在Spring Data JPA中引入Querydsl的实现方式

<!-- Querydsl相关包 -->  
  <dependency>
  <groupId>com.querydsl</groupId>
  <artifactId>querydsl-jpa</artifactId>
 </dependency>
 
 <dependency>
  <groupId>com.querydsl</groupId>
  <artifactId>querydsl-apt</artifactId>
 </dependency>

<plugin>
  <groupId>com.mysema.maven</groupId>
  <artifactId>maven-apt-plugin</artifactId>
  <version>1.0.4</version>
  <executions>
  <execution>
   <phase>generate-sources</phase>
   <goals>
   <goal>process</goal>
   </goals>
   <configuration>
   <outputDirectory>target/generated-sources</outputDirectory>
   <processor>com.querydsl.apt.jpa.JPAAnnotationProcessor</processor>
   </configuration>
  </execution>
  </executions>
  </plugin> 

<dependency>
  <groupId>com.querydsl</groupId>
  <artifactId>querydsl-jpa</artifactId>
 </dependency>
 <dependency>
  <groupId>com.querydsl</groupId>
  <artifactId>querydsl-apt</artifactId>
  <scope>provided</scope>
 </dependency>

<plugin>
  <groupId>com.mysema.maven</groupId>
  <artifactId>apt-maven-plugin</artifactId>
  <version>1.1.3</version>
  <executions>
   <execution>
   <goals>
    <goal>process</goal>
   </goals>
   <configuration>
    <outputDirectory>target/generated-sources/java</outputDirectory>
    <processor>com.querydsl.apt.jpa.JPAAnnotationProcessor</processor>
   </configuration>
   </execution>
  </executions>
  <dependencies>
   <dependency>
   <groupId>com.querydsl</groupId>
   <artifactId>querydsl-apt</artifactId>
   <version>4.1.3</version>
   </dependency>
  </dependencies>
  </plugin>

package com.test.demo.db;
//
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import static javax.persistence.GenerationType.IDENTITY;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
/**
 * User generated by hbm2java
 */
@Entity
@Table(name = "user", catalog = "testdb")
public class User implements java.io.Serializable {
 private Integer id;
 private Department department;
 private String username;
 public User() {
 }
 public User(Department department, String username) {
 this.department = department;
 this.username = username;
 }
 @Id
 @GeneratedValue(strategy = IDENTITY)
 @Column(name = "id", unique = true, nullable = false)
 public Integer getId() {
 return this.id;
 }
 public void setId(Integer id) {
 this.id = id;
 }
 @ManyToOne(fetch = FetchType.LAZY)
 @JoinColumn(name = "fk_depart")
 public Department getDepartment() {
 return this.department;
 }
 public void setDepartment(Department department) {
 this.department = department;
 }
 @Column(name = "username", length = 45)
 public String getUsername() {
 return this.username;
 }
 public void setUsername(String username) {
 this.username = username;
 }
}

package com.test.demo.repo;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.JpaSpecificationExecutor;
import org.springframework.data.querydsl.QueryDslPredicateExecutor;
import org.springframework.stereotype.Repository;
import com.test.demo.db.User;
@Repository
public interface UserRepository extends QueryDslPredicateExecutor<User>, JpaRepository<User, Integer>,JpaSpecificationExecutor<User>{
 
}

package com.test.demo.controller;
import com.querydsl.core.annotations.QueryProjection;
import lombok.Data;
@SuppressWarnings("unused")
public @Data class UserDTO { 
 private String username; 
 private String departname;
}

package com.test.demo.controller;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.annotation.PostConstruct;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import com.querydsl.core.types.Projections;
import com.querydsl.core.types.dsl.BooleanExpression;
import com.querydsl.jpa.impl.JPAQuery;
import com.querydsl.jpa.impl.JPAQueryFactory;
import com.test.demo.db.QUser;
import com.test.demo.repo.UserRepository;
@RestController
@RequestMapping("/")
public class TestController {
 @Autowired
 UserRepository userRepo;
 @Autowired
 @PersistenceContext
 EntityManager em;
 private JPAQueryFactory queryFactory;
 @PostConstruct
 public void init() {
 queryFactory = new JPAQueryFactory(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<UserDTO> 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<UserDTO> result = query.fetch();
 Map<String, Object> map = new HashMap<>();
 map.put("total", userRepo.count(pre));
 map.put("data", result);
 return map;
 }
}