spring data jpa 通過建立方法名來做查詢，只能做簡單的查詢，那如果我們要做複雜一些的查詢呢，多條件分頁怎麼辦，這裡，spring data jpa為我們提供了JpaSpecificationExecutor介面，只要簡單實現toPredicate方法就可以實現複雜的查詢

1.首先讓我們的介面繼承於JpaSpecificationExecutor

public interface TaskDao extends JpaSpecificationExecutor<Task>{

}

2.JpaSpecificationExecutor提供了以下介面

public interface JpaSpecificationExecutor<T
 
> {

    T findOne(Specification<T> spec);

    List<T> findAll(Specification<T> spec);

    Page<T> findAll(Specification<T> spec, Pageable pageable);

    List<T> findAll(Specification<T> spec, Sort sort);

    long count(Specification<T> spec);
}
 

其中Specification就是需要我們傳進去的引數，它是一個介面

public interface Specification<T> {

    Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder cb);
}

2.接下來我們在service bean

@Service
public class TaskService {

    @Autowired TaskDao taskDao ;


    /**
     * 複雜查詢測試
     * @param
 
 page
     * @param size
     * @return
     */
    public Page<Task> findBySepc(int page, int size){

        PageRequest pageReq = this.buildPageRequest(page, size);
        Page<Task> tasks = this.taskDao.findAll(new MySpec(), pageReq);

        return tasks;

    }

     /**
      * 建立分頁排序請求 
      * @param page
      * @param size
      * @return
      */
     private PageRequest buildPageRequest(int page, int size) {
           Sort sort = new Sort(Direction.DESC,"createTime");
           return new PageRequest(page,size, sort);
     }

    /**
     * 建立查詢條件
     * @author liuxg
     * @date 2016年3月30日 下午2:04:39
     */
    private class MySpec implements Specification<Task>{

        @Override
        public Predicate toPredicate(Root<Task> root, CriteriaQuery<?> query, CriteriaBuilder cb) {

     //1.混合條件查詢
          /*Path<String> exp1 = root.get("taskName");
            Path<Date>  exp2 = root.get("createTime");
            Path<String> exp3 = root.get("taskDetail");
            Predicate predicate = cb.and(cb.like(exp1, "%taskName%"),cb.lessThan(exp2, new Date()));
            return cb.or(predicate,cb.equal(exp3, "kkk"));

            類似的sql語句為:
            Hibernate: 
                select
                    count(task0_.id) as col_0_0_ 
                from
                    tb_task task0_ 
                where
                    (
                        task0_.task_name like ?
                    ) 
                    and task0_.create_time<? 
                    or task0_.task_detail=?
            */

    //2.多表查詢
        /*Join<Task,Project> join = root.join("project", JoinType.INNER);
            Path<String> exp4 = join.get("projectName");
            return cb.like(exp4, "%projectName%");

            Hibernate: 
            select
                count(task0_.id) as col_0_0_ 
            from
                tb_task task0_ 
            inner join
                tb_project project1_ 
                    on task0_.project_id=project1_.id 
            where
                project1_.project_name like ?*/ 
           return null ;  
        }
    }
}

3.實體類task程式碼如下

@Entity
@Table(name = "tb_task")
public class Task {

    private Long id ;
    private String taskName ;
    private Date createTime ;
    private Project project;
    private String taskDetail ;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }

    @Column(name = "task_name")
    public String getTaskName() {
        return taskName;
    }
    public void setTaskName(String taskName) {
        this.taskName = taskName;
    }

    @Column(name = "create_time")
    @DateTimeFormat(pattern = "yyyy-MM-dd hh:mm:ss")
    public Date getCreateTime() {
        return createTime;
    }
    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }


    @Column(name = "task_detail")
    public String getTaskDetail() {
        return taskDetail;
    }
    public void setTaskDetail(String taskDetail) {
        this.taskDetail = taskDetail;
    }

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "project_id")
    public Project getProject() {
        return project;
    }
    public void setProject(Project project) {
        this.project = project;
    }

}

通過重寫toPredicate方法，返回一個查詢 Predicate，spring data jpa會幫我們進行查詢。

也許你覺得，每次都要寫一個類來實現Specification很麻煩，那或許你可以這麼寫

public class TaskSpec {

    public static Specification<Task> method1(){

        return new Specification<Task>(){
            @Override
            public Predicate toPredicate(Root<Task> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
                return null;
            }

        };
    }

    public static Specification<Task> method2(){

        return new Specification<Task>(){
            @Override
            public Predicate toPredicate(Root<Task> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
                return null;
            }

        };
    }

}

那麼用的時候，我們就這麼用

Page<Task> tasks = this.taskDao.findAll(TaskSpec.method1(), pageReq);

JpaSpecificationExecutor的介紹就到這裡，下次再看看怎麼通過寫hql或者sql語句來進行查詢