Hibernate学习笔记 查询简介

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。