How to get the type of generic T, that is, T.class

• I want to write a mongodb tool class in spring boot, and simply encapsulate some of mongodb's entries. In the process of writing the tool class, I found that many of the mongodb methods integrated in spring boot need to use entity class types. In the tool class, I use generic T to represent entity classes, so the biggest problem encountered is that I can'T get T.class. I don'T know what it means. Just look at the examples.

• Questions - Examples

  • Unsightly tools

    /**
     * @author zhao_jianbo
     */
    @Component
    public class MongoDbOperImpl<T> implements BaseDbOper<T> {
    
        @Autowired
        private MongoTemplate mongoTemplate;
    
        /**
         * Delete data by id
         *
         * @param id
         * @param entity
         * @return Number of deleted data
         */
        @Override
        public Long deleteById(String id, T entity) {
            Query query = new Query(Criteria.where("id").is(id));
            return mongoTemplate.remove(query,     entity.getClass()).getDeletedCount();
        }
    
    

  • From the above example, we can see that mongoTemplate.remove() needs a class of entity class, that is, T.class. Many articles about how to obtain T.class have been searched on the Internet, but those methods only apply to the type of T that is clear, such as mongodboperimple < user > so we have no choice but to manually pass in the entity class. Although the flexibility of the code is preserved, it looks very ugly. I just Want to delete a piece of data according to the id, when calling the method, it must pass in the entity class, which is very bad!

• Improved method

  • The improved method comes from HibernateDaoImpl class

  • I don't know exactly why it works like this. Let's talk about it

  • The improved example is as follows

  • Tool class

    /**
     * @author zhao_jianbo
     */
    @Component
    public class MongoDbSupport<T> implements BaseDbOper<T> {
    
        @Autowired
        private MongoTemplate mongoTemplate;
    
        /**
         * Get T.class
         *
         * @return
         */
        private Class<T> getEntry() {
            Type type = getClass().getGenericSuperclass();
            Class<T> result = null;
            if (type instanceof ParameterizedType) {
                ParameterizedType pType = (ParameterizedType) type;
                result = (Class<T>) pType.getActualTypeArguments()[0];
            }
            return result;
        }
    
        /**
         * Delete data by id
         *
         * @param id
         * @return Number of deleted data
         */
        @Override
        public Long deleteById(String id) {
            Query query = new Query(Criteria.where("id").is(id));
            return mongoTemplate.remove(query, getEntry()).getDeletedCount();
        }
    

  • Entity class

    /**
     * @author zhao_jianbo
     */
    @Data
    @NoArgsConstructor
    @AllArgsConstructor
    @Document(collection = "book")
    @Repository
    public class Book extends MongoDbSupport<Book> {
    	......
    }
    

  Now understand why hibernate and mybitas need entity classes and inheritance tool classes