使用Hibernate的JPA 2.0标准查询

JPA 2.0中引入了条件查询。借助条件查询，您可以以类型安全的方式编写查询。在进行标准查询之前，开发人员必须通过构建基于对象的查询定义来编写查询。构建查询时，可能会出现语法错误的情况。条件查询API提供了创建具有编译时安全性的结构化和嵌套查询的功能。进入有关标准查询的更多理论可能不是一个好主意，因为可以在网上找到大量相同的页面。让我们举一个简单的例子来了解使用条件查询的查询。在这篇文章中，我将Hibernate作为JPA 2.0的供应商来展示Criteria Query的示例。

在深入探讨标准查询之前，让我们考虑一下可以表示为实体的数据库表和相应的Java类：

数据库表：

例如，下面有两个数据库表：

1. 状态[stateId，stateName]
2. 城市[cityId，stateId（FK＃），CityName]

我们要获取特定州的城市列表，在该州中，城市名称应从“ M”开始，城市列表应按升序排列。如果我们考虑简单的本机SQL，则如下所示：

Select * from City c, State s where c.stateId = s.stateId and c.stateId = ? and c.cityName like "M%" order by c.cityName

JPA实体：

以下是配置为JPA实体以表示州和城市表的两个Java类。这些也可以使用逆向工程工具生成。

STATE表的Java实体

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import java.util.Date;
import java.util.Set;
import javax.persistence.SequenceGenerator;@Entity
@Table(name="STATE")
@SequenceGenerator(sequenceName="STATE_SEQ",name="STATE_SEQ_GEN")
public class State {private Long stateId;private String stateName;private Set citySet;@Id@Column(name="stateId")@GeneratedValue(generator="STATE_SEQ_GEN",strategy=GenerationType.SEQUENCE)public Long getStateId;() {return stateId;}public void setId(long stateId) {stateId = stateId;}@Column(name="stateName")public String getStateName() {return stateName;}public void setStateName(String stateName) {this.stateName = stateName;}public void setCitySet(Set citySet)this.citySet= citySet;}@OneToMany(fetch=FetchType.LAZY,cascade=CascadeType.ALL,mappedBy="state")public Set getCitySet() {return citySet;}
}

CITY表的Java实体

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.SequenceGenerator;
import javax.persistence.Table;@Entity
@Table(name="CITY")
@SequenceGenerator(sequenceName="CITY_SEQ",name="CITY_SEQ_GEN")
public class City {private Long cityId;private String cityName;private State state;public void setId(long cityId) {this.cityId = cityId;}@Id@Column(name="CityId")@GeneratedValue(generator="CITY_SEQ_GEN",strategy=GenerationType.SEQUENCE)public Long getCityId() {return cityId;}@Column(name="CityName")public void setContent(String cityName) {this.cityName = cityName;}public String getCityName() {return cityName;}public void setState(State state) {this.state = state;}@ManyToOne(fetch =  FetchType.LAZY)@JoinColumn(name = "STATEID", nullable = false)public state getState() {return state;}
}

值对象（POJO）：

以下是两个值对象。借助条件查询，您可以直接从结果数据映射值对象。您无需编写代码即可将结果数据从实体类复制到value对象。这确实是条件查询中令人兴奋的功能。

public class StateVO {private Long stateId;private String stateName;private Set cityVOSet;// All getter setters}
public class CityVO {private Long cityId;private String cityName;private StateVO stateVO;public CityVO( Long cityId, String cityName){this.cityId=cicityId;this.cityName=cityName;}// All getter setters
}

DAO实施：

现在是时候使用标准查询来获取数据了。我们将公开一个将输入参数作为StateVO并返回CityVO列表的方法。

import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;public class StateCityDAOImpl{public List getCityList(StateVO searchStateVO)  {// Get the entity manager instanceEntityManagerFactory emf = Persistence.createEntityManagerFactory("StateCityService");EntityManager entityManager= emf.createEntityManager();// Get the criteria builder instance from entity managerfinal CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();// Create criteria query and pass the value object which needs to be populated as resultfinal CriteriaQuery criteriaQuery = criteriaBuilder.createQuery(CityVO.class);// Tell to criteria query which tables/entities you want to fetch// To join the City and State tables, we need to write below codefinal Root stateRoot = criteriaQuery.from(State.class);final Root cityRoot = criteriaQuery.from(City.class);// Time to define where clause in terms of Predicates// This list will contain all Predicates (where clauses)List criteriaList = new ArrayList();// Note: Ensure that whatever string you are passing in root variables// It should be matched with variables' name in entity classes// [1] where condition: State.StateId = City.StateIdPredicate predicate1 = criteriaBuilder.equal(cityRoot. get("state"). get("stateId"),stateRoot.  get("stateId"));criteriaList.add(predicate1);// [2] where condition: City.StateId = ?if (searchStateVO.getStateId() != null) {Predicate predicate2= criteriaBuilder.equal(cityRoot. get("state"). get("stateId"),searchStateVO.getStateId());criteriaList.add(predicate2);}// [3] where condition: City.cityName like 'M%'Predicate predicate3= criteriaBuilder.like(criteriaBuilder.upper(cityRoot. get("cityName")),"M%");criteriaList.add(predicate3);  // This statement maps your CityVO with result data// You have to have a custom constructor in CityVO  (see above) to populate the result data   criteriaQuery.select(criteriaBuilder.construct(CityVO.class, cityRoot. get("cityId"),cityRoot. get("cityName")));// Pass the criteria list to the where method of criteria querycriteriaQuery.where(criteriaBuilder.and(criteriaList.toArray(new Predicate[0])));// Order by clause based on city namescriteriaQuery.orderBy(criteriaBuilder.asc(cityRoot. get("cityName")));// Here entity manager will create actual SQL query out of criteria queryfinal TypedQuery query = entityManager.createQuery(criteriaQuery);// This code can be used to define the row range to fetch the result/* if (CitySize != 0) {query.setFirstResult(startIndex); // starting row indexquery.setMaxResults(endIndex); //  end row index}*/return query.getResultList();}

现在我们有了CityVO清单作为您的答案。我们可以根据上述要求访问相应的城市名称。

虽然，使用条件查询编写查询可能有点笨拙，但是一旦使用方便，您就会喜欢“条件查询”。

参考：我们的JCG合作伙伴 Narendra Verma在NS.Infra博客上的《使用Hibernate进行JPA 2.0标准查询》。