需要查询以检查数据库中是否已存在电子邮件(Java Persistence API)

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Email {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    
    private String to;
    private String from;
    private String subject;
    private String body;
    
    // 构造函数和getter方法
}

import javax.persistence.EntityManager;
import javax.persistence.EntityTransaction;
import javax.persistence.Query;

public class EmailStorage {
    public void storeEmail(Email email) {
        EntityManager entityManager = Persistence.createEntityManagerFactory("email-jpa").createEntityManager();
        EntityTransaction entityTransaction = entityManager.getTransaction();
        
        try {
            entityTransaction.begin();
            entityManager.persist(email);
            entityTransaction.commit();
            
            System.out.println("Email stored successfully.");
        } catch (Exception e) {
            if (entityTransaction.isActive()) {
                entityTransaction.rollback();
            }
            
            System.out.println("Error storing email: " + e.getMessage());
        } finally {
            entityManager.close();
        }
    }
    
    public Email retrieveEmail(Long id) {
        EntityManager entityManager = Persistence.createEntityManagerFactory("email-jpa").createEntityManager();
        Query query = entityManager.createQuery("SELECT e FROM Email e WHERE e.id = :id");
        query.setParameter("id", id);
        
        try {
            return (Email) query.getSingleResult();
        } catch (Exception e) {
            System.out.println("Error retrieving email: " + e.getMessage());
            return null;
        } finally {
            entityManager.close();
        }
    }
}