Spring Data JPA进阶——Specifications和Querydsl

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 

@Service
public 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