JPA 多表左连接多条件分页查询

业务场景：

 主表：订单表，与客户表、产品表、物流表存在一对一关系，映射字段为id，现需要根据订单编号、订单日期、客户名称、客户编号、产品名称、产品编号、快递单号查询该笔订单，需要支持模糊查询和分页。

Order实体类中的需要进行一对一关系映射：

    @OneToOne
    @JoinColumn(name = "express_id")
    private Express express;
    @OneToOne
    @JoinColumn(name = "product_id")
    private Product product;
    @OneToOne
    @JoinColumn(name = "customer_id")
    private Customer customer;

Dao中的需要继承JpaRepository，JpaSpecificationExecuto两个接口:

@Repository
public interface OrderDao extends JpaRepository, JpaSpecificationExecutor {
}

Service中的写法：

 Pageable通过前端传入的pageSize 和 pageNum进行创建

Pageable pageable = PageRequest.of(pageNum - 1, pageSize);

    @Autowired
    OrderDao licenseDao;

    @Autowired
    KeyPairService keyPairService;

    public Page getLicenseList(Order order, Pageable pageable) {
        Specification specification = new Specification() {
            @Override
            public Predicate toPredicate(Root root, CriteriaQuery criteriaQuery, CriteriaBuilder criteriaBuilder) {
                List list = new ArrayList();
                Join expressJoin = root.join("express", JoinType.LEFT);
                Join proJoin = root.join("product", JoinType.LEFT);
                Join customerJoin = root.join("customer", JoinType.LEFT);
                if (null != order.getCustomer() && !StringUtils.isEmpty(order.getCustomer().getCode())) {
                    list.add(criteriaBuilder.like(customerJoin.get("code").as(String.class), "%" + order.getCustomer().getCode() + "%"));
                }
                if (null != order.getCustomer() && !StringUtils.isEmpty(order.getCustomer().getName())) {
                    list.add(criteriaBuilder.like(customerJoin.get("name").as(String.class), "%" + order.getCustomer().getName() + "%"));
                }
                if (null != order.getProduct() && !StringUtils.isEmpty(order.getProduct().getCode())) {
                    list.add(criteriaBuilder.like(proJoin.get("code").as(String.class), "%" + order.getProduct().getCode() + "%"));
                }
                if (null != order.getProduct() && !StringUtils.isEmpty(order.getProduct().getName())) {
                    list.add(criteriaBuilder.like(proJoin.get("name").as(String.class), "%" + order.getProduct().getName() + "%"));
                }
                if (null != order.getExpress() && !StringUtils.isEmpty(order.getExpress().getCode())) {
                    list.add(criteriaBuilder.like(expressJoin.get("code").as(String.class), "%" + order.getExpress().getCode() + "%"));
                }
                if (!StringUtils.isEmpty(order.getCode())) {
                    list.add(criteriaBuilder.like(root.get("code").as(String.class), "%" + order.getCode() + "%"));
                }
                if (null != order.getCreateDate()) {
                    list.add(criteriaBuilder.lessThan(root.get("createDate").as(Date.class), order.getCreateDate()));
                }
                Predicate[] p = new Predicate[list.size()];
                return criteriaBuilder.and(list.toArray(p));
            }
        };
        return orderDao.findAll(specification, pageable);
    }