dao.DataIntegrityViolationException: Cannot create index for 'deviceId' in collection 'T_MDevice' with keys '{ "deviceId" : 1}' and options '{ "name" : "deviceId"}'. Index already defined as '{ "v" : 1 , "unique" : true , "key" : { "deviceId" : 1} , "name" : "deviceId" , "ns" : "appdb.T_MDevice"}'.; nested exception is com.mongodb.MongoCommandException: Command failed with error 85: 'exception: Index with name: deviceId already exists with different options' on server 127.0.0.1:27017. The full response is { "createdCollectionAutomatically" : false, "numIndexesBefore" : 6, "errmsg" : "exception: Index with name: deviceId already exists with different options", "code" : 85, "ok" : 0.0 } at org.springframework.data.mongodb.core.index.MongoPersistentEntityIndexCreator.createIndex(MongoPersistentEntityIndexCreator.java:157) at org.springframework.data.mongodb.core.index.MongoPersistentEntityIndexCreator.checkForAndCreateIndexes(MongoPersistentEntityIndexCreator.java:133) at org.springframework.data.mongodb.core.index.MongoPersistentEntityIndexCreator.checkForIndexes(MongoPersistentEntityIndexCreator.java:125) at org.springframework.data.mongodb.core.index.MongoPersistentEntityIndexCreator.<init>(MongoPersistentEntityIndexCreator.java:91) at org.springframework.data.mongodb.core.index.MongoPersistentEntityIndexCreator.<init>(MongoPersistentEntityIndexCreator.java:68) at org.springframework.data.mongodb.core.MongoTemplate.<init>(MongoTemplate.java:229) at org.bootfoo.BootConfiguration.mongoTemplate(BootConfiguration.java:121) at org.bootfoo.BootConfiguration$$EnhancerBySpringCGLIB$$1963a75.CGLIB$mongoTemplate$2(<generated>) at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) at java.lang.reflect.Method.invoke(Unknown Source) at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:162) ... 58 more Caused by: com.mongodb.MongoCommandException: Command failed with error 85: 'exception: Index with name: deviceId already exists with different options' on server 127.0.0.1:27017. The full response is { "createdCollectionAutomatically" : false, "numIndexesBefore" : 6, "errmsg" : "exception: Index with name: deviceId already exists with different options", "code" : 85, "ok" : 0.0 } at com.mongodb.connection.ProtocolHelper.getCommandFailureException(ProtocolHelper.java:115) at com.mongodb.connection.CommandProtocol.execute(CommandProtocol.java:114) at com.mongodb.connection.DefaultServer$DefaultServerProtocolExecutor.execute(DefaultServer.java:168)
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关键信息：org.springframework.dao.DataIntegrityViolationException: Cannot create index
从异常信息上看，出现的是索引冲突(Command failed with error 85)，spring-data-mongo 组件在程序启动时会实现根据注解创建索引的功能。
查看业务实体定义：
@Document(collection = "T_MDevice")public class MDevice { @Id private String id; @Indexed(unique=true) private String deviceId;
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deviceId 这个字段上定义了一个索引，unique=true 表示这是一个唯一索引。
我们继续 查看 MongoDB 中表的定义：
db.getCollection('T_MDevice').getIndexes() >>[ { "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "appdb.T_MDevice" }, { "v" : 1, "key" : { "deviceId" : 1 }, "name" : "deviceId", "ns" : "appdb.T_MDevice" }]
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发现数据库表中同样存在一个名为 deviceId 的索引，但是并非唯一索引！
三、详细分析
为了核实错误产生的原因，我们尝试通过 Mongo Shell 去执行索引的创建，发现返回了同样的错误。
通过将数据库中的索引删除，或更正为 unique=true 之后可以解决当前的问题。
从严谨度上看，一个索引冲突导致 SpringBoot 服务启动不了，是可以接受的。
但从灵活性来看，是否有某些方式能禁用索引的自动创建，或者仅仅是打印日志呢？
尝试 google spring data mongodb disable index creation
发现 JIRA-DATAMONGO-1201 在 2015 年就已经提出，至今未解决。
图
stackoverflow 找到许多同样问题，
但大多数的解答是不采用索引注解，选择其他方式对索引进行管理。
这些结果并不能令人满意。
尝试查看 spring-data-mongo 的机制，定位到 MongoPersistentEntityIndexCreator 类：
初始化方法中，会根据 MappingContext(实体映射上下文)中已有的实体去创建索引
public MongoPersistentEntityIndexCreator(MongoMappingContext mappingContext, MongoDbFactory mongoDbFactory, IndexResolver indexResolver) { ... for (MongoPersistentEntity<?> entity : mappingContext.getPersistentEntities()) { checkForIndexes(entity); } }
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在接收到MappingContextEvent时，创建对应实体的索引
public void onApplicationEvent(MappingContextEvent<?, ?> event) { if (!event.wasEmittedBy(mappingContext)) { return; } PersistentEntity<?, ?> entity = event.getPersistentEntity(); if (entity instanceof MongoPersistentEntity) { checkForIndexes((MongoPersistentEntity<?>) entity); } }
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MongoPersistentEntityIndexCreator 是通过 MongoTemplate 引入的，如下：
public MongoTemplate(MongoDbFactory mongoDbFactory, MongoConverter mongoConverter) { Assert.notNull(mongoDbFactory); this.mongoDbFactory = mongoDbFactory; this.exceptionTranslator = mongoDbFactory.getExceptionTranslator(); this.mongoConverter = mongoConverter == null ? getDefaultMongoConverter(mongoDbFactory) : mongoConverter; ... mappingContext = this.mongoConverter.getMappingContext(); if (null != mappingContext && mappingContext instanceof MongoMappingContext) { indexCreator = new MongoPersistentEntityIndexCreator((MongoMappingContext) mappingContext, mongoDbFactory); eventPublisher = new MongoMappingEventPublisher(indexCreator); if (mappingContext instanceof ApplicationEventPublisherAware) { ((ApplicationEventPublisherAware) mappingContext).setApplicationEventPublisher(eventPublisher); } } } ... public void setApplicationContext(ApplicationContext applicationContext) throws BeansException { prepareIndexCreator(applicationContext); eventPublisher = applicationContext; if (mappingContext instanceof ApplicationEventPublisherAware) { ((ApplicationEventPublisherAware) mappingContext).setApplicationEventPublisher(eventPublisher); } resourceLoader = applicationContext; } ... private void prepareIndexCreator(ApplicationContext context) { String[] indexCreators = context.getBeanNamesForType(MongoPersistentEntityIndexCreator.class); for (String creator : indexCreators) { MongoPersistentEntityIndexCreator creatorBean = context.getBean(creator, MongoPersistentEntityIndexCreator.class); if (creatorBean.isIndexCreatorFor(mappingContext)) { return; } } if (context instanceof ConfigurableApplicationContext) { ((ConfigurableApplicationContext) context).addApplicationListener(indexCreator); } }
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MongoPersistentEntityIndexCreator 是通过 MongoTemplate 引入的，如下：
public MongoTemplate(MongoDbFactory mongoDbFactory, MongoConverter mongoConverter) { Assert.notNull(mongoDbFactory); this.mongoDbFactory = mongoDbFactory; this.exceptionTranslator = mongoDbFactory.getExceptionTranslator(); this.mongoConverter = mongoConverter == null ? getDefaultMongoConverter(mongoDbFactory) : mongoConverter; ... mappingContext = this.mongoConverter.getMappingContext(); if (null != mappingContext && mappingContext instanceof MongoMappingContext) { indexCreator = new MongoPersistentEntityIndexCreator((MongoMappingContext) mappingContext, mongoDbFactory); eventPublisher = new MongoMappingEventPublisher(indexCreator); if (mappingContext instanceof ApplicationEventPublisherAware) { ((ApplicationEventPublisherAware) mappingContext).setApplicationEventPublisher(eventPublisher); } } } ... public void setApplicationContext(ApplicationContext applicationContext) throws BeansException { prepareIndexCreator(applicationContext); eventPublisher = applicationContext; if (mappingContext instanceof ApplicationEventPublisherAware) { ((ApplicationEventPublisherAware) mappingContext).setApplicationEventPublisher(eventPublisher); } resourceLoader = applicationContext; } ... private void prepareIndexCreator(ApplicationContext context) { String[] indexCreators = context.getBeanNamesForType(MongoPersistentEntityIndexCreator.class); for (String creator : indexCreators) { MongoPersistentEntityIndexCreator creatorBean = context.getBean(creator, MongoPersistentEntityIndexCreator.class); if (creatorBean.isIndexCreatorFor(mappingContext)) { return; } } if (context instanceof ConfigurableApplicationContext) { ((ConfigurableApplicationContext) context).addApplicationListener(indexCreator); } }
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由此可见，MongoTemplate 在初始化时，先通过 MongoConverter 带入 MongoMappingContext，
随后完成一系列初始化，整个过程如下：
实例化 MongoTemplate;
实例化 MongoConverter;
实例化 MongoPersistentEntityIndexCreator；
初始化索引(通过 MappingContext 已有实体);
Repository 初始化 -> MappingContext 发布映射事件；
ApplicationContext 将事件通知到 IndexCreator；
IndexCreator 创建索引
在实例化过程中，没有任何配置可以阻止索引的创建。
四、解决问题
从前面的分析中，可以发现问题关键在 IndexCreator，能否提供一个自定义的实现呢，答案是可以的！
实现的要点如下
实现一个 IndexCreator，可继承 MongoPersistentEntityIndexCreator，去掉索引的创建功能；
实例化 MongoConverter 和 MongoTemplate 时，使用一个空的 MongoMappingContext 对象避免初始化索引；
将自定义的 IndexCreator 作为 Bean 进行注册，这样在 prepareIndexCreator 方法执行时，
原来的 MongoPersistentEntityIndexCreator 不会监听 ApplicationContext 的事件
IndexCreator 实现了 ApplicationContext 监听，接管 MappingEvent 事件处理。
实例化 Bean
@Bean public MongoMappingContext mappingContext() { return new MongoMappingContext(); } @Bean public MongoTemplate mongoTemplate(MongoDbFactory mongoDbFactory, MongoMappingContext mappingContext) { MappingMongoConverter converter = new MappingMongoConverter(new DefaultDbRefResolver(mongoDbFactory), mappingContext); converter.setTypeMapper(new DefaultMongoTypeMapper(null)); MongoTemplate mongoTemplate = new MongoTemplate(mongoDbFactory, converter); return mongoTemplate; }
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自定义 IndexCreator
@Component public static class CustomIndexCreator extends MongoPersistentEntityIndexCreator { public CustomIndexCreator(MongoMappingContext mappingContext, MongoDbFactory mongoDbFactory) { super(mappingContext, mongoDbFactory); } public void onApplicationEvent(MappingContextEvent<?, ?> event) { PersistentEntity<?, ?> entity = event.getPersistentEntity(); if (entity instanceof MongoPersistentEntity) { System.out.println("Detected MongoEntity " + entity.getName()); } } }
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在这里 CustomIndexCreator 继承了 MongoPersistentEntityIndexCreator，将自动接管 MappingContextEvent 事件的监听。
在业务实现上可以根据需要完成索引的处理！
小结
spring-data-mongo 提供了非常大的便利性，但在灵活性支持上仍然不足。上述的方法实际上有些隐晦，在官方文档中并未提及这样的方式。
ORM-Mapping 框架在实现 Schema 映射处理时需要考虑校验级别，比如 Hibernate 便提供了 none/create/update/validation 多种选择，毕竟这对开发者来说更加友好。
期待 spring-data-mongo 在后续的演进中能尽快完善 Schema 的管理功能！
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