Hibernate学习笔记查询简介

不念不忘少年蓝@ 2022-07-13 13:06 173阅读 0赞

## 创建实体类 ##

在介绍Hibernate查询语言之前，首先我们来建立一下数据库。这里直接使用了MySQL自带的样例数据库world。如果你没有安装MySQL那么需要安装一下，并且在安装的时候选择安装样例数据库。

安装完成之后，应该能在MySQL中看到一个名为world的数据库，其中有三个表，country、city以及countrylanguage表。然后我们来建立这三个表对应的实体类。需要注意，由于这一次是针对已经存在的数据库，所以在`hibernate.cfg.xml`中`<property name="hbm2ddl.auto">update</property>`这一行应当设置为update，避免Hibernate重新创建表覆盖掉原有的数据。

这三个表有点长，所以会影响到阅读。由于countrylanguage表存在两个主键，而且Hibernate要求复合主键的实体类必须实现Serializable接口，所以这里也实现了这个接口。

@Entity
    public class City { 
        private int id;
        private Country country;
        private String district;
        private String name;
        private Integer population;
    
        @Id
        @Column(name = "ID")
        public int getId() {
            return id;
        }
    
        public void setId(int id) {
            this.id = id;
        }
    
        @ManyToOne
        @JoinColumn(name = "CountryCode", foreignKey = @ForeignKey(name = "countryLanguage_ibfk_1"))
        public Country getCountry() {
            return country;
        }
    
        public void setCountry(Country country) {
            this.country = country;
        }
    
        @Basic
        @Column(name = "District")
        public String getDistrict() {
            return district;
        }
    
        public void setDistrict(String district) {
            this.district = district;
        }
    
        @Basic
        @Column(name = "Name")
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        @Basic
        @Column(name = "Population")
        public Integer getPopulation() {
            return population;
        }
    
        public void setPopulation(Integer population) {
            this.population = population;
        }
    
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            City city = (City) o;
            return id == city.id &&
                    Objects.equals(country, city.country) &&
                    Objects.equals(district, city.district) &&
                    Objects.equals(name, city.name) &&
                    Objects.equals(population, city.population);
        }
    
        @Override
        public String toString() {
            return "City{" +
                    "id=" + id +
                    ", country=" + country.getName() +
                    ", district='" + district + '\'' +
                    ", name='" + name + '\'' +
                    ", population=" + population +
                    '}';
        }
    
        @Override
        public int hashCode() {
            return Objects.hash(id, country, district, name, population);
        }
    }
    
    
    
    @Entity
    public class Country { 
        private String code;
        private String name;
        private String continent;
        private String region;
        private double surfaceArea;
        private Short indepYear;
        private int population;
        private Double lifeExpectancy;
        private Double gnp;
        private Double gnpOld;
        private String localName;
        private String governmentForm;
        private String headOfState;
        private Integer capital;
        private String code2;
    
        @Id
        @Column(name = "Code")
        public String getCode() {
            return code;
        }
    
        public void setCode(String code) {
            this.code = code;
        }
    
        @Basic
        @Column(name = "Name")
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        @Basic
        @Column(name = "Continent")
        public String getContinent() {
            return continent;
        }
    
        public void setContinent(String continent) {
            this.continent = continent;
        }
    
        @Basic
        @Column(name = "Region")
        public String getRegion() {
            return region;
        }
    
        public void setRegion(String region) {
            this.region = region;
        }
    
        @Basic
        @Column(name = "SurfaceArea")
        public double getSurfaceArea() {
            return surfaceArea;
        }
    
        public void setSurfaceArea(double surfaceArea) {
            this.surfaceArea = surfaceArea;
        }
    
        @Basic
        @Column(name = "IndepYear")
        public Short getIndepYear() {
            return indepYear;
        }
    
        public void setIndepYear(Short indepYear) {
            this.indepYear = indepYear;
        }
    
        @Basic
        @Column(name = "Population")
        public int getPopulation() {
            return population;
        }
    
        public void setPopulation(int population) {
            this.population = population;
        }
    
        @Basic
        @Column(name = "LifeExpectancy")
        public Double getLifeExpectancy() {
            return lifeExpectancy;
        }
    
        public void setLifeExpectancy(Double lifeExpectancy) {
            this.lifeExpectancy = lifeExpectancy;
        }
    
        @Basic
        @Column(name = "GNP")
        public Double getGnp() {
            return gnp;
        }
    
        public void setGnp(Double gnp) {
            this.gnp = gnp;
        }
    
        @Basic
        @Column(name = "GNPOld")
        public Double getGnpOld() {
            return gnpOld;
        }
    
        public void setGnpOld(Double gnpOld) {
            this.gnpOld = gnpOld;
        }
    
        @Basic
        @Column(name = "LocalName")
        public String getLocalName() {
            return localName;
        }
    
        public void setLocalName(String localName) {
            this.localName = localName;
        }
    
        @Basic
        @Column(name = "GovernmentForm")
        public String getGovernmentForm() {
            return governmentForm;
        }
    
        public void setGovernmentForm(String governmentForm) {
            this.governmentForm = governmentForm;
        }
    
        @Basic
        @Column(name = "HeadOfState")
        public String getHeadOfState() {
            return headOfState;
        }
    
        public void setHeadOfState(String headOfState) {
            this.headOfState = headOfState;
        }
    
        @Basic
        @Column(name = "Capital")
        public Integer getCapital() {
            return capital;
        }
    
        public void setCapital(Integer capital) {
            this.capital = capital;
        }
    
        @Basic
        @Column(name = "Code2")
        public String getCode2() {
            return code2;
        }
    
        public void setCode2(String code2) {
            this.code2 = code2;
        }
    
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Country country = (Country) o;
            return Double.compare(country.surfaceArea, surfaceArea) == 0 &&
                    population == country.population &&
                    Objects.equals(code, country.code) &&
                    Objects.equals(name, country.name) &&
                    Objects.equals(continent, country.continent) &&
                    Objects.equals(region, country.region) &&
                    Objects.equals(indepYear, country.indepYear) &&
                    Objects.equals(lifeExpectancy, country.lifeExpectancy) &&
                    Objects.equals(gnp, country.gnp) &&
                    Objects.equals(gnpOld, country.gnpOld) &&
                    Objects.equals(localName, country.localName) &&
                    Objects.equals(governmentForm, country.governmentForm) &&
                    Objects.equals(headOfState, country.headOfState) &&
                    Objects.equals(capital, country.capital) &&
                    Objects.equals(code2, country.code2);
        }
    
        @Override
        public int hashCode() {
            return Objects.hash(code, name, continent, region, surfaceArea, indepYear, population, lifeExpectancy, gnp, gnpOld, localName, governmentForm, headOfState, capital, code2);
        }
    
        @Override
        public String toString() {
            return "Country{" +
                    "code='" + code + '\'' +
                    ", name='" + name + '\'' +
                    ", continent='" + continent + '\'' +
                    ", region='" + region + '\'' +
                    ", surfaceArea=" + surfaceArea +
                    ", indepYear=" + indepYear +
                    ", population=" + population +
                    ", lifeExpectancy=" + lifeExpectancy +
                    ", gnp=" + gnp +
                    ", gnpOld=" + gnpOld +
                    ", localName='" + localName + '\'' +
                    ", governmentForm='" + governmentForm + '\'' +
                    ", headOfState='" + headOfState + '\'' +
                    ", capital=" + capital +
                    ", code2='" + code2 + '\'' +
                    '}';
        }
    }
    
    
    
    @Entity
    public class Countrylanguage implements Serializable { 
        private String language;
        private Country country;
        private String isOfficial;
        private Double percentage;
    
        @Id
        @Column(name = "Language")
        public String getLanguage() {
            return language;
        }
    
        public void setLanguage(String language) {
            this.language = language;
        }
    
        @Id
        @ManyToOne
        @JoinColumn(name = "CountryCode", foreignKey = @ForeignKey(name = "countryLanguage_ibfk_1"))
        public Country getCountry() {
            return country;
        }
    
        public void setCountry(Country country) {
            this.country = country;
        }
    
        @Basic
        @Column(name = "IsOfficial")
        public String getIsOfficial() {
            return isOfficial;
        }
    
        public void setIsOfficial(String isOfficial) {
            this.isOfficial = isOfficial;
        }
    
        @Basic
        @Column(name = "Percentage")
        public Double getPercentage() {
            return percentage;
        }
    
        public void setPercentage(Double percentage) {
            this.percentage = percentage;
        }
    
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Countrylanguage that = (Countrylanguage) o;
            return Objects.equals(language, that.language) &&
                    Objects.equals(country, that.country) &&
                    Objects.equals(isOfficial, that.isOfficial) &&
                    Objects.equals(percentage, that.percentage);
        }
    
        @Override
        public String toString() {
            return "Countrylanguage{" +
                    "language='" + language + '\'' +
                    ", country=" + country.getName() +
                    ", isOfficial='" + isOfficial + '\'' +
                    ", percentage=" + percentage +
                    '}';
        }
    
        @Override
        public int hashCode() {
            return Objects.hash(language, country, isOfficial, percentage);
        }
    }

## HQL ##

HQL是Hibernate的数据库查询语言，看名字可以知道这种查询语言和SQL类似。其实呢，这种查询语言，其实就是SQL中把表名和列名换成了实体类名和属性名。而且如果使用IDEA这样的智能集成开发环境，还会贴心的把SQL和HQL等查询语言高亮显示，特别方便。

首先我们需要创建一个Session，然后使用session.createQuery方法创建一个Query对象，然后调用query.list方法获取查询结果。这是一个简单的例子，演示了一下HQL的基本用法。如果需要更详细的用法还是查阅相关资料更好。

@Test
        public void test() {
            try (Session session = factory.openSession()) {
                //查询所有国家
                List<Country> countries = session.createQuery("from Country", Country.class).list();
                //查询c开头的所有国家
                List<Country> countriesStartWithC = session.createQuery("from Country c where c.name like 'c%'", Country.class).list();
                //countries.forEach(country -> logger.info(country.toString()));
                countriesStartWithC.forEach(country -> logger.info(country.toString()));
                //查询中国的国家代码
                List<String> countryCodes = session.createQuery("select c.code from Country c where c.name='China'", String.class).list();
                countryCodes.forEach(c -> logger.info(c));
                //查询中国的所有城市
                List<City> cities = session.createQuery("from City c where c.country.name='China'", City.class).list();
                cities.forEach(c -> logger.info(c.toString()));
            }
    
        }

## Criteria ##

Criteria是Hibernate提供的另外一种查询语言。Criteria有两个版本，`org.hibernate.Criteria`属于旧版本的，虽然还没有标记为过时，Hibernate官方已经不推荐我们使用这种了。相应的，我们应该使用`javax.persistence.criteria.CriteriaBuilder`接口来创建和使用查询。

首先需要调用getCriteriaBuilder方法生成一个CriteriaBuilder。然后使用Builder的createQuery方法创建一个查询。Root对象代表查询的根，也就是要查询的表，然后可以使用查询对象提供的各种方法来查询我们要的数据。详细的使用方法还是需要查阅文档。

@Test
        public void testCriteria() {
            try (Session session = factory.openSession()) {
                //查询人口最少的20个城市
                CriteriaBuilder builder = session.getCriteriaBuilder();
                CriteriaQuery<City> query = builder.createQuery(City.class);
                Root<City> root = query.from(City.class);
                query.select(root);
                query.orderBy(builder.asc(root.get("population")));
                List<City> cities = session.createQuery(query).setMaxResults(20).list();
                cities.forEach(city -> logger.info(city.toString()));
            }
        }

## Native SQL ##

除了使用上面所说的查询语言之外，还可以直接使用SQL查询数据。调用Session.createNativeQuery即可创建一个SQL查询。需要注意返回的结果是一个List

@Test
        public void testSQL() {
            try (Session session = factory.openSession()) {
                //东亚所有国家和地区
                NativeQuery query = session.createNativeQuery("select code,name,population from country where Region='Eastern Asia'");
                List<Object[]> countries = query.getResultList();
                for (Object[] country : countries) {
                    String code = (String) country[0];
                    String name = (String) country[1];
                    int population = (int) country[2];
                    logger.info("Code:{} Name:{} Population:{}", code, name, population);
                }
            }
        }

以上就是Hibernate提供的几种查询方式。我这里只做了一点简单介绍，详细的使用方法还是需要查看更详细的资料。不过这些方法基本大同小异，只要熟悉SQL数据库的查询语言，学会这几种查询方法也很简单。