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)

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;

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"
    }
]

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：

1. 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) {
        ...
        //根据已有实体创建
        for (MongoPersistentEntity<?> entity : mappingContext.getPersistentEntities()) {
            checkForIndexes(entity);
        }
    }

1. 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();

        // Double check type as Spring infrastructure does not consider nested generics
        if (entity instanceof MongoPersistentEntity) {
            //创建单个实体索引
            checkForIndexes((MongoPersistentEntity<?>) entity);
        }
    }

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;
        ...

        // We always have a mapping context in the converter, whether it's a simple one or not
        mappingContext = this.mongoConverter.getMappingContext();
        // We create indexes based on mapping events
        if (null != mappingContext && mappingContext instanceof MongoMappingContext) {
            indexCreator = new MongoPersistentEntityIndexCreator((MongoMappingContext) mappingContext, mongoDbFactory);
            eventPublisher = new MongoMappingEventPublisher(indexCreator);
            if (mappingContext instanceof ApplicationEventPublisherAware) {
                ((ApplicationEventPublisherAware) mappingContext).setApplicationEventPublisher(eventPublisher);
            }
        }
    }


    ...
    //MongoTemplate实现了 ApplicationContextAware，当ApplicationContext被实例化时被感知
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {

        prepareIndexCreator(applicationContext);

        eventPublisher = applicationContext;
        if (mappingContext instanceof ApplicationEventPublisherAware) {
            //MappingContext作为事件来源，向ApplicationContext发布
            ((ApplicationEventPublisherAware) mappingContext).setApplicationEventPublisher(eventPublisher);
        }
        resourceLoader = applicationContext;
    }

    ...
    //注入事件监听
    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) {
            //使 IndexCreator 监听 ApplicationContext的事件
            ((ConfigurableApplicationContext) context).addApplicationListener(indexCreator);
        }
    }

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

1. 实现一个IndexCreator，可继承MongoPersistentEntityIndexCreator，去掉索引的创建功能；
2. 实例化 MongoConverter和 MongoTemplate时，使用一个空的 MongoMappingContext对象避免初始化索引；
3. 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
4. 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();
    }

    // 使用 MappingContext 实例化 MongoTemplate
    @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;
    }

自定义IndexCreator

    // 自定义IndexCreator实现
    @Component
    public static class CustomIndexCreator extends MongoPersistentEntityIndexCreator {

        // 构造器引用MappingContext
        public CustomIndexCreator(MongoMappingContext mappingContext, MongoDbFactory mongoDbFactory) {
            super(mappingContext, mongoDbFactory);
        }

        public void onApplicationEvent(MappingContextEvent<?, ?> event) {
            PersistentEntity<?, ?> entity = event.getPersistentEntity();

            // 获得Mongo实体类
            if (entity instanceof MongoPersistentEntity) {
                System.out.println("Detected MongoEntity " + entity.getName());
                
                //可实现索引处理..
            }
        }
    }

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！