EntityManagerFactory是threadsafe、EntityManager是non-threadsafe

https://docs.jboss.org/hibernate/stable/entitymanager/reference/en/html/transactions.html#transactions-basics

 
Source Code:
https://github.com/javaee/jpa-spec/blob/master/javax.persistence-api/src/main/java/javax/persistence/EntityManagerFactory.java

https://github.com/eclipse-ee4j/jpa-api/blob/master/api/src/main/java/jakarta/persistence/EntityManagerFactory.java