如果参数为空，则选择所有记录，否则在JPA条件查询中返回特定项目

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<Entity> query = cb.createQuery(Entity.class);
Root<Entity> root = query.from(Entity.class);

if (param != null) {
    // 添加条件
    Predicate predicate = cb.equal(root.get("property"), param);
    query.where(predicate);
}

List<Entity> result = entityManager.createQuery(query).getResultList();

String jpql = "SELECT e FROM Entity e";
if (param != null) {
    jpql += " WHERE e.property = :param";
}

TypedQuery<Entity> query = entityManager.createQuery(jpql, Entity.class);
if (param != null) {
    query.setParameter("param", param);
}

List<Entity> result = query.getResultList();