spring mongodb converter

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通过预实例化+预转换，解决Spring Data MongoDB 在首次加载实体类时的反射初始化锁问题

问题所在：

当你使用spring data mongodb的api操作mongodb时，例如 mongoTemplagte.findbyId()

该方法内部会去调用

MongoPersistentEntity<?> entity = mappingContext.getPersistentEntity(entityClass);

而getPersistentEntity这个方法会加写锁去注册 TypeInformation

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@Nullable
	@Override
	public E getPersistentEntity(TypeInformation<?> type) {

		Assert.notNull(type, "Type must not be null");

		try {
      
			read.lock();

			Optional<E> entity = persistentEntities.get(type);

			if (entity != null) {
				return entity.orElse(null);
			}

		} finally {
			read.unlock();
		}

		if (!shouldCreatePersistentEntityFor(type)) {

			try {
				write.lock();
				persistentEntities.put(type, NONE);
			} finally {
				write.unlock();
			}

			return null;
		}

		if (strict) {
			throw new MappingException("Unknown persistent entity " + type);
		}

		return addPersistentEntity(type).orElse(null);
	}


------------------------//addPersistentEntity //---------------------------------

	/**
	 * Adds the given {@link TypeInformation} to the {@link MappingContext}.
	 *
	 * @param typeInformation must not be {@literal null}.
	 * @return
	 */
	protected Optional<E> addPersistentEntity(TypeInformation<?> typeInformation) {

		Assert.notNull(typeInformation, "TypeInformation must not be null");

		try {

			read.lock();

			Optional<E> persistentEntity = persistentEntities.get(typeInformation);

			if (persistentEntity != null) {
				return persistentEntity;
			}

		} finally {
			read.unlock();
		}

		E entity;

		try {

			write.lock();

			Optional<E> userTypeEntity = persistentEntities.get(typeInformation.getUserTypeInformation());

			if (userTypeEntity != null) {
				persistentEntities.put(typeInformation, userTypeEntity);
				return userTypeEntity;
			}

			entity = doAddPersistentEntity(typeInformation);

		} catch (BeansException e) {
			throw new MappingException(e.getMessage(), e);
		} finally {
			write.unlock();
		}

		// Inform listeners
		if (applicationEventPublisher != null) {
			applicationEventPublisher.publishEvent(new MappingContextEvent<>(this, entity));
		}

		return Optional.of(entity);
	}

而jdk的写锁就有问题

获取写锁失败的线程会一直阻塞，而不是直接返回

总结：

通过「预实例化 + 预转换」的办法，解决了 Spring Data MongoDB 在首次加载实体类时的反

射初始化锁问题，提升了项目首次数据访问时的并发性能

解决方案：

ps：https://blog.csdn.net/dch9210/article/details/116131039