说明: QueryDSL是以函数连接的方式将SQL调用进行拆分，比较spring data jpa中的criteria查询方法还是简洁了不少。 (转载请注明来源:cnblogs coder-fang)

用例:通过服务调用，使用querydsl进行查询并直接返回DTO对象(自定义传输对象(根据业务需求)，注意区别于Entity)

实践步骤:

1.　　创建user与depart表，使用外键进行关联，并插入一些模拟数据。

2.　　创建sprintboot项目，在pom文件中加入以下依赖:

com.querydsl

querydsl-jpa

com.querydsl

querydsl-apt

provided

3.　　在pom文件中-->节点下加入plugin：

com.mysema.maven

apt-maven-plugin

1.1.3

process

target/generated-sources/java

com.querydsl.apt.jpa.JPAAnnotationProcessor

com.querydsl

querydsl-apt

4.1.3

4.　　生成相关entity与repository对象,这里以user为例:

packagecom.test.demo.db;//

importjavax.persistence.Column;importjavax.persistence.Entity;importjavax.persistence.FetchType;importjavax.persistence.GeneratedValue;import staticjavax.persistence.GenerationType.IDENTITY;importjavax.persistence.Id;importjavax.persistence.JoinColumn;importjavax.persistence.ManyToOne;importjavax.persistence.Table;/*** User generated by hbm2java*/@Entity

@Table(name= "user", catalog = "testdb")public class User implementsjava.io.Serializable {privateInteger id;privateDepartment department;privateString username;publicUser() {

}publicUser(Department department, String username) {this.department =department;this.username =username;

}

@Id

@GeneratedValue(strategy=IDENTITY)

@Column(name= "id", unique = true, nullable = false)publicInteger getId() {return this.id;

}public voidsetId(Integer id) {this.id =id;

}

@ManyToOne(fetch=FetchType.LAZY)

@JoinColumn(name= "fk_depart")publicDepartment getDepartment() {return this.department;

}public voidsetDepartment(Department department) {this.department =department;

}

@Column(name= "username", length = 45)publicString getUsername() {return this.username;

}public voidsetUsername(String username) {this.username =username;

}

}

View Code

packagecom.test.demo.repo;importorg.springframework.data.jpa.repository.JpaRepository;importorg.springframework.data.jpa.repository.JpaSpecificationExecutor;importorg.springframework.data.querydsl.QueryDslPredicateExecutor;importorg.springframework.stereotype.Repository;importcom.test.demo.db.User;

@Repositorypublic interface UserRepository extends QueryDslPredicateExecutor, JpaRepository,JpaSpecificationExecutor{

}

View Code

注意：repository需要继承 QueryDslPredicateExecutor接口。

5.　　生成业务传输对象DTO：

packagecom.test.demo.controller;importcom.querydsl.core.annotations.QueryProjection;importlombok.Data;

@SuppressWarnings("unused")public @Data classUserDTO {privateString username;privateString departname;

}

6.　　创建controller进行测试:

packagecom.test.demo.controller;importjava.util.HashMap;importjava.util.List;importjava.util.Map;importjavax.annotation.PostConstruct;importjavax.persistence.EntityManager;importjavax.persistence.PersistenceContext;importorg.springframework.beans.factory.annotation.Autowired;importorg.springframework.web.bind.annotation.RequestMapping;importorg.springframework.web.bind.annotation.RequestParam;importorg.springframework.web.bind.annotation.RestController;importcom.querydsl.core.types.Projections;importcom.querydsl.core.types.dsl.BooleanExpression;importcom.querydsl.jpa.impl.JPAQuery;importcom.querydsl.jpa.impl.JPAQueryFactory;importcom.test.demo.db.QUser;importcom.test.demo.repo.UserRepository;

@RestController

@RequestMapping("/")public classTestController {

@Autowired

UserRepository userRepo;

@Autowired

@PersistenceContext

EntityManager em;privateJPAQueryFactory queryFactory;

@PostConstructpublic voidinit() {

queryFactory= newJPAQueryFactory(em);

}

@RequestMapping("/users")

Object getUsers(@RequestParam(value= "page", required = false, defaultValue = "1") Integer page,

@RequestParam(value= "size", required = false, defaultValue = "10") Integer size,

@RequestParam(value= "name", required = false) String name,

@RequestParam(value= "depart", required = false) String depart) {

QUser user=QUser.user;

JPAQuery query =queryFactory

.select(Projections.bean(UserDTO.class, user.username, user.department.name.as("departname")))

.from(user);

BooleanExpression pre= null;if (name!=null && !name.isEmpty()) {

pre=user.username.startsWith(name);

}if (depart!=null && !depart.isEmpty()) {

pre=user.department.name.startsWith(depart);

}

query.where(pre);

query.limit(size);

query.offset((page-1)*size);

List result =query.fetch();

Map map = new HashMap<>();

map.put("total", userRepo.count(pre));

map.put("data", result);returnmap;

}

}

View Code

注：这里就是使用querydsl进行查询，并直接转换需要的属性至DTO。并且代码中的pre是可以根据参数动态拼接的。

7.　　测试结果:

这是查询日志:

完。