spring data jpa 利用JpaSpecificationExecutor做复杂查询

spring data jpa 通过创建方法名来做查询，只能做简单的查询，那如果我们要做复杂一些的查询呢，多条件分页怎么办，这里，spring data jpa为我们提供了JpaSpecificationExecutor接口，只要简单实现toPredicate方法就可以实现复杂的查询

1.首先让我们的接口继承于JpaSpecificationExecutor

public interface TaskDao extends JpaSpecificationExecutor<Task>{}
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2.JpaSpecificationExecutor提供了以下接口

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);}
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其中Specification就是需要我们传进去的参数，它是一个接口

public interface Specification<T> {    Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder cb);}
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提供唯一的一个方法toPredicate，我们只要按照JPA 2.0 criteria api写好查询条件就可以了，关于JPA 2.0 criteria api的介绍和使用，欢迎参考
http://blog.csdn.net/dracotianlong/article/details/28445725
http://developer.51cto.com/art/200911/162722.htm

2.接下来我们在service bean

@Servicepublic class TaskService {    @Autowired TaskDao taskDao ;    /**     * 复杂查询测试     * @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;    }     /**      * 建立分页排序请求       * @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);     }    /**     * 建立查询条件     * @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.混合条件查询          /*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"));            类似的sql语句为:            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.多表查询        /*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 ;          }    }}
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3.实体类task代码如下

@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;    }}
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通过重写toPredicate方法，返回一个查询 Predicate，spring data jpa会帮我们进行查询。

也许你觉得，每次都要写一个类来实现Specification很麻烦，那或许你可以这么写

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;            }        };    }}
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那么用的时候，我们就这么用

Page<Task> tasks = this.taskDao.findAll(TaskSpec.method1(), pageReq);
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JpaSpecificationExecutor的介绍就到这里，下次再看看怎么通过写hql或者sql语句来进行查询