Spring Data CrudRepository类中的S参数是什么？

Question

有人可以从org.springframework.data.repository告诉我CrudRepository接口中的S吗？

我创建了一个名为“ DBBooks”的类，它是我的数据库表“ books”的支持对象。

  

DBBooks.java

SNIPPET 
        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        @Column(name="book_index")
        private Integer book_index;
        public Integer getBook_index(){
            return book_index;
        }

我知道T是我的DBBooks对象，而ID是我表中的book_index键对象类型：

SNIPPET 
@Repository
public interface BookRepository extends CrudRepository<DBBook, Integer> {

}

---

SNIPPET 
@NoRepositoryBean
    public interface CrudRepository<T, ID> extends Repository<T, ID> {

        /**
         * Saves a given entity. Use the returned instance for further operations as the save operation might have changed the
         * entity instance completely.
         *
         * @param entity must not be {@literal null}.
         * @return the saved entity will never be {@literal null}.
         */
        <S extends T> S save(S entity);

        /**
         * Saves all given entities.
         *
         * @param entities must not be {@literal null}.
         * @return the saved entities will never be {@literal null}.
         * @throws IllegalArgumentException in case the given entity is {@literal null}.
         */
        <S extends T> Iterable<S> saveAll(Iterable<S> entities);

但是S是什么？我不明白。