A specification, the full name is: Java Persistence API (java persistence application interface), which was developed by Sun Corporation. Since there are currently many Java persistence layer frameworks, such as Hibernate, TopLink, OpenJPA, etc., problems arise with so many choices, causing Java programmers to have to learn multiple persistence layer frameworks. The emergence of JPA frees programmers from learning persistence layer frameworks. For persistence layer frameworks that implement JPA specifications, they only need to develop persistence layer code in accordance with JPA specifications without caring about which framework implements it; in addition , JPA also encapsulates the persistence layer frameworks that implement its specifications, making these frameworks more convenient to use. Currently, the three persistence layer frameworks mentioned above have all implemented JPA specifications.